Transistor - Overview

After knowing the details about a single PN junction, or simply a diode, let us try to go for the
two PN junction connection. If another P-type material or N-type material is added to a single
PN junction, another junction will be formed. Such a formation is simply called as a Transistor.

A Transistor is a three terminal semiconductor device that regulates current or voltage flow and
acts as a switch or gate for signals.

Uses of a transistor

A transistor acts as an Amplifier, where the signal strength has to be increased.

A transistor also acts as a switch to choose between available options.
It also regulates the incoming current and voltage of the signals.

Constructional Details of a Transistor

The Transistor is a three terminal solid state device which is formed by connecting two diodes
back to back. Hence it has got two PN junctions. Three terminals are drawn out of the three
semiconductor materials present in it. This type of connection offers two types of transistors.
They are PNP and NPN which means an N-type material between two Ptypes and the other is a
P-type material between two N-types respectively.
The following illustration shows the basic construction of transistors

The three terminals drawn from the transistor indicate Emitter, Base and Collector terminals.
They have their functionality as discussed below.

Emitter

The left-hand side of the above shown structure can be understood as Emitter.

This has a moderate size and is heavily doped as its main function is to supply a
number of majority carriers, i.e. either electrons or holes.

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 As this emits electrons, it is called as an Emitter.
This is simply indicated with the letter E.

Base

The middle material in the above figure is the Base.

This is thin and lightly doped.
Its main function is to pass the majority carriers from the emitter to the collector.
This is indicated by the letter B.

Collector

The right side material in the above figure can be understood as a Collector.
Its name implies its function of collecting the carriers.

This is a bit larger in size than emitter and base. It is moderately doped.
This is indicated by the letter C.

The symbols of PNP and NPN transistors are as shown below.

The arrow-head in the above figures indicated the emitter of a transistor. As the collector of a
transistor has to dissipate much greater power, it is made large. Due to the specific functions of
emitter and collector, they are not interchangeable. Hence the terminals are always to be kept
in mind while using a transistor.

In a Practical transistor, there is a notch present near the emitter lead for identification. The PNP
and NPN transistors can be differentiated using a Multimeter. The following image shows how
different practical transistors look like.

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We have so far discussed the constructional details of a transistor, but to understand the
operation of a transistor, first we need to know about the biasing.

Transistor Biasing

As we know that a transistor is a combination of two diodes, we have two junctions here. As one
junction is between the emitter and base, that is called as Emitter-Base junction and likewise,
the other is Collector-Base junction.

Biasing is controlling the operation of the circuit by providing power supply. The function of both
the PN junctions is controlled by providing bias to the circuit through some dc supply. The figure
below shows how a transistor is biased.

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By having a look at the above figure, it is understood that

The N-type material is provided negative supply and P-type material is given positive
supply to make the circuit Forward bias.
The N-type material is provided positive supply and P-type material is given negative
supply to make the circuit Reverse bias.

By applying the power, the emitter base junction is always forward biased as the emitter
resistance is very small. The collector base junction is reverse biased and its resistance is a
bit higher. A small forward bias is sufficient at the emitter junction whereas a high reverse bias
has to be applied at the collector junction.
The direction of current indicated in the circuits above, also called as the Conventional
Current, is the movement of hole current which is opposite to the electron current.

Operation 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.

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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 re-combine 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 −

The conduction in a PNP transistor takes place through holes.

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

Operation 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.

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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 re-combine 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 −

The conduction in a NPN transistor takes place through electrons.

The collector current is higher than the emitter current.

The increase or decrease in the emitter current affects the collector current.

Advantages of Transistors

There are many advantages of using a transistor, such as −

High voltage gain.

Lower supply voltage is sufficient.
Most suitable for low power applications.
Smaller and lighter in weight.
Mechanically stronger than vacuum tubes.
No external heating required like vacuum tubes.
Very suitable to integrate with resistors and diodes to produce ICs.
There are few disadvantages such as they cannot be used for high power applications due to
lower power dissipation. They have lower input impedance and they are temperature
dependent.

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 Transistor Configurations

Any transistor has three terminals, the emitter, the base, and the collector. Using these 3
terminals the transistor can be connected in a circuit with one terminal common to both input
and output in three different possible configurations.
The three types of configurations are Common Base, Common Emitter and Common
Collector configurations. In every configuration, the emitter junction is forward biased and the
collector junction is reverse biased.

Common Base (CB) Configuration

The name itself implies that the Base terminal is taken as common terminal for both input and
output of the transistor. The common base connection for both NPN and PNP transistors is as
shown in the following figure.

For the sake of understanding, let us consider NPN transistor in CB configuration. When the
emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel
the emitter electrons and current flows through the emitter and base to the collector to
contribute collector current. The collector voltage VCB is kept constant throughout this.
In the CB configuration, the input current is the emitter current IE and the output current is the
collector current IC.

Current Amplification Factor (α)

The ratio of change in collector current (ΔI C) to the change in emitter current (ΔI E) when
collector voltage VCB is kept constant, is called as Current amplification factor. It is denoted
by α.

ΔIC
α = at constant VCB
ΔIE

Expression for Collector current

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 With the above idea, let us try to draw some expression for collector current.
Along with the emitter current flowing, there is some amount of base current IB which flows
through the base terminal due to electron hole recombination. As collector-base junction is
reverse biased, there is another current which is flown due to minority charge carriers. This is
the leakage current which can be understood as Ileakage. This is due to minority charge carriers
and hence very small.
The emitter current that reaches the collector terminal is

αIE

Total collector current

IC = αIE + Ileakage

If the emitter-base voltage VEB = 0, even then, there flows a small leakage current, which can
be termed as ICBO (collector-base current with output open).

