PAMIN, MICHELLE P.JULY 25, 2014Wireless Communications ENGR. OCHOCO

MICROWAVE DEVICES

MICROWAVE TRANSISTOR

1. Microwave Bipolar Junction Transistors

BJT Modes of OperationThere are two junctions in bipolar junction transistor. Each junction can be forward or reverse biased independently. Thus there are four modes of operations:1. Forward Active2. Cut off3. Saturation4. Reverse activeFORWARD ACTIVEIn this mode of operation, emitter-base junction is forward biased and collector base junction is reverse biased. Transistor behaves as a source. With controlled source characteristics the BJT can be used as an amplifier and in analog circuits.CUTT OFFWhen both junctions are reverse biased it is called cut off mode. In this situation there is nearly zero current and transistor behaves as an open switch.SATURATIONIn saturation mode both junctions are forward biased large collector current flows with a small voltage across collector base junction. Transistor behaves as an closed switch.REVERSE ACTIVEIt is opposite to forward active mode because in this emitter base junction is reverse biased and collector base junction is forward biased. It is called inverted mode. It is no suitable for amplification.However the reverse active mode has application in digital circuits and certain analog switching circuits.Applications: Analog or mixed signal ICs Bipolar Complementary Metal-Oxide Semiconductor (BiCMOS)

2. Heterojunction Bipolar Transistor

Operation:The operation of an HBT is fundamentally the same as that of a BJT. An HBT has the same npn structure as a BJT, although its implementation is very different. An HBT uses a BE heterojunction instead of a simple pn junction. The heterojunction employs dissimilar semiconductor materials to provide a barrier between the emitter and base, allowing heavy base doping, which minimizes base resistance and maximizes cutoff frequency. Advanced fabrication techniques used for HBTs, which would make no economic sense for conventional BJTs, contribute to improved performance as well. Unlike BJTs, HBTs are rarely available as discrete devices; almost all are used in IC technologies. While conventional BJTs are invariably silicon devices, HBTs are realized in many III-V technologies. Silicon HBTs are also possible; silicon-germanium HBTs provide high performance at lower cost than III-V devices. In contrast to the planar BJT, its mesa structure is decidedly nonplanar. Although more complicated (and, of course, more expensive) to fabricate than the planar BJT, the structure provides better definition of the emitter,lower parasitic resistances, and lower fringing capacitance. As with BJTs, power HBTs can be fabricated by paralleling a number of devices having long, narrow emitters.

Applications:It is commonly used in modern ultrafast circuits, mostly radio-frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones.

3. Tunnel Diode

Operation:

Forward bias operation

Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the p-n junction barrier because electron states in the conduction band on the n-side become aligned with valence band hole states on the p-side of the pn junction. As voltage increases further these states become more misaligned and the current drops this is called negative resistance, because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the pn junction, and no longer by tunneling through the pn junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region.

Reverse bias operationWhen used in the reverse direction they are called back diodes and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals. (That is, they have an accurate square law characteristic in the reverse direction.)

Under reverse bias at sufficiently high reverse voltage, electrons flow in the opposite direction, as now different electron states on each side of the pn junction become increasingly aligned and tunnel through the pn junction barrier in reverse direction this is the Zener effect that also occurs in zener diodes.

Applications: local oscillators for UHF television

Oscillator circuits :Tunnel diodes can be used as high frequency oscillators as the transition between the high electrical conductivity is very rapid. They can be used to create oscillation as high as 5Gz. Even they are capable of creativity oscillation up to 100 GHz in a appropriate digital circuits.

Used in microwave circuits: Normal diode transistors do not perform well in microwave operation. So, for microwave generators and amplifiers tunnel diode are. In microwave waves and satellite communication equipments they were used widely, but now a days their uses is decreasing rapidly as transistor for working in wave frequency area available in market.

Resistant to nuclear radiation :Tunnel diodes are resistant to the effects of magnetic fields, high temperature and radioactivity. Thats why these can be used in modern military equipment. These are used in nuclear magnetic resource machine also. But the most important field of its use satellite communication equipments.

