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ELECTRONICS I

CHAPTER I

1.1 INTRODUCTION TO ELECTRONICS 1.1.1 GENERAL OVERVIEW OF ELECTRONICS COURSE STRUCTURE

Everyone today is exposed to electronic devices in one way or another. True or False? The computer and digital revolutions are good examples. Everyone can benefit from additional knowledge of electronics. The study of Electronics-I start with a background in electricity, magnetism and Basic Electronics. These include Ohm's Law and other basic principles of Applied Electricity. 1.1.2 WHAT IS ELECTRONICS? Electronics is a branch of Physics that deals with scientific studies of the Emission of Electrons, Effects of electrons and the Use of electronic Devices

As usual,

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Physics is the scientific study of the interactions between physical systems. In other words, Electronics is the scientific study of the conduction of electricity in a vacuum, in gases and in semiconductors and the design and applications of devices or apparatus that control the movement of electrons. One reason for this is that all electrical phenomena involve the interaction between positive and negative charges (i.e., actions and reactions of electrons). The concept of electronics is built in the use of electronic components, integrated circuits and electronic systems. The field of electronics refers to the study and use of systems that operate by controlling the flow of electrons (or other charge carriers) in devices such as thermion valves and semiconductors. The design and construction of electronic circuits to solve practical problems is an integral technique in the field of electronics engineering and is

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equally important in hardware design for computer engineering. All applications of electronics involve the transmission of either information or power. 1.1.3 WHY ELECTRONICS Prior to the invention of semiconductor diode devices, vacuum tubes were the available choice of device for the rectification of alternating current. These days, and even as at today, new digital life is built on the development of miniaturized electronic circuits (microchips), broadband telephones and data transmission through optical fiber and wireless networks. The computer has been a common tool both at work and at home. By continued miniaturization of digital electronic components and circuits, the PC and other advanced electronics have been commercially available for people in general. The capacity of the computers almost doubles every year. This expansion is achievable because of tighter packaging of the components onto the microchip. But as we get closer to the atomic limit where each component on the microchip is just a few atomic lengths, we need innovative developments for the future. Modern cars have been exposed by a tremendous development where the main parts of the functions have been controlled by the electronics. The cars are equipped with electronics like airbag systems, ABS brakes, anti spinning system and burglar alarm. Within transportation, we have obtained advanced electronic navigation systems, instrument landing system for airplanes and anti collision systems for ships and cars. Automatic toll rings around the largest cities provides money for new roads and attempts for environmentally friendly traffic. Furthermore, modern electronics have revolutionized medical diagnosis by introducing new techniques like Computer Tomography (CT), Magnetic Resonance (MR) and Ultrasonic Imaging Systems (UIS). Common for the realization of these new technical developments, besides digital circuits, are the sensors that can feel sound, light, pressure, temperature, acceleration, shadow, etc., and actuators that can act, i.e. carry out specific operation like switch on a knob, or transmit sound- or light signals. The study of electronics ensures that students get a general knowledge so that they can contribute to the electronics of the future. 1.1.4 The Origin and Historical Development of Electronics Why Historical Background? Students studying Engineering are urged to study, first of all, the History of Science, Mathematics and Engineering. It is observed that, Engineering Students possessing senses of History of technological evolutions, developments or advancements acquire and develop a deeper and faster understanding of new concepts and are more creative problem solvers. Pre-Electronics. About 100 years ago, to keep people entertained at home there were only Music boxes, phonographs (Edison 1886) and gramophones (pianoforte), books, chat, in front of a fireplace, etc. To write, we could use a quill, a fountain pen or a typewriter (1829) forerunner of the modern laptop. To communicate upon long distance, we had to use the telegraph (1833) using keys and wires. The results were printed on a strip of paper. To communicate voice at long distance, we used a telephone (1876). As the voltage applied to the grid of a vacuum tube was varied from negative to positive, the amount of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive amplifier of voltages. DeForest called his invention the "Audion". In 1907, De Forest filed U.S. Patent 879532 for a three-electrode version of the Audion for use in radio communications. The device is now known as the Triode Valve.

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1.2 ELECTRONIC DEVICES What Is A Device? Technically, a device is anything made or adapted for a purpose. Therefore, Electronic Devices are electrical units/systems operated by the movement of electrons in electric circuits and apparatus consisting of vacuum, gas or semiconductor materials. There are two types of electronic devices. These are: - Vacuum Devices and Semiconductor Devices. 1.2.1 Vacuum Devices What Are Vacuum Devices Vacuum is a space where all air or other gases has been removed. Thus, vacuum devices are sealed tubes, made of glasses or metals, containing arrangements of electrodes extended out for external electrical connections. The air inside the tube is removed by air evacuators. 1.2.2 The Vacuum Tube Vacuum tube also called electron tube is a sealed glass or metal-ceramic enclosure used in electronic circuitry to control the flow of electrons between the metal electrodes, sealed inside the tubes. Differently put, Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use materials like ceramic and metal. It is an electronic device in which the flow of electron is through a vacuum. Electronically, the vacuum tube is a voltage-controlled device, which means that the relationship between the input and output circuits is determined by a trans-conductance function. In electronics, a vacuum tube is a device generally used to amplify, or otherwise modify, a signal by controlling the movement of electrons in an evacuated space. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a tube socket for easy replacement. There are several ways of classifying vacuum tubes according to the number of electrodes. Vacuum tube is classified as follows: - Vacuum Diode - Vacuum Triode - Vacuum Tetrode - Vacuum Pentode, etc. The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air. When hot, the filament releases electrons into the vacuum: a process called thermionic emission. The resulting negatively-charged cloud of electrons is called a space charge. These electrons will be drawn to a metal "plate" inside the envelope if the plate (also called the anode) is positively charged relative to the filament (or cathode). The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. This very simple example described can thus be seen to operate as a diode: a device that conducts current only in one direction. It is very important that the vacuum inside the envelope be as perfect, or "hard", as possible. Any gas atoms remaining will be ionized at operating voltages, and will conduct electricity between the elements in an uncontrolled manner. This can lead to erratic operation or even catastrophic destruction of the tube and associated circuitry. Unabsorbed free air sometimes ionizes and becomes visible as a pink-purple glow discharge between the tube elements. To prevent any remaining gases from remaining in a free state in the tube, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, with barium being the most common. While the tube envelope is being

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evacuated, the internal parts except the getter are heated by RF induction heating to extract any remaining gases from the metal. The tube is then sealed and the getter is heated to a high temperature, again by Radio frequency induction heating causing the material to evaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. The getter continues to absorb any gas molecules that leak into the tube if a tube develops a crack in the envelope, this deposit turns a white color when it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getters. Early gathered tubes used phosphorous based getters and these tubes are easily identifiable as the phosphorous leaves a characteristic orange deposit on the glass. The use of Phosphorous was short lived and was quickly replaced by the superior barium getters. Unlike the barium getters, the phosphorous did not absorb any further gasses once it had fired working life. 1.2.3 Vacuum Tube Applications For most purposes, the vacuum tube has been replaced by the much smaller and less expensive transistor, either as a discrete device or in an integrated circuit. However, tubes are still used in several specialized applications such as audio systems and high power RF transmitters, as the display device in cathode ray tube television sets, and to generate microwaves in microwave ovens. Generally, vacuum tubes are used for: - Amplification of a weak current; Rectification of an alternating current to direct current (AC to DC), Generation of oscillating radio-frequency (RF) power for radio and radar, etc. Tubes were ubiquitous (everywhere in the electronic mark) in the early generations of electronic devices, such as radios, televisions, and early computers such as the Colossus which used 2000 tubes, the ENIAC which used nearly 18,000 tubes, and the IBM 700 series. Vacuum tubes inherently have higher resistance to the electromagnetic pulse effect of nuclear explosions. This property kept them in use for certain military applications long after transistors had replaced them elsewhere. Vacuum tubes are still used for very high-powered applications such as microwave ovens, industrial radio-frequency heating, and power amplification for broadcasting. Tubes are also considered by many people in the audiophile, professional audio, and musician communities to have superior audio characteristics over transistor electronics, due to their warmer, more natural tone. There are many companies which still make specialized audio hardware featuring tube technology. 1.2.4 The Vacuum Diode In 1904 John Ambrose Fleming developed the "oscillation valve" or Kenoton when he was hired by Edison as a Scientific Adviser to the Marconi Company. Fleming later renamed the Kenotron as the Vacuum Diode. Lemings Vacuum Diode allowed electric current to flow in only one direction. This principle made rectification or conversion of Alternating Current to a Direct Current a reality. 1.2.5 So What's a Thermionic Diode? The name diode comes from the Greek word for two (2). Diode has two elements, an anode and a cathode. The simplest diodes, possibly, the earliest ones, had cathodes which consisted of filaments that could be heated to red hot or even orange hot, by passing an electric current through them. The anode is a flat metal plate, often called the plate. Circuit Symbols of Thermionic Diodes

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1.2.6 The Elements of a Vacuum Diode As shown below, the Vacuum Diode consists of Two Electrodes: the Anode or the Plate, and the Cathode. The electrode called Filament/Heater (normally not numerated) gets red hot when connected to an a.c. source, to heat the cathode before it (cathode) can emit wanted electrons.

