Transistor

A transistor is a type of semiconductor device that can be used to conduct and insulate electric current
or voltage. A transistor basically acts as a switch and an amplifier. A typical transistor is composed of
three layers of semiconductor materials or, more specifically, terminals which help to make a connection
to an external circuit and carry the current.

Advantages

There are many advantages of a transistor such as

1. High voltage gain.

2. Lower supply voltage is sufficient.
3. Most suitable for low power applications.
4. Smaller and lighter in weight.
5. Mechanically stronger than vacuum tubes.
6. No external heating required like vacuum tubes.
7. Very suitable to integrate with resistors and diodes to produce ICs.

Parts of a transistor
1. Base: This is used to activate the transistor.
2. Collector: It is the positive lead of the transistor.
3. Emitter: It is the negative lead of the transistor.

Emitter: The section that supplies the large section of majority charge carrier is called emitter. The
emitter is always connected in forward biased with respect to the base so that it supplies the majority
charge carrier to the base. The emitter-base junction injects a large amount of majority charge carrier
into the base because it is heavily doped and moderate in size.

Collector: The section which collects the major portion of the majority charge carrier supplied by the
emitter is called a collector. The collector-base junction is always in reverse bias. Its main function is to
remove the majority charges from its junction with the base. The collector section of the transistor is
moderately doped, but larger in size so that it can collect most of the charge carrier supplied by the
emitter.
Base: The middle section of the transistor is known as the base. The base forms two circuits, the input
circuit with the emitter and the output circuit with the collector. The emitter-base circuit is in forward
biased and offered the low resistance to the circuit. The collector-base junction is in reverse bias and
offers the higher resistance to the circuit. The base of the transistor is lightly doped and very thin due to
which it offers the majority charge carrier to the base.

Transistor Types:

Bipolar junction transistor (BJT)

A bipolar junction transistor is a three-terminal semiconductor device that consists of two p-n junctions
which are able to amplify or magnify a signal. It is a current controlled device. The three terminals of the
BJT are the base, the collector, and the emitter.

Types:

1. NPN Transistor
2. PNP Transistor

NPN Transistor

An NPN transistor is the one in which two layers of N-type semiconductor material are separated by a
thin layer of P-type semiconductor material. Hence, in case of NPN transistor, the emitter and collector
are of N-type while the base is of P-type.
Fig : NPN Transistor Symbol

PNP Transistor

When a thin layer of N-type semiconductor material is sandwiched between two layers of P-type
semiconductor material, the resulting transistor is known as a PNP transistor. In a PNP transistor, the
emitter and collector are made of P-type semiconductor material while the base is made of N-type
material.

Fig : PNP Transistor Symbol

Working of NPN Transistor

The operation of an NPN transistor can be explained by having a look at the following figure, in which
emitter-base junction is forward biased and collector-base junction is reverse biased.
The voltage VEE provides a negative potential at the emitter which repels the electrons in the N-type
material and these electrons cross the emitter-base junction, to reach the base region. There a very low
percent of electrons recombines with free holes of P-region. This provides very low current which
constitutes the base current IB. The remaining holes cross the collector-base junction, to constitute the
collector current IC.

As an electron reaches out of the collector terminal, and enters the positive terminal of the battery, an
electron from the negative terminal of the battery VEE enters the emitter region. This flow slowly
increases and the electron current flows through the transistor.

Hence, we can understand that −

1. The conduction in a NPN transistor takes place through electrons.

2. The collector current is higher than the emitter current.
3. The increase or decrease in the emitter current affects the collector current.

Working of PNP Transistor

The operation of a PNP transistor can be explained by having a look at the following figure, in which
emitter-base junction is forward biased and collector-base junction is reverse biased.
The voltage VEE provides a positive potential at the emitter which repels the holes in the P-type material
and these holes cross the emitter-base junction, to reach the base region. There a very low percent of
holes recombine with free electrons of N-region. This provides very low current which constitutes the
base current IB. The remaining holes cross the collector-base junction, to constitute collector current IC,
which is the hole current.