FIELD-EFFECT TRANSISTORS

4. Junction Field Effect Transistor (JFET)

Operation:In the N channel device, the N channel is sandwiched between two P type regions (the gate and the substrate) that are connected together electrically to form the gate. The N type channel is connected to the source and drain terminals via more heavily doped N+ type regions. The drain ic connected to a positive supply, and the source to zero volts. N+ type silicon has a lower resistivity than N type. This gives it a lower resistance, increasing conduction and reducing the effect of placing standard N type silicon next to the aluminium connector, which because aluminium is a trivalent material, having three valence electrons whilst silicon has four, would tend to create an unwanted junction, similar in effect to a PN junction at this point.

The P type gate is at 0V and is therefore negatively biased compared to the channel, which has a potential gradient on it, as one end is connected to 0 volts (the source), and the other end to a positive voltage (the drain). Any point on the channel (apart from the extreme end near the source terminal) must therefore be more positive than the gate. Therefore the two PN junctions formed between the N type channel and the P type areas of the gate and the substrate are both reverse biased, and so have a depletion layer that extends into the channel.

The shape of the depletion layer is not symmetrical. It is generally thicker towards the drain end of the channel, because the voltage on the drain is more positive than that on the source due to voltage gradient that exists along the channel. This causes a larger potential across the junctions nearer the drain, and so a thickening of the depletion layer. The effect becomes more marked when the voltage between drain and source is greater than about 1volt or so.

When a voltage is applied between drain and source (VDS) current flows and the silicon channel acts rather like a conventional resistor. Now if VDS is increased (with VGS held at zero volts) towards what is called the pinch off value VP, the drain current ID also at first, increases. The transistor is working in the "ohmic region".

Applications:JFET are used in: RF amplifiers in FM tuners and communication equipment for the low noise lever. mixer circuits in FM and TV receivers, and communication dquipment because inter modulation distortion is low.

5. Metal-Semiconductor Field Effect Transistor (MESFET)

Operation:Like other forms of field effect transistor the GaAs Fet or MESFET has two forms that can be used: Depletion mode MESFET: If the depletion region does not extend all the way to the p-type substrate, the MESFET is a depletion-mode MESFET. A depletion-mode MESFET is conductive or "ON" when no gate-to-source voltage is applied and is turned "OFF" upon the application of a negative gate-to-source voltage, which increases the width of the depletion region such that it "pinches off" the channel. Enhancement mode MESFET: In an enhancement-mode MESFET, the depletion region is wide enough to pinch off the channel without applied voltage. Therefore the enhancement-mode MESFET is naturally "OFF". When a positive voltage is applied between the gate and source, the depletion region shrinks, and the channel becomes conductive. Unfortunately, a positive gate-to-source voltage puts the Schottky diode in forward bias, where a large current can flow.

Application: military communications As front end low noise amplifier of microwave receivers in both militaryradardevices and communication commercialoptoelectronics satellite communications As power amplifier for output stage of microwave links. As a power oscillator.

6. MetalOxideSemiconductor Field-Effect Transistor (MOSFET)

Operation:MOSFET transistors have metal gates which are insulated from the semiconductor by a layer of SiO2 or other dielectric. In enhancement type MOSFETs, the application of a gate voltage activates the channel (by inducing a layer of carriers between source and drain under the gate. In depletion type MOSFETs, there is a small strip of semiconductor of the same type as that of the source and drain, and the gate voltage can either reduce (by depleting carriers) or increase (by increasing carriers) the channel current. In an n channel MOSFET, the conducting channel exists in a p type substrate.

Application:MOSFETs in motor control applicationsMotor control is another application where power MOSFETs find use and where the most important selection criteria might again differ. A motor-control circuit doesnt switch at the high frequencies found in modern switching power supplies. A typical half-bridge control circuit employs two MOSFETs (a full bridge uses four). But both of the MOSFETs spend a fair amount of time switched off dead time. Reverse Recovery Time (trr) becomes very important in such applications. When a control circuit switches a MOSFET in a bridge circuit to the off state when controlling an inductive load such as a motor winding, the other switch in the bridge conducts current in the reverse direction temporarily, via the body diode in the MOSFET--hence recirculating the current to continue to supply the motor. When the first MOSFET turns on again, the stored charge in the other MOSFET diode must be removed and discharged through the first, and that is a loss of energy, a short trr period minimizes such losses.

TRANSFERRED ELECTRON DEVICES

7. Gunn DiodeOperation:At microwave frequencies, it is found that the dynamic action of the diode incorporates elements resulting from the thickness of the active region. When the voltage across the active region reaches a certain point a current is initiated and travels across the active region. During the time when the current pulse is moving across the active region the potential gradient falls preventing any further pulses from forming. Only when the pulse has reached the far side of the active region will the potential gradient rise, allowing the next pulse to be created.