2) Cathode put on the centre of diode b) Simplified structure and surrounded by plate and heater schematic diagram of a inserted inside cathode. When cathode Diode heated by heater element, electrons will move from cathode to plate and produce plate current.

In vacuum tube no matter how careful air is evacuated, gas molecules will still always be present, which may be in the form of ion by impact with electrons. Under the influence of electric field these Positive ions will strike the cathode and if the high voltage is being used, the cathode is subject to considerable bombardment and can be damaged. 1.2.7 What Does a Diode Do? The hot cathode emits electrons in droves (moving cloud of electrons). Many more than are needed. They build up in the volume around the cathode and form a large pool of electrons just waiting for something positive to happen. This pool of electrons is called the space charge. When the plate (anode) is made positive some of the electrons are attracted to it. They impact on it, and are absorbed into the metal and electrons flow out of the plate connection into the battery or what ever provided the positive voltage. Heater current does not flow through the cathode, but through the heater element that is known as filament. The heat energy from filament conduct into cathode through insulator placed between cathode and heater. Material used for cathode must have the following properties : a. Low work function, so that the electron emission could occur using only small amount of energy b. High melting point, as the thermionic emission occurs at high temperature so the substance used as cathode must have high melting point. c. High mechanical strength, Substances used as cathode must have strong mechanical strength to withstand the bombardment of positive ions. The other end of

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the voltage source must be connected back to the cathode in some way. If the polarity is reversed which makes the plate negative with respect to the cathode the electrons in the space charge are repelled away from the plate and no conduction takes place. The plate is cold and is made of a metal that is a very poor emitter of electrons. This makes the diode conduct current in only one direction. 1.2.8 How the Vacuum Diode Functions To get a clear idea about how diode works let us observe 3 situations as follows:

a. Diode supplied by zero voltage b. Diode supplied by negative voltage c. Diode supplied by positive voltage

1.2.9 Diode Supplied By Zero Voltage When there is no voltage difference between plate and cathode, heated electrons from cathode could never have enough energy to reach plate. These electrons will start to accumulate near the cathode by cathode forming-electron cloud. This is known as space charge. In this case, there will be no plate current flowing.

1.2.10 Diode Supplied with Negative Voltage If plate is made negative with respect to cathode, the negative charge from plate will push back the electron to cathode so the heated electron can not reach plate and still no current indication appear at the amperemeter.

1.2.11 Diode Supplied with Positive Voltage If plate is made positive with respect to cathode, the positive charge from plate will attract heated electron from cathode to reach plate and generate plate current. As the plate voltage increases, the plate current will also increase, but in only one direction due to the property of

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diodes. This is the reason why diodes are applicable for current rectifier application.

1.2.12 Diode Plate Characteristic The most important diode characteristic is the plate characteristic. It shows the correlation between plate voltage and plate current. Test circuit to achieve plate characteristic and the example of plate characteristic is shown below. The heater voltage is supplied to the filament in order to heat cathode until it reaches a certain temperature (T1). At the same time, the anode voltage (Eb) increases from 0 until it reaches maximum permissible anode voltage. The anode current (Ib) increases as the plate voltage is increased. When plate voltage reaches a certain value, anode current remains constant. However, the plate voltage can still be increased. This situation is known as the saturation point. If diode temperature is increased from T1 to T2, anode voltage will increase and a corresponding increase in anode current will occur. This situation tells us that, a change in diode temperature has direct effect on the flow of anode flow current. Increasing the diode temperature from T1, T2, T3, ..Tn, and varying the anode Voltage Eb will create a family of I-V characteristics. This is known as the Anode or Plate Characteristics of a Diode. Voltage Current Temperature Characteristics of a Diode

1.2.13 Diode Resistance We have seen that plate current flowing through a vacuum diode varies as the plate voltage is changed. Therefore diode may be considered as having internal resistance that limits the amount of plate current flow. This internal resistance offered by diode is known as its plate or anode resistance. The resistance is not the same for direct current as for the alternating current. Accordingly like any vacuum tube, diode has two types resistances, namely, dc plate

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resistance and ac plate resistance. As the plate characteristic is not a straight line, therefore DC plate resistance is not constant but depends upon operating point. Thus, in real diode applications, DC plate resistance must be determined at the actual operating point. AC plate resistance offered by diode to alternating current and defined as the ratio of a small change in plate voltage across a diode to the resulting change in plate current. Figure 12.B shows us the way to measure AC plate resistance. According to the definition of AC plate resistance, the ac plate resistance rb is given by rb = BC/YZ. As tubes are generally used with AC voltage than DC voltage so the ac plate resistance is more important than DC plate resistance.

1.2.14 RECTIFICATION & RECTIFIERS What Is Rctification? Rectification is an electronics process whereby alternating current (AC) is converted into direct current (DC). Rectification is commonly performed by Vacuum Tubes & Semiconductor Diodes. Before the development of solid state rectifiers, vacuum tube diodes and copper oxide or selenium rectifier stacks were used. Early radios, called crystal sets, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point contact rectifier / diode or "crystal detector. In gas heating systems "flame rectification" can be used to detect a flame. 1.2.15 THE RECTIFIER What Is A Rectifier? A rectifier is an electronic device, comprising one or more semiconductor devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. When just one diode is used to rectify AC (by blocking the negative or positive portions of the waveform) the difference between the term diode and the term rectifier is merely one of usage, e.g., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with just a single diode. 1.2.16 Types Of Rectifier There are Two Main types of Rectifier. They are: Half Wave Rectifier and Full Wave Rectifier 1.2.17 The Vacuum diode As A Rectifier

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The fig. below shows a basic experimental vacuum diode rectifier circuit. A transformer has its secondary side connected to common. The other side is connected to the plate of a vacuum diode. The primary is not shown as connected to anything but it is assumed that it is connected to a source of AC. The filament of the tube is powered by a battery. The side of the filament which connects to the batterys positive also connects to a resistor. The other end of the resistor goes to common (ground).

Animatronics of a Rectifier in action. There are two voltmeters in the circuit. One is connected from the plate of the diode to common and the other is connected across the load resistor. Both meters have zero marked at the center with positive voltage to the right and negative voltage to the left. These unreal meters can follow individual cycles of the input AC. As the animation runs, the pointer of the meter on the plate of the tube moves slowly to the right and returns to zero continuing on to the negative extreme and returning to zero again. This keeps repeating. The meter across the load resistor indicates the same voltage as on the plate when that meter is positive only. When the meter on the plate goes negative, the one across the load resistor remains at zero. In the tube small dots represent electrons. When both meters are positive the electrons come out of the filament and are absorbed by the plate. When the plate meter is negative the electrons come out of the filament go about half way to the plate then turn around and go back to the filament where they are reabsorbed. 1.2.18 Vacuum Diode Half-Wave Rectification The fig. below shows a fundamental Half-Wave Vacuum Diode Rectifier circuit. It has a power transformer with two secondary windings. The top winding has many more turns than the other one. This top winding is the high voltage winding of the transformer. The bottom of the high voltage winding goes to ground. The top of this winding goes to the plate of an indirectly heated cathode type diode. Above the lead from the transformer to the tube is a wave indicating a normal sine wave on this lead. Each side of the filament connects to its own side of the other winding on the transformer. One side of this heater winding is grounded. The separate cathode connects to a resistor. The other end of the resistor connects to the bottom of the high voltage winding and ground. A wave form indicates that the voltage across the resistor consists of only the positive halves of the sine wave in which the diode conducts. When the sine wave at the plate of the tube is negative, the output wave stays at zero due to the property of a diode.