As a hole reaches the collector terminal, an electron from the battery negative terminal fills the space in
the collector. This flow slowly increases and the electron minority current flows through the emitter,
where each electron entering the positive terminal of VEE, is replaced by a hole by moving towards the
emitter junction. This constitutes emitter current IE.

Hence we can understand that −

1. The conduction in a PNP transistor takes place through holes.

2. The collector current is slightly less than the emitter current.
3. The increase or decrease in the emitter current affects the collector current.

Some Regions of Transistor Operations(Active, Saturation and cut-off regions)

Cut off : The point where the load line intersects the IB = 0 curve is known ascut off. At this point, IB = 0
and only small collector current (i.e. collector leakage current ICEO) exists. At cut off, the base-emitter
junction no longer remains forward biased and normal transistor action is lost. The collector-emitter
voltage is nearly equal to VCC i.e. VCE (cut off) = VCC

Saturation region:The point where the load line intersects the IB = IB(sat) curve is called saturation. At
this point, the base current is maximum and so is the collector current. At saturation, collector-base
junction no longer remains reverse biased and normal transistor action is lost.

Active region: The region between cut off and saturation is known as active region. In the active region,
collector-base junction remains reverse biased while base-emitter junction remains forward biased.
Consequently, the transistor will function normally in this region.

BJT Configuration

The three basic configurations of a BJT are

1. common emitter (CE),

2. common base (CB),
3. common collector (CC)

Common Emitter (CE) Configuration

Input and Output Characteristics
The curve plotted between base current IB and the base-emitter voltage VEB is called Input
characteristics curve. For drawing the input characteristic the reading of base currents is taken through
the ammeter on emitter voltage VBE at constant collector-emitter current. The curve for different value
of collector-base current is shown in the figure. The curve for common base configuration is similar to a
forward diode characteristic. The base current IB increases with the increases in the emitter-base voltage
VBE. Thus the input resistance of the CE configuration is comparatively higher that of CB configuration.

The effect of CE does not cause large deviation on the curves, and hence the effect of a change in VCE on
the input characteristic is ignored.

In CE configuration the curve draws between collector current IC and collector-emitter voltage VCE at a
constant base current IB is called output characteristic. The characteristic curve for the typical NPN
transistor in CE configuration is shown in the figure.

In the active region, the collector current increases slightly as collector-emitter VCE current increases.
The slope of the curve is quite more than the output characteristic of CB configuration. The output
resistance of the common base connection is more than that of CE connection.

The value of the collector current IC increases with the increase in VCE at constant voltage IB, the value β
of also increases.

When the VCE falls, the IC also decreases rapidly. The collector-base junction of the transistor always in
forward bias and work saturate. In the saturation region, the collector current becomes independent and
free from the input current IB

Common Base (CB) Configuration

The curve plotted between emitter current IE and the emitter-base voltage VEB at constant collector
base voltage VCB is called input characteristic curve. The following points are taken into consideration
from the characteristic curve.

1. For a specific value of VCB, the curve is a diode characteristic in the forward region. The PN
emitter junction is forward biased.
2. When the value of the voltage base current increases the value of emitter current increases
slightly. The junction behaves like a better diode. The emitter and collector current is
independent of the collector base voltage VCB.
3. The emitter current IE increases with the small increase in emitter-base voltage VEB. It shows
that input resistance is small.

In common base configuration, the curve plotted between the collector current and collector base
voltage VCB at constant emitter current IE is called output characteristic. The CB configuration of PNP
transistor is shown in the figure below. The following points from the characteristic curve are taken into
consideration.

