Presented by: Joel S. Sagala

Introduction:

• Transistor is a solid state semiconductor device

• Transistor allows a variable current, from an external source, to flow between two of its terminals depending on the smaller voltage or current applied to a third terminal.

• Transistor can be used for amplification, switching, voltage stabilization, signal modulation and many other functions.

• Transistors are made either as separate components or as part of an integrated circuit.

History:

• 1928 the first patents for the transistor principle were registered in Germany by Julius Edgar Lilienfeld.

• 1934 German physicist Dr. Oskar Heil patented the field-effect transistor. It is not clear whether either design was ever built, and this is generally considered unlikely.

• On December 22, 1947 William Shockley, John Bardeen, and Walter Brattain succeeded in building the first practical point-conctact transistor at Bell Labs for radar units as a frequency mixer element in microwave radar receivers.

• Bell Telephone Laboratories needed a generic name for the new invention, "transistor," coined by John R. Pierce, won an internal ballot. This is an abbreviated combination of the words “transconductance" or "transfer", and “varistor“ since device logically belongs in the varistor family,

• In August 1948 German physicists Herbert F. Mataré (1912– ) and Heinrich Walker (ca. 1912–1981) applied for a patent on an amplifier based on the minority carrier injection process which they called the "transistron." Since Bell Labs did not make a public announcement of the transistor until June 1948, the transistron was considered to be independently developed. Transistrons were commercially manufactured for the French telephone company and military.

Transistors are categorized by:

• Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide

• Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"

• Polarity: NPN, PNP, N-channel, P-channel

• Maximum Power Rating: low, medium, high

• Maximum Operating Frequency: low, medium, high, radio frequency (RF), micorwave(The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for

"frequency of transition." The frequency of transition is the frequency at which the transistor yields unity gain).

• Application: switch, general purpose, audio, high voltage, super-beta, matched pair

• Physical Packaging: through hole metal, through hole plastic, surface mount, ball grid array

Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch.

Semiconductor Material Characteristics

Semiconductor Material

Junction Forward Voltage

at 25 °C (V)

Electron Mobility at 25 °C

(m/s)

Hole Mobility at 25 °C (m/s)

Max. Junction Temp. 25 °C

Ge 0.27 0.39 0.19 70 to 100

Si 0.71 0.14 0.05 150 to 200

GaAs 1.03 0.85 0.05 150 to 200

Al-Si junction 0.3 - - 150 to 200

• The junction forward voltage is the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. The lower the better (less power is required to "drive" the transistor). The junction forward current decreases with temperature.

•The electron and hole mobility columns show the average speed that electrons and holes diffuse through the semiconductor material with an electric field of 1 volt per meter applied across the material (the higher the electron mobility the faster the transistor).

• Max. Junction Temperature (taken from various manufacturers' data sheets) This temperature should not be exceeded or the transistor may be destroyed.

• Unipolar Transistors - FET Field Effect Transistoruses either electrons or holes as carrier.varies the current by varying the shape of the conducting volume.It is a voltage controlled device.Voltage on gate controls the current flowing through the device.

• Bipolar Transistors - BJT Bipolar Junction Transistoruses both electrons and holes as carrier.controls the current by varying the number of charge carriers.It is a current controlled device.The output current is controlled by the input current

2 Types of Transistor: Transistors are active circuit elements and are typically made from silicon or germanium and come in two types:

Bipolar Transistor:

• Was the first type of transistor to be mass-produced.

• Bipolar transistors are so named because the main conduction channel employs both electrons and holes to carry the main electric current.

• It is a three-terminal device constructed from doped semiconductor material.

• There are two types of BJT – NPN and PNP transistors (schematic symbol below)

NPN PNP

• The BJT is commonly described as a current-operated device because the emitter/collector current is controlled by the current flowing between base and emitter terminals.

NPN TransistorWhen a layer of a P-type material is sandwiched between two layers of N-type material

(figure 1) an NPN transistor is formed.

The layers of semiconductor material behaves like 2 diodes connected back to back (figure 2). Figure 3 shows the circuit symbol for an NPN transistor. The arrow points to the direction of conventional current flow.