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Full-wave This schematic is, by far, the most used as it uses both the half-waves to produce DC. Filtering is much easier. We use two diodes or a bi-plate diode to acheive a full rectification.

Voltage Doubler This schematic diagram uses two diodes with their outputs serially connected to obtain a DC that is the double of its original AC voltage.

Filters for Rectification To "fill the gaps" of the DC coming from the diodes, we must filter it through various cells (capacitors). All the following formulae correspond to a full-wave rectifier. Filtering is much easier and effective in such configuration. A Capacitor Only With No Load - Filter In such a configuration, the voltage will reach progressively (but quickly!) the Vmax value. That's the reason why we should always use capacitors with a nominal voltage higher than the Vmax.

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A capacitor only with a load - Filter That's the simplest way to produce DC current. Pay attention to the value of the capacitor: if too large, it will destroy the diode by a high current appeal when powering ON and some diodes (gas diodes) do not accept a capacitor at all, we should use a choke as first element!

In this configuration the voltage never reaches Umax but varies between a minimum and a maximum. The difference between these two voltages is the ripple, we can calculate it with the following formula: Vrip0 = 1 / 2 FsC1 [Volts], Where Vrip is the ripple voltage Fs is the mains supply frequency = 50 Hz (Europe); 60 Hz (USA). C1 is the reserviour capacitor. We can also easily understand why a bi-plate diode is advantageous: the half sine waves are twice which results in a much lower ripple (2 times less: that's the "2" in the formula)! A Pi filter (self/capacitors) It's a common type and used very often. It could be considered as a capacitor filter plus a self/capacitor filter. The ripple will go down furthermore by a factor a1. W corresponds to the pulsation.

Cascade Pi filters Each cell will reduce the ripple by its own factor (a1, a2, ...).

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Cascade Resistors Filters Each cell will reduce the ripple by its own factor (a1, a2, ...). Mixed filters Of course we can mix self-capacitors, resistors and cells, each cell having its own ripple reducing factor. Low Voltage Capacitors It's sometimes difficult to find capacitors with high voltage specifications. We can put 2 or more capacitors serially, their final capacitance will decrease:

1 1 1 ----- = ----- + ----- Ctot C1 C2 For example: two 50uF/200V caps will result in a block of 25uF, but their voltage acceptance will increase to 400V. A good thing is to put resistors (high values) in parallel with each capacitor to equilibrate the voltage that each capacitor will face, as shown below.

Stored Energy In the previous examples, only ripple was considered. Another aspect should be considered: the amount of energy stored in the last supply stage. This stage will furnish the instantaneous current to the power tubes. As music is mainly composed by transients, it shouldn't be neglected. The energy stored by a capacitor is calculated by: J = [CV2] Where J = energy in Joules C = capacity in Farads V = voltage in Volts 1 Joule corresponds to the energy spent by a power of 1 Watt during 1 second. To maintain a power of 10 Watts during 10 seconds, we need 100 Joules which means, for a 500V supply, a capacity of 800uF. But remember, this energy is given by such a cap fully discharged, thus with a final voltage (the one supplying the power tubes !) down to 0 ...The previous supply stages charge continuously these caps but nonetheless if we don't want the supply to collapse, the final capacity should be much higher. It's rarely the case in commercial

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products, mainly for costs reasons, and increasing the size of the final caps is nearly always advisable. Some comments about huge power supplies: A discharging circuit should be used to avoid maintaining a high voltage on the tubes that do not conduct any more, which would reduce their life unnecessarily. This circuit will also prevent injuries to the audiophile ... A two-step charging procedure (through a power resistor, bypassed once the caps are charged enough) is advisable to avoid to blow the rectifying diode. Silicium diodes are more resistant but it's usually the fuse that will blow. Increasing the value of the fuse is dangerous as it could not blow any more when a real problem occurs. The Vacuum Triode Valve In 1906 Dr Lee De Forest (1873-1961), an American scientist placed the third electrode between cathode and plate of a vacuum diode. The resulting devices, as shown below, were called a Triode Valve.

What Is a Triode Valve?

The original three-element device was invented in 1906 by Lee De Forest who called it an Audion. The Audion did incorporate, in an imperfect form, the key principle of allowing amplification. As its name implies, the Triode Valve has three electrodes. These are cathode, grid and Anode. As shown above, the Cathode stands on the centre surrounded by grid which is in the form of wire mesh. There are spaces between that wire mesh, plate cover both cathode and grid. Heated electrons from cathode move to plate through the gap between those wire meshes. In the real application of triode, grid controls the electron flow inside triode. A triode valve is a type of vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electrical amplification device. The name triode appeared later, probably when it became necessary to distinguish it from other generic kinds of vacuum tubes with more elements [Tetrodes (4), Pentodes (5) etc]. The original Audion tubes were not vacuum tubes however, as they deliberately contained some gas at low pressure. The name triode is only applied to vacuum tubes. Why Vacuum Triode Valve As stated above, the Triode Vacuum Tube was the first electrical amplification device. Triodes are said to be largely obsolete today, having been replaced by the transistor, However, Triodes can still be found in applications where power consumption and overall size are not concerns, but low component count and high power capacity are. They are also still valued by musicians and audiophiles for amplification purposes, as some claim that triodes are still more linear at audio-frequency ranges and have less musical distortion characteristics.

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How the Vacuum Triode Valve Operates The principle of its operation is that, like in a vacuum tube based diode, the heated filament causes flow of electrons that hit the plate and create an electric charge to it. The control grid is then charged negatively to repel some of the electrons back towards the filament: the larger the charge on the grid, the smaller the charge created on the plate. The Elements of a Vacuum Triode Basically, The Vacuum Triode consists of Three Electrodes (Triode is a Greek word Meaning 3 electrodes). These are: Anode (positive potential), Grid (negative potential) and Cathode (negative electrode). Again there is a Filament/Heater which ensures that the cathode is thermally heated enough to release electrons.

To get a clear idea about how the control grid works, let us see the graphicals below. When grid stands on negative voltage fig. a, or less than zero, the negative charge of grid will push back heated electron from cathode to emit and reach plate, so under this condition there is no electron emit from cathode and reach plate. Regarding to this situation there is a certain grid voltage that is to totally unabled heated electrons from cathode to emit and this grid voltage known as rid cutt-off voltage. When grid stands on zero volt fig.b, the heated electron from cathode start to emit and reach the plate. This is because, zero voltage is still positive compared to negative charge of electron. Zero voltage is enough to attract heated electron to emit from cathode. When grid stand on positive voltage fig.C, positive charge from grid will attract strongly heated electron from cathode to emit through gap between wire mesh and reach plate. As grid positive voltage increases up, the amount of emitted electron will also increase up.

Triode Characteristics There are two important characteristics of triode, Plate Characteristic and Mutual Characteristics. Test circuit for obtaining anode characteristics shown below. There are two different variable voltage supplies for grid and plate. Grid voltage, Plate voltage and plate current are measured by different meters. By this way both plate and mutual characteristic will be achieved.