1. The active region of the collector-base junction is reverse biased, the collector current IC is
almost equal to the emitter current IE. The transistor is always operated in this region.
2. The curve of the active regions is almost flat. The large charges in VCB produce only a tiny
change in IC The circuit has very high output resistance ro.
3. When VCB is positive, the collector-base junction is forward bias and the collector current
decrease suddenly. This is the saturation state in which the collector current does not depend on
the emitter current.
4. When the emitter current is zero, the collector current is not zero. The current which flows
through the circuit is the reverse leakage current, i.e., ICBO. The current is temperature depends
and its value range from 0.1 to 1.0 μA for silicon transistor and 2 to 5 μA for germanium
transistor.

Common Collector (CC) Configuration

The input characteristic of the common collector configuration is drawn between collector base
voltage VCE and base current IB at constant emitter current voltage VCE. The value of the output
voltage VCE changes with respect to the input voltage VBC and IB With the help of these values, input
characteristic curve is drawn.

The output characteristic of the common emitter circuit is drawn between the emitter-collector
voltage VEC and output current IE at constant input current IB. If the input current IB is zero, then the
collector current also becomes zero, and no current flows through the transistor.

Transistor as an Amplifier

A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to
the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained
regardless of the polarity of the signal. The below figure shows how a transistor looks like when
connected as an amplifier.

The low resistance in input circuit, lets any small

change in input signal to result in an appreciable
change in the output. The emitter current caused
by the input signal contributes the collector
current, which when flows through the load
resistor RL, results in a large voltage drop across
it. Thus a small input voltage results in a large
output voltage, which shows that the transistor
works as an amplifier.

Transistor as a Switch

Cut Off State (Open Switch)

When transistor operates in the cut off region shows the following characteristics −

1. The input is grounded i.e. at zero potential.

2. The VBE is less that cut – in voltage 0.7 V.
3. Both emitter – base junction and collector – base junction are reverse biased.
4. The transistor is fully – off acting as open switch.
5. The collector current IC = 0 A and output voltage Vout = VCC.

Saturation State (Closed Switch)

1. The transistor operating in the saturation region exhibits following characteristics −

2. The input is connected to VCC.
3. Base – Emitter voltage is greater than cut – in voltage (0.7 V).
4. Both the base – emitter junction and base – collector junction are forward biased.
5. The transistor is fully – ON and operates as closed switch.
6. The collector current is maximum
Field effect transistor
FET Working

When a voltage is applied to the gate electrode, an electric field is created across the insulating layer,
which in turn creates a depletion region in the channel. The depletion region reduces the number of free
charge carriers in the channel, and thus the conductivity of the channel is reduced. This effect is known
as the field-effect, and it is the basis of the FET operation. In the case of an n-type FET, a negative voltage
applied to the gate electrode creates a depletion region in the channel, which reduces the flow of
electrons from the source to the drain. In contrast, a positive voltage applied to the gate electrode of a
p-type FET creates a depletion region that reduces the flow of holes from the source to the drain. Thus,
by varying the voltage applied to the gate electrode, the conductivity of the channel can be controlled,
and the flow of current through the FET can be modulated.

To explain the working principle of FET, the analogy of a water pipe and vessel can be used. In this
analogy, the source of water can be considered as the source terminal of FET, while the vessel that
collects water can be analogous to the drain terminal of FET. The gate terminal can be compared to the
controlling tap that regulates the flow of water. Similar to how the tap modulates the flow of water, the
voltage applied at the gate terminal controls the flow of current from the source to the drain terminal of
FET. Thus, the FET operates by controlling the flow of current through the channel by modulating the
number of charge carriers in the channel using the voltage applied at the gate terminal.

Types

FET Configurations
Common source: This FET configuration is probably the most widely used. The common source circuit
provides a medium input and output impedance levels. Both current and voltage gain can be described
as medium, but the output is the inverse of the input, i.e. 180° phase change. This provides a good
overall performance and as such it is often thought of as the most widely used configuration.