It can be seen that the time taken for the current pulse to traverse the active region largely determines the rate at which current pulses are generated, and hence it determines the frequency of operation.

To see how this occurs, it is necessary to look at the electron concentration across the active region. Under normal conditions the concentration of free electrons would be the same regardless of the distance across the active diode region. However a small perturbation may occur resulting from noise from the current flow, or even external noise - this form of noise will always be present and acts as the seed for the oscillation. This grows as it passes across the active region of the Gunn diode.

Applications:Because of their high frequency capability, Gunn diodes are mainly used at microwave frequencies and above. They can produce some of the highest output power of any semiconductor devices at these frequencies. Their most common use is in oscillators, but they are also used in microwave amplifiers to amplify signals. Because the diode is a one-port (two terminal) device, an amplifier circuit must separate the outgoing amplified signal from the incoming input signal to prevent coupling. One common circuit is a reflection amplifier which uses a circulator to separate the signals. A bias tee is needed to isolate the bias current from the high frequency oscillations.

AVALANCHE TRANSIT TIME DEVICES8. IMPact ionization Avalanche Transit-Time (IMPATT) DiodeOperation:Once the carriers have been generated the device relies on negative resistance to generate and sustain an oscillation. The effect does not occur in the device at DC, but instead, here it is an AC effect that is brought about by phase differences that are seen at the frequency of operation. When an AC signal is applied the current peaks are found to be 180 out of phase with the voltage. This results from two delays which occur in the device: injection delay, and a transit time delay as the current carriers migrate or drift across the device.

The voltage applied to the IMPATT diode has a mean value where it is on the verge of avalanche breakdown. The voltage varies as a sine wave, but the generation of carriers does not occur in unison with the voltage variations. It might be expected that it would occur at the peak voltage. This arises because the generation of carriers is not only a function of the electric field but also the number of carriers already in existence.

As the electric field increases so does the number of carriers. Then even after the field has reached its peak the number of carriers still continues to grow as a result of the number of carriers already in existence. This continues until the field falls to below a critical value when the number of carriers starts to fall. As a result of this effect there is a phase lag so that the current is about 90 behind the voltage. This is known as the injection phase delay.

When the electrons move across the N+ region an external current is seen, and this occurs in peaks, resulting in a repetitive waveform.

Application:These diodes are used in a variety of applications from low power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications. A main advantage is their high power capability. They operate at frequencies between about 3 and 100 GHz or more.

9. TRApped Plasma Avalanche Transit-time (TRAPATT) DiodeOperation:The criterion for operation in TRAPATT operation is that the avalanche front advances fasrer than the saturation velocity of the carriers. In general it exceeds the saturation value by a factor of around three.

The TRAPATT mode does not depend upon the injection phase delay.

Although the TRAPATT diode provides a much higher level of efficiency than the IMPATT, its major disadvantage is that the noise levels on the signal are even higher than they are when using an IMPATT. A balance needs to be made according to the application required.

Applications:1.They are used in microwave beacon. 2.They are used in low power Doppler radar as local oscillator. 3.They are used in landing system.

10. Barrier Injection Transit Time (BARITT) DiodeOperation:Essentially the BARITT diode consists of two back to back diodes. When a potential is applied across the device, most of the potential drop occurs across the reverse biased diode.

If the voltage is then increased until the edges of the depletion region meet, then a condition known as punch through occurs.

After a charge is injected, it travels to the substrate with the saturation velocity.

As seen from the diagram, it can be seen that the injection current is in phase with the RF voltage waveform. This results in a non-ideal current waveform situation which flows in the positive resistance region and therefore losses are higher in the BARITT than in an IMPATT.

The terminal current pulse width is determined by the transit time which is L/vsat (Where the electrodes are spaced L apart and vsat is the saturation velocity). This constitutes around three quarters of the cycle.

In view of the physical restraints of the BARITT diode, the power capability decreases approximately as the square of the frequency because higher frequencies require a smaller separation between the electrodes and this in turn limits the voltages that can be used.Also the efficiency falls away with increasing frequency. For low frequency operation it may be around 5% or a little more.

Applications:Used in specialist microwave RF signal applications but with lower noise.