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a) Anode / Plate Characteristics Anode / Plate Characteristics show the correlation between plate voltage and the resulting plate current at the same grid voltage. b) `Mutual Characteristics. Mutual Characteristics show correlation between grid voltage and the resulting plate current at the same plate voltage. On real applications both Mutual and Plate characteristic are widely used to design tube electronic circuits. Valve Parameters

While a transistor is a current controlled current amplifier, an amplifying valve behaves more like a voltage controlled resistance. Yet, as in the case of the transistor, the valve is employed in circuits in which it is the change in current produced by a change in voltage that is of interest. The change takes place around an operating point on the curve, so that for a change of i the corresponding change in e will be found along the slope of the curve. As the grid goes negative it repels the electrons from the cathode, reducing the anode current. Cut-off occurs when the grid negative potential is large enough to prevent the flow of any electrons to the anode. For small positive grid potential, electrons are speeded up towards the anode and Ia increases. For these small positive bias voltages, the transfer characteristics remain linear. However, higher positive bias will attract the electrons to the grid itself causing grid current to flow, a situation that usually must be avoided (but there are special exceptions). For this reason the control grid's DC potential is nearly always maintained negative with respect to the cathode, and this is called negative bias. Characteristics of Valve Amplifiers Valves are high voltage/low current devices in comparison with transistors (and especially MOSFETs) and their transfer characteristics show very flat anode current vs. anode voltage indicating high output impedances. The high working voltage makes them well suited for radio transmitters, for example, and valves remain in use today for very high power radio transmitters, where there is still no other technology available. However, for most applications requiring an appreciable output current, a matching transformer is required. The transformer is a critical component and heavily influences the performance (and cost) of the amplifier. Many power valves have good open-loop linearity, but only modest gain or transconductance. As a result, valve amplifiers usually need only modest levels of feedback. Signal amplifiers using tubes are capable of very high frequency response ranges - up to radio frequency. Indeed, many of the Directly Heated Single Ended Triode (DH-SET) audio amplifiers are in fact radio transmitting tubes designed to operate in the megahertz range. In practice, however, tube amplifier designs typically "couple" stages either capacitively, limiting bandwidth at the low end, or inductively with transformers, limiting the bandwidth at high end. Circuit Advantages of Valves Good for high power systems. Electrically very robust, they can tolerate overloads for minutes which would destroy bipolar transistor systems in milliseconds. Disadvantages of Valves

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Heater supplies are required for the cathodes. Dangerously high voltages are required for the anodes. Valve audio equipment is normally heavy because of the weight of transformers. Valves often have a shorter working life than solid state parts because the heaters tend to fail. Valves are fragile and break if hit, since they are usually made of glass. Solid state components don't have this problem. Applications of the Vacuum Triode Valve A valve amplifier (UK and Aus.) or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical signal, typically (but not exclusively) sound or radio frequency signals. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s, and replacement valves are no longer produced in the same large quantities as they were in the past. Specially constructed valves are still in use at high power levels, especially at microwave frequencies; see the Microwave amplifiers section. The Cathode Follower An amplifier with the anode as the common terminal is called a cathode follower. The name is logical because the cathode potential varies with and is almost equal to the input voltage. The circuit and its equivalent are shown below, the bias resistances are replaced by their parallel equivalent Rg. Neglecting the valve capacitances, the mesh equations are: Eg = Es IpRk; mEg = 1p (rp + Rk). The circuit is used as an impedance-matching device, to couple a high impedance source to a lower impedance circuit, analogous to the function of the emitter-follower with the transistor.

The Disadvantages of Triode Valve When the first time triode was developed by Dr. Lee De Forest it gave much idea for further development that had never been imagined before, but triode still have two disadvantages: a) Low Amplification Factor (mu). The mu of triodes is still considered as not high

enough (max. 1000) for many electronic applications. b) Inter-electrode capacitance between the triode electrodes make it become insufficient

for high frequency application. c) Bulkiness. d) Occupies large space. The inter-electrode capacitance are: Cgk (between grid cathode), Cgp (between grid and plate), and Cpk (between plate and cathode). Generally value of the inter-electrode capacitance is in the range of 2 12 picrofarad. Cgp especially acts as Miller Feedback capacitance that limits the high frequency performance of the Triode Tube.

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1.3 ELECTRON EMISSIONS What Is Electron Emission Electron emission is actually the basics of tube working principal; it is defined as liberation of free electron from a surface of a substance caused by the external energy transferred to the electrons. Electron emission tends to occur on metal, because metal is a substance with much free electron in between its molecule. Nucleus attracting force is not strong enough to put the electron standstill. Every time the free electrons move around from one molecule to another but it can't leave out from metal surface.

In order to emit from the metal surface these free electrons require additional external energy. The amount of outside energy required by electron to emit from the metal surface is known as work function. The work function is usually defined in electron volt (eV) unit. The additional external energy required by the electron to emit from the metal surface could come from few source such as heat energy, energy stored in the electron field, light energy or kinetic energy. Types of Electron Emission There are Four Main types of Electron Emission. These are: - Thermal / Thermionic Emission - Photo Emission - Secondary Emission - Field Emission What Is Thermal Emission? Thermal / Thermionic Emission is the flow of electrons from a metal or metal oxide surface, caused by thermal vibrated-energy overcoming the electrostatic forces holding electrons to the surface. The effect increases dramatically with increasing temperature of about (10003000 K). The science dealing with this phenomenon is thermionics. The charged particles are called thermions. In this method the additional energy come to the electron in the form of heat energy, by the electrons the energy trasnferred into kinetic energy. As the kinetic energy of electron increase its movement becomes uncertain and then finally there will be electrons that leave out from the metal surface. The substance where the electrons emit from is known as emitter or cathode. In case of vacuum tube it is preferably to call as cathode. And the substance that receive electron is known as anode or plate. With regard to thermionic emission there are two types of cathode

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a) Direct Heated Cathode ( in short DHC) b) Indirect Heated Cathode ( is short IHC). In this type of cathode, both heater current and also emitted electrons come from it. PHOTO EMISSION In this type of emission the additional energy comes to the cathode by photons. When a beam of light strikes the surface of the cathode, the energy from photons will be transferred to free electrons within the cathode. If the energy from photons is greater than the metal work function the free electron will knock out from the cathode surface. The emitted electron is called as photo electron. The amount of photo electron depends of the light intensity.

SECONDARY EMISSION Electron emission from a metallic surface by the bombardment of high speed electrons or other particles is known as secondary emission. When high speed electrons suddenly strike a metallic surface, they may give some or all of their kinetic energy to the free electrons in the metal. If the energy of the striking Electrons is sufficient enough, the free electrons will escape from the metal surface. This phenomenon is called Secondary Emission.

The principle of secondary emission is described below. A glass envelope consists of electron source, cathode and plat. When electrons from electron source strike cathode they will knock out secondary electron from cathode which are attracted to plate by positive voltage from

19

plate. The effect of secondary emission is very undesirable in many electron devices for example in Tetrode Valve where secondary emission is responsible for negative resistance. Field Emission This type of emission of additional energy comes in the form of electric field. When a conductor put in a place very close to high voltage conductor, the electric field from the conductor will exert attractive force on the free electron in the metal. If the positive field is big enough the free electron will succeed in overcoming restraining of the metal surface and it will emit from the metal surface.

Very intense electric field is required to produce Field emission. Usually a voltage of the order of a million volts per centimetre distance between the emitting surface and the positive conductor is necessary to cause field emission. Field emission can be obtained at temperature much lower than required for thermionic emission and therefore it is also sometimes called as cold cathode emission or Auto electronic emission.

CHAPTER 2.

FUNDAMENTALS OF SEMICONDUCTOR DEVICES 2.1 THE NATURE OF ATOM The Constitution of Matter What Is Matter? Matter is anything that has Mass, Volume & can occupy Space. Matter is made up of very Tiny Particles called Molecules. A Molecule is the smallest (indivisible) particle of a substance or compound that can exist on its own, take part in chemical reactions and can maintain the physical and chemical properties of that substance / compound. A Molecule is made up of two or more Atoms. Matter is constituted by: Elements, Compounds and Molecules. There are approximately 110 basic substances known to man and these are simply called elements or chemical elements and they are fundamental to nature, and cannot be reduced further by chemical process. The chemical elements are made up of a number of smaller particles known as atoms. Compounds are two or more different elements combined to form a chemical compound.

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Matter Can Exist In Three Different Forms:

1. Solids 2. Liquids 3. Gases. For this course, we will study only solid state materials which will finally lead us to the study of semiconductor devices. Solids materials have Fixed Definite Sizes and Shapes. Examples are: Stone, A Piece of Chalk, Iron, Etc. What is A Molecule The smallest particle of a compound is a molecule. Each molecule has the atoms contained in the compound. Thus the word molecule also describes the combined atoms of the same element e.g. Ozone (O3) which is triatomic, i.e. it contains three atoms of oxygen. Certain elements are very reactive and react readily with other substances e.g. sodium (Na). The reason being that, hey are either short of electrons, or have too many for an ideal atomic design. E.g. inert gases, such as helium (He), neon (Ne), argon (Ar), Krypton (Kr), xenon (Xe), and radon (Rn). They do not combine with other elements under normal conditions, not even atoms of their kind. ATOMIC STRUCTURE OF MATTER According to Modern Theory of Atomic Structure of different materials, every Element of any Material is composed of Molecules. Molecules are the smallest divisible particles of any element. A Molecule is made up of Atoms. The smallest unit of all matter is the Atom. An Atom consists of Two Parts: Nucleus & Electrons. The First Part of an Atom is made up of The NUCLEUS.