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N Type Material

P type Material

N type Material

Emitter

Base

Collector

Emitter

Base

Collector

PNP TransistorThe other bipolar type is the PNP Transistor: a layer of a N-type material is sandwiched

between two layers of P-type material (see illustration 1). It can be considered a two diodes head to head (see illustration 2)

The arrow points to the direction of conventional current flow (refer to PNP transistor symbol)

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P Type Material

N type Material

P type Material

Emitter

Base

Collector

Emitter

Base

Collector

Simplified cross section of an NPN bipolar junction transistors

• The Emitter is higly doped to reduce resistance.• The Collector is lightly doped to reduce the juntion capacitance of the collector-base junction. It sorrounds the emitter region making it almost impossible for the electrons injected into the base region to escape being collected.• The Base is lightly doped central region (physically located between the emitter and the collector and is made from lightly doped, high resistivity material) has majority charge carriers of the oppposite polarity to those in the sorrounding material.

ConfigurationsThere are three configurations of connecting transistors.

• The common – emitter mode produces high voltage and current gains but it has a quite low input impedance thus it requires a relatively high base current. It also has high output impedance.

• The common-collector configuration sometimes called the emitter follower, has high input impedance and low output impedance so it is often used as a buffer stage to match a high impedance source with a low impedance load.

• The common base configuration has a low input and high output impedance so it is used to match a low impedance source to drive a high impedance load. (a.k.a Grounded Base Amplifier).

Characteristics Common Emitter Common Collector Common Base

Voltage Gain >1 (yes) <1 (no) >1 (yes)

Current Gain >1 (yes) >1 (yes) <1 (no)

Power gain >1(yes) >1 (yes) >1 (yes)

Input Resistance 3.5k ohms 580k ohms 30 ohms

Output Resistance infinity 35 ohms Infinity

Phase Change yes no No

Die of a KSY34 high-frequency NPN transistor, base and emitter connected via bonded wires

Regions of OperationBipolar transistors have several different regions of operation:• Linear Region - collector-emitter current is approximately proportional to the base current but many times larger, making this the ideal mode of operation for current amplification.

•The BJT enters Saturation when the base current is increased to a point where the external circuitry prevents the collector current from growing any larger. At this point, the C-B junction also becomes forward biased. A residual voltage drop of approximately 100 mV to 300 mV (depending on the amount of base and collector current) then remains between collector and emitter.

•In the Cut-off Region the base-emitter voltage is too small for any significant current to flow. In typical BJTs manufactured from silicon, this is the case below 0.6 V or so. BJTs that are operated only in 'cut off' and 'saturation' regions can be viewed as electronic switches.

Field Effect Transistor

The Field-Effect Transistor (FET) is a transistor that relies on an electric field to control the shape and hence the conductivity of a 'channel' in a semiconductor material. FET are sometimes used as voltage-controlled resistors. FET has three connectors: the SOURCE, the DRAIN, and the GATE. The current flows along a narrow channel between source and drain. The voltage on the gate creates an electric field that controls the current flow.

As with BJT, FET are used in both amplifying and switching circuits. The input resistance of the gate is very high (10M ohms+), so FET are preferred when the input can not supply much current.

The FET is simpler in concept than the bipolar transistor and can be constructed from a wide range of materials. The channel region of any FET is either doped to produce n-type semiconductor, giving an "N-channel" device, or with p-type to give a "P-channel" device. The doping determines the polarity of gate operation. The different types of field-effect transistors can be distinguished by the method of insulation between channel and gate. The two main classes of FET: JFET (Juntion) and MOSFET (Metal Oxide Semiconductor). They are devided as follows:

Types of Junction Field Effect Transistor

• There are two types of JFET: N-channel and the P-channel (refer to figures below).

• The designation refers to the polarity of the majority charge carriers in the bar of semiconductor that connects the DRAIN to SOURCE.

• Channel is formed from a single-polarity (unipolar) material.

• JFET operates with all PN junctions reversed-biased so as to obtain a high input input to the gate.