Common drain: This FET configuration is also known as the source follower. The reason for this is that
the source voltage follows that of the gate. Offering a high input impedance and a low output impedance
it is widely used as a buffer. The voltage gain is unity, although current gain is high. The input and output
signals are in phase.
Common gate: This transistor configuration provides a low input impedance while offering a high
output impedance. Although the voltage is high, the current gain is low and the overall power gain is also
low when compared to the other FET circuit configurations available. The other salient feature of this
configuration is that the input and output are in phase.
FET as an amplifier

FET Common Source Amplifier Circuit

1. The common source amplifier circuit diagram with N-channel FET coupling and biasing capability
is shown below. This circuit will be similar to a Bipolar Junction transistor’s common-emitter
follower. The polarity of the input voltage will be reversed if we use a P-channel FET.
2. Among other JFET configurations, the common source amplifier configuration is widely used
because it can provide both high voltage gain and a large input impedance.
 FET Common Source Amplifier Circuit

There are three terminals on the common source FET amplifier. They are as follows: source, gate, and
drain.

Source:

This terminal receives the majority of the carriers required by the device. Current enters the channel via
a source terminal, which is denoted by IS.

Drain:

This terminal is where the majority of the carriers in the channel exit. That is exhausting. As a result,
conventional current enters the channel, denoted by ID.

Gate:

This terminal is always in charge of the channel’s conductivity. As a result, the flow of current in the
output is controlled by a voltage level across the gate.

This amplifier can provide medium input Impedance, medium output impedance, medium current gain,
medium voltage gain, and reverse output concerning input which means the output signal will be in 180
degrees phase change. From these characteristics, we can conclude that this amplifier can give high-level
performance over other amplifier circuits like a common drain (source follower) and common gate.
Hence it is most widely used than other amplifier circuits.

Common Source Amplifier Working

1. This amplifier can function as a transconductance amplifier or a voltage amplifier. If the amplifier
is acting as a transconductance amplifier, the input signals are amplified and modulate the
 current flowing to the load. If the amplifier is acting as a voltage amplifier, the input signal is
amplified and modulated, changing the voltage across the load resistor according to Ohm’s law.
2. The above circuit diagram explains how a common source amplifier works. Its operation is
similar to that of a common-emitter follower in a BJT circuit.
3. When the input signal is routed through the capacitor C1 to the gate terminal. This capacitor is
used to determine whether the gate terminal is affected by any DC voltage from the previous
stage. The potential is held by the resistor R2 of around 1Mega ohms located between the gate
and the ground. The voltage is generated across the resistor R2, which keeps the source above
ground. The bypass capacitor C2 adds gain to the alternating current signal.
4. The amplified output voltage is obtained by crossing the resistor R3 at the load at the circuit’s
drain terminal. The capacitor C3 couples the amplified output voltage to the AC signal of the
next stage by blocking or eliminating the DC components. This amplifier’s amplified output signal
is 180 degrees out of phase concerning the input signal and produces a high power gain.
5. The operation of the P-channel common source amplifier FET is similar to that of the N-channel
common source amplifier FET, with the exception that the voltage polarities are reversed. There
will be no current flowing between the gate and the source in the reverse-biased state. As a
result, the gate current is zero. The gate voltage (DC) is then given as.
Vg = Ig x Rg
6. The DC voltage at the source is given by,
Vs = Id x Rs
7. Then the gate to source voltage is given by,Vgs = Vg – Vs = 0- Id x RsSince Ig = 0
Vgs = – Id x Rs
Where,
Vgs = gate-source voltage
Vg = gate voltage
Rs = resistor at the source
Rg = resistor at Gate
Ig = gate current
8. The flow of DC components in the drain current can cause the resistor Rs to self-bias and provide
feedback from the output to the input.

Applications of FET Common Source Amplifier Circuit

1. Used in sensor signal amplification.

2. RF signal amplification with low noise.
3. Used in communication systems such as television and FM receivers.
4. In op-amps, they are used as voltage-controlled devices.
5. Cascade amplifiers and RF amplifier circuits are examples of their applications.