No. of Electrons = 2 x n2

The Nucleus is the central part of an Atom. The Nucleus consists of a number of PROTONS and NEUTRONS. The Protons are POSITIVELY Charged Particles. The Neutron does not have any ELECTRICAL Charge (Neutral). The presence of the Protons around the Nucleus makes the Nucleus POSITIVELY Charged. The Nucleus constitutes almost the entire weight of an Atom. This weight is called the ATOMIC WEIGHT of an ATOM The second part of an Atom is made up of ELECTRONS. Electrons are NEGATIVELY Charged Particles. A number of Electrons revolve around the Nucleus of an Atom. The Circular-territorial revolving or movements of Electrons around the Nucleus are called ELECTRON ORBITS. Multiples of Electron Orbits are called ELECTRON SHELLS. The Distribution of Electrons in different Orbits is determined by the formula:

Where n is the Number or order of Orbits counted from the Nucleus. The Maximum No. of Electrons that different Electrons can contain are given below: First Orbit: 2 x 12 Second Orbit: 2 x 2

= 2 2

Third Orbit: 2 x 3 = 8

2

Fourth Orbit: 2 x 4 = 18

2

The nth Orbit: 2 x n = 32

2

Orbital-Electrons

= .

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IMPORTANT POINTS TO REMEMBER Neutrons & Protons, the heaviest Particles, make up the Nucleus (the core) of the Atom. Neutrons have no Electrical Charges. Protons are Positively Charge. Electrons which are negatively charged have a weight of about 1/1800 that of a Neutron or Proton, very light weight. An Atom contains an Equal Number of Electrons & Protons. IMPORTANT POINTS TO NOTE! 1. The composition of Nucleus, The of No. of Electrons & The shape of Orbits are all Different for different Elements. 2. The Positive Charge in the Nucleus equals the total Negative Charge in the Electrons, so that the Atom as a whole is Electrically NEUTRAL. 3. The Electrons carry Large Charges as compared to their mass, i.e., Charge / Mass equals to 1.77 X 10-11

As a theory, the Bohr Model can be derived as a first-order approximation of the hydrogen atom in the broader and much more accurate

Coulombs/kg (very small mass). 4. This makes electron very mobile. 5. The Nucleus being many times heavier than an Electron is quite immobile. THE BOHR ATOM The Most Fundamental Unit of all Matter is the Atom. We shall consider materials leading to semiconductors by studying the single isolated atom. Investigation of the properties of Solid Materials containing many combined Atoms shall then flow. The Atom consists of THREE DISTINCT types of PARTICLES. These are: NEUTRONS, PROTONS AND ELECTRONS.

quantum mechanics, and thus may be considered to be an obsolete scientific theory. However, because of its simplicity, the Bohr model is still commonly taught to introduce students to quantum mechanics. In 1913, Niels Bohr incorporated this idea into Bohr model of the atom, in which the electrons could only orbit the nucleus in particular circular orbits with fixed angular momentum and energy, their distances from the nucleus being proportional to their respective energies. They were not allowed to spiral into the nucleus, because they could not loose energy in a continuous manner; they could only make quantum leaps between fixed energy levels.

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The borh atom The Bohr model of the hydrogen atom is as shown above. The atomic nucleus is shown in green, the electron in blue and the emitted photon in red. The electronic orbital are shown as dashed black circles; their radii grow like n2, where n is the principal quantum number. A hydrogen atom is an atom of the chemical element hydrogen. It is composed of a single negatively-charged electron circling a single positively-charged proton which is the nucleus of the hydrogen atom. The electron is bound to the proton by the Coulomb force. The hydrogen atom has special significance in quantum mechanics and quantum field theory as a simple two-body problem physical system which has yielded many simple analytical solutions in closed-form. In 1913, Niels Bohr obtained the spectral frequencies of the hydrogen atom after making a number of simplifying assumptions. These assumptions were not fully correct, but did yield the correct energy answers (see The Bohr Model).

The Bohr model Of The Hydrogen Atom In atomic physics, the Bohr model depicts the atom as a small, positively charged nucleus surrounded by electrons that travel in circular orbits around the nucleus similar in structure to the solar system, but with electrostatic forces providing attraction, rather than gravity. The Bohr model of the hydrogen atom, where negatively charged electrons confined to atomic shells encircle a small positively charged atomic nucleus, and that an electron jump between orbits must be accompanied by an emitted or absorbed amount of electromagnetic energy h. The orbits that the electrons travel in are shown as grey circles; their radii increase n2, where n is the principal quantum number. The 32 transition depicted here produces the first line of the Balmer series, and for hydrogen (Z = 1) results in a photon of wavelength 656 nm (red).

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2.2 ENERGY-BAND IN SEMICONDUCTORS Electron Energies in Solids Energy Bands The energy levels available to the single electron of the Hydrogen Atom are numbered n=1,2; in increasing order of energy. There are an infinite number of energies between the various levels. However the hydrogen electron can exist only at one of the permissible levels. The higher the energy level, the further the electrons orbit is away from the nucleus. The atom is made up of a heavy nucleus around which one or more electrons revolve in orbits. For each isolated atom, however, there are only a certain number of orbits available. Theses available orbits represent energy levels for the electrons. Each orbit corresponds to a certain value of total electron energy. No more than two electrons may exist in any one level or orbit. The energy unit which we will employ in all our work on atomic theory and semiconductor will be the electron volt. It is defined as that amount of energy gained or lost when an electron moves with or against a potential difference of one volt. In terms of joules, a common unit of energy, an electron volt (abbreviated eV is equivalent to: 1.6 X 10-19

The electrons of a single free-standing atom occupy

joules. Why Energy-bands Occur

atomic orbital, which form a discrete set of energy levels. If several atoms are brought together into a molecule, their atomic orbital split due to the Pauli Exclusion Principle. This produces a number of molecular orbital proportional to the number of atoms. When a large number of atoms (of order 1020 or more) are brought together to form a solid, the number of orbits becomes exceedingly large, and the difference in energy between them becomes very small. However, some intervals of energy contain no orbits, no matter how many atoms are aggregated. These energy levels are so numerous as to be indistinct. First, the separation between energy levels in a solid is comparable with the energy that electrons constantly exchange with phonons (atomic vibrations). Second, it is comparable with the energy uncertainty due to the Heisenberg uncertainty principle, for reasonably long intervals of time. However the Hydrogen Electron can exist only at One of the Permissible Levels. The Higher the Energy Level, the further is the Nucleus away from the Electrons Orbit.

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Any solid has a large number of bands. In theory, it can be said to have infinitely many bands (just as an atom has infinitely many energy levels). However, all but a few lie at energies so high that any electron that reaches those energies escapes from the solid. These bands are usually disregarded. Bands have different widths, based upon the properties of the atomic orbital from which they arise. Also, allowed bands may overlap, producing (for practical purposes) a single large band. Metals contain a band that is partly empty and partly filled regardless of temperature. Therefore they have very high conductivity. The uppermost occupied band in an insulator or semiconductor is called the valence band by analogy to the valence electrons of individual atoms. The lowermost unoccupied band is called the conduction band because only when electrons are excited to the conduction band can current flow in these materials. Energy Band Theory of Solids A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically. An important parameter in the band theory is the Fermi level, the top of the available electron energy levels at low temperatures. The position of the Fermi level with the relation to the conduction band is a crucial factor in determining electrical properties.

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Insulator Energy Bands Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band). E.g., Glass is an insulating material which may be transparent to visible light for reasons closely correlated with its nature as an electrical insulator. While the doping of insulators can dramatically change their optical properties, it is not enough to overcome the large band gap to make them good conductors of electricity. However, the doping of semiconductors has a much more dramatic effect on their electrical conductivity and is the basis for solid electronics. Index

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Semiconductor Energy Bands For intrinsic semiconductors like silicon and germanium, the Fermi level is essentially halfway between the valence and conduction bands. Although no conduction occurs at 0 K, at higher temperatures a finite number of electrons can reach the conduction band and provide some current. In doped semiconductors, extra energy levels are added. The increase in conductivity with temperature can be modelled in terms of the Fermi function, which allows one to calculate the population of the conduction band

Conductor Energy Bands

.