MICROWAVE TUBES

11. KlystronOperation:Klystrons amplify RF signals by converting the kinetic energy in DC electron beam into radio frequency power. A beam of electrons is produced by a thermionic cathode (a heated pellet of low work function material), and accelerated by high voltage electrodes. (typically in the tens of kilovolts). This beam is then passed through an input cavity. RF energy is fed into the input cavity at, or near, its natural frequency to produce a voltage which acts on the electron beam. The electric field causes the electrons to bunch: electrons that pass through during an opposing electric field are accelerated and later electrons are slowed, causing the previously continuous electron beam to form bunches at the input frequency. To reinforce the bunching, a klystron may contain additional "buncher" cavities. The electron bunches increase in magnitude, as the overall drift velocity of the beam decreases, and this in effect represents the sum of an RF current in the beam along with the the DC component. An RF current of course will produce a magnetic field, and this will in turn excite a voltage across the gap of the output cavity, thus allowing the transfer of RF energy developed flows out through a waveguide. The spent electron beam, which now contains less energy than it started with, is captured in a collector.

Applications:CloudSat Satellite, Radar

12. MagnetronOperation:All cavity magnetrons consist of a hot filament (cathode) kept at, or pulsed to, a high negative potential by a high-voltage, direct-current power supply. The cathode is built into the center of an evacuated, lobed, circular chamber. Circular cathode that emits electrons when heated. In a normal diode vacuum tube, the electrons would flow directly from the cathode straight to the anode, causing a high current to flow. In a magnetron tube, however, the direction of the electrons is modified because the tube is surrounded by a strong magnetic field. The field is usually applied by a C-shape permanent magnet centered over the interaction chamber.

The magnetic fields of the moving electrons interact with the strong field supplied by the magnet. The result is that the path for the electron flow from the cathode is not directly to the anode, but instead is curved. By properly adjusting the anode voltage and the strength of the magnetic field, the electrons can be made to bend such that they rarely reach the anode and cause current flow. The path becomes circular loops. Eventually , the electrons do reach the anode and cause current flow. By adjusting the dc anode voltage and the strength of the magnetic field, the electron path is made circular. In making their circular passes in the interaction chamber, electrons excite the resonant cavities into oscillation. A magnetron, therefore, is an oscillator, not an amplifier. A takeoff loop in one cavity provides the output.

Applications:Microwave Oven, Radar

13. Travelling Wave (TWT) TubeOperation:The travelling wave tube, TWT is contained within a glass vacuum tube. This obviously maintains the vacuum that is required for the operation of the TWT.

Within the travelling wave tube the first element is the electron gun comprising primarily of a heated cathode and grids. This produces and then accelerates a beam of electrons that travels along the length of the tube.

In order that the electrons are made to travel as a tight or narrow beam along the length of the travelling wave tube, a magnet and focussing structure is included. The field from the magnet keeps the beam as narrow as required and in this way ensures that the beam travels along the length of the TWT.

The RF input consists of a direction coupler which may either be in the form of a waveguide or an electromagnetic coil. This is positioned near the electron gun emitter and it induces current into the helix (see below).

A helix is an essential part of the traveling wave tube. It acts as a delay line, in which the RF signal travels at near the same speed along the tube as the electron beam. The electromagnetic field due to the current in the helix interacts with the electron beam, causing bunching of the electrons in an effect known as velocity modulation and the electromagnetic field resulting from the beam current then induces more current back into the helix. In this way the current builds up and the signal is therefore amplified.

The RF output from the traveling wave tube consists of a second directional coupler. Again this may either be an electromagnetic coil of a waveguide. This is positioned near the collector and it receives the amplified version of the signal from the far end of the helix from the electron gun or emitter.

An attenuator is included on the helix, usually between the input and output sections of the TWT helix. This is essential to prevent the reflected wave from travelling back to the cathode of the electron gun.

The collector finally collects and absorbs the electron beam. It is in this area that high levels of power may be dissipated and therefore this section of the travelling wave tube can become very hot and will require cooling.

Applications:There are many areas in which TWT amplifiers are used. They are an ideal form of RF amplifier for satellites and as a result they are extensively used for satellite transponders where low levels signals are received and need to be retransmitted at much higher levels. In addition to this TWT amplifiers are used in microwave radar systems where they are able to produce the high levels of power required. Traveling wave tube, TWT technology is also used for electronic warfare applications. In these applications the grid on the travelling wave tube may be used to pulse the transmission.