• Electrons move faster than the holes, the reason n-channel JFET is more efficient, thus used more often.

Depletion areas occur at the junction due to electron-hole collisions. If a battery is connected, electron will flow through the channel from source to drain. As the voltage increases, the depletion areas widen and distort, and the channels narrows.

If we connect a second battery, the channel is at its widest when the gate-source voltage is zero.

- - - - - - - n-type material (excess of electrons)o o o o o p-type material (excess of holes)

Closing the switch: If the gate is more negative than the source, the battery will supply electrons to the gate. These electrons will attract holes to the gate. So the depletion layers widen until the channel is blocked preventing electrons flowing from the source to the drain. At this point, the gate-source voltage is called the “pinch-off voltage”.

Metal Oxide Semiconductor Field Effect Transistor

MOSFET also called IGFET (Insulated Gate FET), are very important devices in the electronics industry. They can be packed onto a very small area of wafer so are widely used for VLSI (Very Large Scale Integration).

There are two types of MOSFET, ‘enhancement’ MOSFET and ‘depletion’ MOSFET.

One juntion will be depleted when the gate voltage is zero, no current will flow between source and drain.

The MOSFET gate is insulated from the semiconductor by a thisn layer of SiO2 which has a very high resistance. If a sufficient negative voltage is applied to the gate, holes are attracted to the area under the gate. A channel is created allowing electrons to flow from source to drain.

The gate voltage controls the source drain current.

In n-channel enhancement MOSFET thepolarization will be reversed.

The ‘depletion’ MOSFET has a region of less heavily doped semiconductor under the gate: conduction can take place when there is no voltage on the gate.

If the negative voltage is applied to the gate, holes are attracted to the gate so the channel becomes blocked, stopping conduction.

The gate voltage controls the source drain current.

Depletion mode MOSFET are rare.

Field Effect Transistor uses either electrons or holes as carrier.It is a voltage controlled device.Voltage on gate controls the current flowing through the device.Input Impedance – HighTransconductance - Low Linearity – WorseIntegration Scale – Much LargerTemperature Tolerance – Better

Comparison Between BJT and FET

Bipolar Junction Transistor uses both electrons and holes as carrier.It is a current controlled device.The output current is controlled by the input currentInput-impedance - LowTransconductance - High Linearity – BetterIntegration Scale – Much SmallerTemperature Tolerance – Worse

Other Types of Transistor

• Unijunction Transistors can be used as simple pulse generators. They comprise a main body of either P-type or N-type semiconductor with ohmic contacts at each end (terminals Base1 and Base2). A junction with the opposite semiconductor type is formed at a point along the length of the body for the third terminal (Emitter).

• Dual Gate FET have a single channel with two gates in cascode; a configuration that is optimized for high frequency amplifiers, mixers, and oscillators.

• Transistor Arrays are used for general purpose applications, function generation and low-level, low-noise amplifiers. They include two or more transistors on a common substrate to ensure close parameter matching and thermal tracking, characteristics that are especially important for long tailed pair amplifiers.

• Darlington transistors comprise a medium power BJT connected to a power BJT. This provides a high current gain equal to the product of the current gains of the two transistors. Power diodes are often connected between certain terminals depending on specific use.

• Insulated gate bipolar transistors (IGBTs) use a medium power IGFET, similarly connected to a power BJT, to give a high input impedance. Power diodes are often connected between certain terminals depending on specific use. IGBTs are particularly suitable for heavy-duty industrial applications.

• Single-electron transistors (SET) consist of a gate island between two tunnelling junctions. The tunnelling current is controlled by a voltage applied to the gate through a capacitor.

Other Types of Transistor (cont . . . )

• Heterojunction Bipolar Transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems.

•The MESFET (Metal-Semiconductor Field-Effect Transistor) substitutes the p-n-junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.

• Using bandgap engineering in a ternary semiconductor like AlGaAs gives a HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET). The fully depleted wide-band-gap material forms the isolation.

Through Hole plastic (JFET) Surface Mount (FET)Through Hole plastic MOSFET

Through Hole plastic BJT

Ball Grid Array (BGA)