In terms of the band theory of solids, metals are unique as good conductors of electricity. This can be seen to be a result of their valence electrons

Silicon Energy Bands

being essentially free. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material.

At finite temperatures, the number of electrons which reach the conduction band and contribute to current can be modelled by the Fermi function. That current is small compared to that in doped semiconductors under the same conditions. Germanium Energy Bands At finite temperatures, the number of electrons which reach the conduction band and contribute to current can be modelled by the Fermi function. That current is small compared to that in doped semiconductors under the same conditions.

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The difference between insulators and semiconductors is only that the forbidden band gap between the valence band and conduction band is larger in an insulator, so that fewer electrons are found there and the electrical conductivity is less. Because one of the main mechanisms for electrons to be excited to the conduction band is due to thermal energy, the conductivity of semiconductors is strongly dependent on the temperature of the material. This band gap is one of the most useful aspects of the band structure, as it strongly influences the electrical and optical properties of the material. Electrons can transfer from one band to the other by means of carrier generation and recombination processes. The band gap and defect states created in the band gap by doping can be used to create semiconductor devices such as solar cells, diodes, transistors, laser diodes

The difference between insulators and semiconductors is only that the forbidden

, etc.

band gap between the valence band and conduction band is larger in an insulator, so that fewer electrons are found there and the electrical conductivity is less. Because one of the main mechanisms for electrons to be excited to the conduction band is due to thermal energy, the conductivity of semiconductors is strongly dependent on the temperature of the material. This band gap is one of the most useful aspects of the band structure, as it strongly influences the electrical and optical properties of the material. Electrons can transfer from one band to the other by means of carrier generation and recombination processes. The band gap and defect states created in the band gap by doping can be used to create semiconductor devices such as solar cells, diodes, transistors, laser diodes

In a solid, the atoms are so close to each other that certain important changes occur in the state of the energy levels. When atoms are brought into close proximity, as in a solid, the energy levels which existed for single isolated atoms, split up to form bands of energy levels. Within each band there are still discrete permissible energy levels rather than a continuum. There are many bands below the level of the valence band shown in this figure, however, only the two upper bands are of interest in considering electrical properties. The upper most energy band is the conduction band. Separating these two hands is a forbidden energy band, which may not be occupied by any electron in the silicon crystal. The valence band is the band of electron energies which contains all the energy levels available to the valence electrons in the structure. These valence electrons are more or less attached to the individual atoms and are not free to move about as are the electrons in the conduction band. Every valence electron has energy in the valence band. In

, etc.

solid state physics, the electronic band structure (or simply band structure) of a solid describes ranges of energy that an electron is "forbidden" or "allowed" to have. The band structure determines a material's electronic properties, optical properties, and a variety of other properties.

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Valence Band In solids, the valence band is the highest range of electron energies where electrons are normally present at absolute zero. In semiconductors and insulators, there is a band gap above the valence band, followed by a conduction band above that. In metals, the conduction band has no energy gap separating it from the valence band. The rest of this article refers to the valence band in semiconductors and insulators.

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The Conduction Band

: It is a band of energies in which the level of energy of the electrons is high enough so that electrons in these levels are not attached or bound to any atom but rather is mobile and capable of being influenced by an external force. The electrons that move to the conduction band are those electrons from the valence band that gains sufficient energy through some form of excitation to be elevated to the conduction band. For example, in silicon, an electron existing at an energy level near the top of the valence band needs to gain 0.7ev of energy in order to jump the gap and reach the bottom of the conduction band. It is this energy difference across the forbidden energy band that determines whether a solid behaves as a conductor, insulator or semiconductor. The valence band is the band of electron energies which contains all the energy levels available to the valence electrons in the structure. These valence electrons are more or less attached to the. Individual atoms and are not free to move about as are the electrons in the conduction band.

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ELECTRIC CURRENT: Electric Current is defined as The Movement of Charges. Since electrons are negatively charged particles, then it is logical to conclude that the ability of a material to conduct electricity depends upon the availability of free valence-band electrons, or conduction-band electrons within the material. A conductor is a solid material containing many electrons in the conduction band at room temperatures. There is no forbidden region between the valence and conduction bands on a good conductors energy-band diagram. The two energies overlap. An insulator material has an energy band diagram with a very wide forbidden energy band. It is so wide that, practically no electrons can be given sufficient energy to jump the gap from the valence band to the conduction band. SEMICONDUCTOR Is a solid material which has a forbidden energy-band which is very narrow. Its forbidden energy-band is much smaller than that of an insulator, but larger than that of a conductor. Resistivity: It is a measure of the degree to which a material opposes or resists the flow of electric current. is the symbol (the Greek letter rho)

i.e., R = ( x l ) / a

Conductivity: This is the degree to which a material allows current to flow through it. The Greek letter sigma is the symbol ofr conductivity. Different materials have different values of resistivity: the resistivity of rubber, which is an insulator, is far greater than that of copper, which is a conductor. The resistivity of silicon, which is a semiconductor, is between that of rubber and copper. Different materials have different values of resistivity: The resistivity of rubber, which is an insulator, is far greater than that of copper, which is a conductor. The resistivity of silicon, which is a semiconductor, is between that of rubber and copper. The resistivity of a material is determined by a number of factors: a) The atomic structure of the material. b) Its temperature c) The density of free charge carriers available to move under an external force such as

electric potential or light energy. Different materials have different values of resistivity: The resistivity of rubber, which is an insulator, is far greater than that of copper, which is a conductor. The resistivity of silicon, which is a semiconductor, is between that of rubber and copper. Resistivity is a measurable quantity and is independent of the geometrical shape of the material. It is an intrinsic property in the same sense that colour is, for example resistivity has the units of ohm-metre or ohm-centimetre (ohm-c). Ohms are the familiar units of electrical resistance which is also a measure of opposition to current but depends on both the resistivity of a material and on its geometry (length and cross- sectional area).

Where: - Is The Resistivity Of The Material L - Is the Length of the Material A - Is The Cross-Sectional Area Of The Material. Conductivity is the exact opposite of resistivity and mathematically, it is equal to the reciprocal of resistivity; i.e., = 1/. The units for are: 1/ohm-cm or mho/cm. Since a mho is the reciprocal of an ohm (1/r). SEMICONDUCTOR PRINCIPLES

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Solid materials are classified according to their energy-band structures as conductors, semiconductors or insulators. What determines whether a material has the electrical characteristics of a good, fair or poor conductor? Obviously the number of free charge carriers available within a material is a major factor. This number is determined by the complexity of atom, the number of valence electrons and the type of bonding between atoms. Atoms with few valence electrons tend to give them up more readily than atoms with many valence electrons. Atom possessing few valence electrons has them more loosely bound to the nucleus than atoms with many valence electrons. Low-valence atoms have narrow forbidden energy bands between conduction-bands and are thus good conductors. A simple high valence atoms have wide forbidden energy bands, making them poor conductors. GERMANIUM AND SILICON SEMICONDUCTOR MATERIALS Germanium has an atomic number (the number of electrons) of 32 and silicon has an atomic number of 14. Both have the same number of valence electrons. Since germanium is more complex, we can expect it to be a better conductor than silicon. Germanium forbidden band gap is: 0.5 eV or 0.7eV. Silicon forbidden band gap is: 0.7 eV or 1.1eV.

Basic Information

COVALENT BONDING IN SEMICONDUCTORS In semiconductor materials, the bonding between atoms is covalent. Taking germanium as an example, the covalent bonding process is illustrated in the figure below. Germanium has a valence of four and shares its 4 valence atoms with four adjacent atoms of germanium. Each germanium atom thus appears to have a valence of 8. A covalent bond is made up of one electron from each of two atoms strongly bonded together. Silicon and germanium are elements which are semiconductors. There are other semiconductor materials made up of compounds such as gallium arsenide. Generally, materials made up of 3, 4 or 5 valence atoms in a covalent bond are semiconductors. INTRINSIC SEMICONDUCTORS When external energy is applied, such as Heat, to an intrinsic semiconductor it Increases the energy of each atom in the crystal. This increase in energy may be imported to some of the valence electrons; Particularly one at an energy level near the top of valence band, may acquire Sufficient additional energy to break away from its atom and become a free Electron. In so doing, it must break it covalent bond and acquire enough energy to jump the forbidden gap. Then it will exit at an energy level in the conduction band as a free conduction electron. Liberation of this valence electron has left a vacancy in the valence structure. This vacancy is called a hole. A hole, by virtue of its positive charge, has a great attraction for an electron, if one should wander by.

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Name: Germanium Symbol: Ge Atomic Number: 32 Atomic Mass: 72.61 amu Melting Point: 937.4 C (1210.55 K, 1719.3201 F) Boiling Point: 2830.0 C (3103.15 K, 5126.0 F) Number of Protons/Electrons: 32 Number of Neutrons: 41 Classification: Metalloid Crystal Structure: Cubic Density @ 293 K: 5.323 g/cm3 Colour: grayish

Atomic Structure Number of Energy Levels: 4 First Energy Level: 2 Second Energy Level: 8 Third Energy Level: 18 Fourth Energy Level: 4

Date of Discovery: 1823 Discoverer: Jons Berzelius Name Origin: From the Latin word silex (flint) Uses: glass, semiconductors Obtained From: Second most abundant element. Found in clay, granite, quartz, sand

Facts

A Brief Explanation of Atomic Structure And Electrical Conduction

Electrical conduction is caused by electrons breaking free of their atoms and moving around. Atoms of some elements let go of their outer electrons pretty easily, which makes these elements good conductors. In other elements, the atoms hold on to their electrons, so these elements don't conduct electricity as well. Copper and silicon are used here as examples. The same general ideas apply to other elements. In these sketches, positive charge is shown in red, negative in black, and neutral in green.

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The atomic number of copper is 29, which means it has 29 protons in the middle and 29 electrons moving around the outside. (The 29 negative charges of the electrons and the 29 positive charges of the protons balance out, so the atom is neutral when all of its electrons are in place.). Copper has two electrons in the innermost shell, eight in the next shell, eighteen in the third shell, and one in the fourth shell. This means that the first three shells each have as many electrons as they can hold, and the fourth shell has one lonely electron. (The fourth shell can hold up to 32 electrons.) Because this one lonely electron is all by itself in the outer shell, it can easily separate from the rest of the atom and go roaming around, which makes copper a very good conductor.

The atomic number of silicon is 14, which means it has 14 protons in the middle and 14 electrons moving around the outside. Silicon has two electrons in the innermost shell, eight in the next shell, and four in the third shell. This means that the first two shells are completely full, and the third shell has four electrons, out of the 18 that can fit in the third shell of an atom. Something about having four electrons in that outer shell makes the shell more stable than copper's outer shell with its one lonely electron, so the electrons in the silicon atom don't wander off as easily. Since the silicon atom has a fairly firm grip on its electrons, silicon is not as good a conductor as copper is. Most house wiring is made of copper, because copper is a very good conductor of electricity and is not as expensive as other good conductors, such as gold and platinum. Silicon can conduct electricity, but not nearly as well as copper does. The conductivity of silicon depends a great deal on what is mixed with it, because this affects how tightly it holds on to its outer electrons. Silicon is the main ingredient in glass, which does not conduct electricity. Silicon is also used to make semi-conductors, which do conduct electricity, but still not as well as copper does. (That's why they're called semi-conductors instead of conductors.) 2.3 ELECTRONS AND HOLES (ELECTRON-HOLE PAIRS)

The excitation of a valence electron into a conduction band is always accompanied by the appearance of a hole. The freed electron and the hole it left behind are called an electron-hole pair. When heat is the form of energy, the generated electron hole pair are said to be thermally generated and they depend on temperature. What happens to all thermally generated holes? The answer to this is that, they also become current carriers though not in the same way as the electrons. It appears that the holes move from one atom to another. This means that, current flow in a semiconductor is composed of: - Electron movements and - hole generations.

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Basic Information Name: Silicon Symbol: Si Atomic Number: 14 Atomic Mass: 28.0855 amu Melting Point: 1410.0 C (1683.15 K, 2570.0 F) Boiling Point: 2355.0 C (2628.15 K, 4271.0 F) Number of Protons/Electrons: 14 Number of Neutrons: 14 Classification: Metalloid Crystal Structure: Cubic

35

Density @ 293 K: 2.329 g/cm3 Colour: grey Atomic Structure Number of Energy Levels: 3 First Energy Level: 2 Second Energy Level: 8 Third Energy Level: 4 Isotopes IsotopeHalf LifeSi-28StableSi-29StableSi-30StableSi-312.62 hoursSi-32100.0 years Facts Date of Discovery: 1823 Discoverer: Jons Berzelius Name Origin: From the Latin word silex (flint) Uses: glass, semiconductors Obtained From: Second most abundant element. Found in clay, granite, quartz, sand

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RECOMBINATION (Refer to diagram above) It may not be apparent just what happens to the holes as they reach the edge of the semiconductor material. What actually takes place is called the process of recombination in which some of the electrons flow from the negative terminal of the battery and fill the holes as they enter the conductor. Therefore recombination is the filling of holes by free electrons wandering through the crystal, and may encounter holes and recombine with them, thus annihilating (wipe out) electron-hole pairs. What, then, keeps all the electron-hole pairs from disappearing? The result is that, electron-hole pairs are continuously being generated thermally. These thermally generated electron-hole pairs compensate for the recombination losses so that at a given temperature the number of electron-hole pairs in a semiconductor crystal is essentially constant. The electrons that cause hole current are valence electrons which jumps from hole to hole and do not have enough energy to become free electrons. Free electrons move freely through the crystal without being bound to any atom. The resistivity and resistance of an intrinsic semiconductor decrease as its temperature increases because more valence electrons are able to break away from their covalent bonds and become free electrons in intrinsic semiconductor. The total number of free electrons equals the total number of holes. EXTRINSIC SEMICONDUCTOR Pure silicon (or germanium) is of little use as a semiconductor, except maybe as a heat or light-sensitive resistance device. The process of adding impurities to the semiconductor material is called doping. This is performed after the semiconductor material has been refined to a high degree of purity. The impurity material added to the semiconductor is termed as dopant. The concentration of the added impurity, (the dopant), is typically very minute, in the order of one part of impunity per ten million parts of pure semiconductor. The doped semiconductor is referred to as an extrinsic semiconductor. N-TYPE & P-TYPE IMPURITIES The effect of the impurities is to produce a predominance of either free electrons or holes. Doping impurities which add free electrons to the semiconductor material are called n -type impurities. , since they add negative carriers. Hole- producing impurities are called p-type Impurities since they add positive carries tithe semiconductor crystal.

37

38

N-TYPE IMPURITIES The element arsenic is an example of this type of impurity. It has an atomic number of 33 and falls in the valence v column. The arsenic atoms, since they have five valence electrons, do not fit in exactly with the silicon crystal structure. Only 4 of the valence electrons are required in the crystal structure. The 5th valence electron does not enter a covalent bound and is thus only loosely bound to its parent arsenic atom. For each arsenic atom present in the crystal, one virtually free electron is donated to the semiconductor material. For this reason, arsenic and all impurities with a valence greater than 4 are called donor impurities. At absolute zero temperature, the fifth Valence electron of each arsenic atom is bound to its parent atom even though it is not part of a covalent bond. At a room temperature all these electrons have absorbed small amount of energy needed to become conduction electrons. With the addition of a donor impurity, a new energy level, the donor level is introduced.

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These free electrons leave no holes behind since they have broken no covalent bonds. Thus, there are more free electrons in the conduction band than there are holes in the valence band. For this reason, in an n-type semiconductor, one doped with n-type donor impurities, electrons are the majority current carries and holes are the minority carriers. Donor impurities: phosphorous, antimony, arsenic. It is important to note that donor atoms become positive ion when they donate their electrons. The positive ions are locked in the crystal structure and cannot move to conduct current. The element indium

P-TYPE IMPURITIES

is a common p-type impurity. It has an atomic number of 49 and a valence of three.

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SEMICONDUCTORS: CHEMICAL STRUCTURE Starting with a silicon substrate: Silicon has 4 valence electrons, and therefore a lattice structure: Each atom bonds with 4 neighbors. No free electrons (poor conductor) unless you heat it up (semi

Other Group IV elements can be used for substrate, but they are harder to come by (carbon lattice for instance.)

conductor).

Si Si Si Si

Si Si Si

Si Si Si Si

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DOPING

Si Si Si Si

Si Ga Si

Si Si Si Si

Make silicon a better conductor by adding Group III or Group V elements:process called doping

hole

Si Si Si Si

Si As Si

Si Si Si Si

e

Add Group III elements like gallium or indium to get p-type material

Note that the Ga atom has only 3 valence electrons with which to bond; missing bond is called a hole

Add Group V elements like arsenic or phosphorus to get n-type material

Note that the As atom has 5 valence electrons; it has an unbonded electron

Materials are electrically neutral! Equal number of protons and electrons!

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P-N JUNCTION: THE BASIS FOR ELECTRONICS

Put p-type and n-type material together:

Essential Property: Make current flow (or not flow) by applying electric field (voltage) to metal ends.

aluminum

e e e e e e e e e

h h h h h h h h h

n-typeextra e-

p-typeextra holes

This is a diode.

Voltage Controlled Switch

+ -

PHYSICS OF P-N JUNCTIONCASE: Open Circuit

Free electrons from n-type are chemically attracted to holes in p-type.

Free electrons move across junction to fill holes: process called diffusion

Unbalanced protons are left in n-type, unbalanced electrons now in p-type.

e e e e e e

h h h h h h h h h

n-typep-type

+ e e + e e + e e

h h -h h -h h -

n-typep-type

depletionregion

Area near p-n junction now has no charge carriers (free electrons or holes): called depletion region

The charged atoms in the depletion region create an electric field, and thus a difference in electric potential.

V

dist fromjunction

When the potential drop becomes steep, the free electrons no longer cross: drift takes over

e ee

43

At metal-semiconductor junction, potential changes to balance device. Electrons in metal can redistribute easily to do this.

Is there a current when I short a diode? Is KVL violated? NO.

h h -h h -h h -

n-typep-type

+ e e + e e + e e

dist from junction

metal contact

metal contact

V

PHYSICS OF P-N JUNCTIONCASE: Short Circuit

PHYSICS OF P-N JUNCTIONCASE 2: Reverse Bias

Electrons bunch up by positive metal contact, but few cross through wire because of potential drop between contacts.

A diode is in reverse bias mode when the + (p-type) terminal is at a (moderately) lower potential than the (n-type) terminal.

e

e e

h h h

+++

+++

+

depletion region(no free e, h)

n-typep-type

VS > 0

metal contact

metal contactV

VS dist from junction

A tiny leakage currentflows due to these few stray electrons, but basically zero current flow.

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e

e e

h h h

+++

+++

+

depletion region(no free e, h)

PHYSICS OF P-N JUNCTIONCASE: Reverse Breakdown

n-typep-type When the diode + terminal is at a much lower potential than the - terminal, reverse breakdown occurs.

Reverse breakdown begins when the potential rise across the junction becomes so great that electrons from the p-type material travel across the rise.

The difference in potential has to be great enough to overcome the chemical bonds in the p-type lattice.

VS > VZK

Voltage necessary for breakdown: VZK Zener knee

metal contact

metal contactV

VS dist from junction

ee

There are diodes called Zener diodes that are designed to operate in reverse breakdown. The voltage across the diode in the reverse breakdown mode is about constant, as once the voltage gets past VZK, the depletion layer does not really increasethe current increases dramatically (also known as avalanche current). As the reverse voltage is increased, there will be a limit to the current flow: reverse saturation current A Zener diode is used to regulate voltage within a circuit, since it provides about the same voltage (VZK) for a whole range of reverse current conditions. Zener diodes can be obtained for a variety of VZK values, anywhere from 0.5 V to 200 V.

PHYSICS OF P-N JUNCTIONCASE: Forward Bias

ee

+++

+

n-typep-type

VS > VF

dist from junction

metal contact

metal contact

V

VS

h e The difference in potential created

by the voltage source makes the drop across the junction less steep.

If VS is large enough, greater than the diode forward voltage VF, electron diffusion (movement to fill holes) overcomes electron drift (movement due to electric field).

Electrons flow across junction and combine with holes.

The need to redistribute charge at metal ends ensures continuous supply of electrons and holes.

Current flows from + terminal to terminal

h

CASE: Forward Bias

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When the voltage across the forward-biased diode is increased past VF, the current increases dramatically. As the forward voltage is increased, there will be a limit to the current flow: saturation current When operating in forward-bias mode in a circuit, diode voltage is nearly constant (equal to VF). A voltage around VF occurs for a whole range of forward current conditions. Many diodes have a VF of 0.6 to 0.7 V, but light emitting diodes (LEDs) often have higher VF values. An LED emits light when it is forward biased. Doping The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic. A dopant, also called doping agent and dope, is an impurity element added to a semiconductor lattice in low concentrations in order to alter the optical/electrical properties of the semiconductor. The process of introducing dopants into a semiconductor is called doping. The addition of a dopant to a semiconductor has the effect of shifting the Fermi level within the material. This results in a material with predominantly negative (n type) or positive (p type) charge carriers depending on the dopant species. Pure semiconductors altered by the presence of dopants are known as extrinsic semiconductors (cf. intrinsic semiconductor). Dopants are introduced into semiconductors in a variety of ways including ion implantation and surface diffusion. Examples: Boron, arsenic, phosphorus, antimony, among other substances, are commonly used dopants in the semiconductor industry. The medical field has some use for Erbium in as a dopant for lasers used in surgery. Europium is used to dope plastics in lasers. (Source: Strategic Rare Earth Metals Inc.) . Artificially produced gemstones (such as rubies) sometimes contain a dopant to identify them as such. Dopants The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly-bound valence electrons to the material, creating excess negative charge carriers. These weakly-bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier. For example, the pure semiconductor silicon has four valence electrons. In silicon, the most common dopants are IUPAC group 13 (commonly known as column III) and group 15 (commonly known as column V) elements. Group 13 elements all contain three valence electrons, causing them to function as acceptors when used to dope silicon. Group 15 elements have five valence electrons, which allow them to act as a donor. Therefore, a silicon crystal doped with boron creates a p-type semiconductor whereas one doped with phosphorus results in an n-type material. N-type Semiconductor

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An N-type semiconductor is obtained by carrying out a process of doping, that is, by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers. When the doping material is added, it gives away (donates) weakly-bound outer electrons to the semiconductor atoms. This type of doping agent is also known as donor material since it gives away some of its electrons. The purpose of n-type doping is to produce an abundance of mobile or "carrier" electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. N-type Semiconductor An N-type semiconductor is obtained by carrying out a process of doping, that is, by adding an impurity of valence-five elements to a valence-four semiconductor in order to increase the number of free (in this case negative) charge carriers. When the doping material is added, it gives away (donates) weakly-bound outer electrons to the semiconductor atoms. This type of doping agent is also known as donor material since it gives away some of its electrons. The purpose of n-type doping is to produce an abundance of mobile or "carrier" electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. P-type semiconductor A P-type semiconductor is obtained by carrying out a process of doping, that is, adding a certain type of atoms to the semiconductor in order to increase the number of free (in this case positive) charge carriers. When the doping material is added, it takes away (accepts) weakly-bound outer electrons from the semiconductor atoms. This type of doping agent is also known as acceptor material and the semiconductor atoms that have lost an electron are known as holes. The purpose of P-type doping is to create an abundance of holes. In the case of silicon, a trivalent atom (typically from group IIIA of the periodic table, such as boron or aluminium) is substituted into the crystal lattice. The result is that one electron is missing from one of the four covalent bonds normal for the silicon lattice. Thus the dopant atom can accept an electron from a neighboring atoms' covalent bond to complete the fourth bond. Such dopants are called acceptors. The dopant atom accepts an electron, causing the loss of half of one bond from the neighboring atom and resulting in the formation of a "hole". Each hole is associated with a nearby negative-charged dopant ion, and the semiconductor remains electrically neutral

However, once each whole has wandered away into the lattice, one proton in the atom at the hole's location will be "exposed" and no longer cancelled by an electron. For this reason a hole behaves as a quantity of positive charge. When a sufficiently large number of acceptor atoms are added, the holes greatl