The collector current therefore can be expressed as

IC = αIE + ICBO

IE = IC + IB

IC = α(IC + IB ) + ICBO

IC (1 − α) = αIB + ICBO

α ICBO
IC = IB +
1− α 1 −α

IC = ( α
1−α
) IB + (1− )
1
α
ICBO

Hence the above derived is the expression for collector current. The value of collector current
depends on base current and leakage current along with the current amplification factor of that
transistor in use.

Characteristics of CB configuration

This configuration provides voltage gain but no current gain.

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Being VCB constant, with a small increase in the Emitter-base voltage VEB, Emitter
current IE gets increased.
Emitter Current IE is independent of Collector voltage VCB.
Collector Voltage VCB can affect the collector current IC only at low voltages, when VEB
is kept constant.
The input resistance Ri is the ratio of change in emitter-base voltage (ΔVEB) to the
change in emitter current (ΔIE) at constant collector base voltage VCB.

ΔVEB
Ri at constant VCB
ΔIE
=

As the input resistance is of very low value, a small value of V EB is enough to produce a
large current flow of emitter current I E.
The output resistance Ro is the ratio of change in the collector base voltage (ΔVCB) to
the change in collector current (ΔIC) at constant emitter current IE.

ΔVCB
Ro = at constant IE
ΔIC

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 As the output resistance is of very high value, a large change in VCB produces a very
little change in collector current IC.
This Configuration provides good stability against increase in temperature.
The CB configuration is used for high frequency applications.

Common Emitter (CE) Configuration

The name itself implies that the Emitter terminal is taken as common terminal for both input and
output of the transistor. The common emitter connection for both NPN and PNP transistors is as
shown in the following figure.

Just as in CB configuration, the emitter junction is forward biased and the collector junction is
reverse biased. The flow of electrons is controlled in the same manner. The input current is the
base current IB and the output current is the collector current IC here.

Base Current Amplification factor (β)

The ratio of change in collector current (ΔI C) to the change in base current (ΔIB) is known as
Base Current Amplification Factor. It is denoted by β.

ΔI C
β=
ΔI B

Relation between β and α

Let us try to derive the relation between base current amplification factor and emitter current
amplification factor.

ΔI C
β=
ΔIB

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 ΔIC
α=
ΔI E

IE = IB + IC

ΔI E = ΔI B + ΔI C

ΔI B = ΔI E − ΔI C

We can write

ΔI C
β=
ΔI E − ΔI C

Dividing by ΔIE

ΔIC /ΔIE
β=
−
ΔIE ΔIC
ΔIE ΔIE

We have

α = ΔIC /ΔIE

Therefore,

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 α
β=
1 −α

From the above equation, it is evident that, as α approaches 1, β reaches infinity.

Hence, the current gain in Common Emitter connection is very high. This is the reason this
circuit connection is mostly used in all transistor applications.

Expression for Collector Current

In the Common Emitter configuration, I B is the input current and IC is the output current.
We know

IE = IB + IC

And

IC = αIE + I CBO

= α(IB + IC ) + ICBO

IC (1 − α) = αIB + ICBO

α 1
IC = IB + ICBO
1 −α 1 −α

If base circuit is open, i.e. if IB = 0,

The collector emitter current with base open is I CEO

1
ICEO = ICBO
1 −α

Substituting the value of this in the previous equation, we get

α
IC = IB + ICEO
1 −α

IC = βIB + ICEO

Hence the equation for collector current is obtained.

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Knee Voltage

In CE configuration, by keeping the base current I B constant, if VCE is varied, IC increases nearly
to 1v of VCE and stays constant thereafter. This value of VCE up to which collector current IC
changes with VCE is called the Knee Voltage. The transistors while operating in CE
configuration, they are operated above this knee voltage.

Characteristics of CE Configuration

This configuration provides good current gain and voltage gain.

Keeping VCE constant, with a small increase in VBE the base current IB increases
rapidly than in CB configurations.
For any value of VCE above knee voltage, IC is approximately equal to βIB.
The input resistance Ri is the ratio of change in base emitter voltage (ΔVBE) to the
change in base current (ΔIB) at constant collector emitter voltage VCE.

ΔVBE
Ri at constant VCE
ΔIB
=

As the input resistance is of very low value, a small value of V BE is enough to produce a
large current flow of base current IB.
The output resistance Ro is the ratio of change in collector emitter voltage (ΔVCE) to the
change in collector current (ΔIC) at constant IB.

ΔVCE
Ro = at constant IB
ΔIC

As the output resistance of CE circuit is less than that of CB circuit.

This configuration is usually used for bias stabilization methods and audio frequency
applications.

Common Collector (CC) Configuration

The name itself implies that the Collector terminal is taken as common terminal for both input
and output of the transistor. The common collector connection for both NPN and PNP
transistors is as shown in the following figure.

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 Just as in CB and CE configurations, the emitter junction is forward biased and the collector
junction is reverse biased. The flow of electrons is controlled in the same manner. The input
current is the base current IB and the output current is the emitter current IE here.

Current Amplification Factor (γ)

The ratio of change in emitter current (ΔI E) to the change in base current (ΔIB) is known as
Current Amplification factor in common collector (CC) configuration. It is denoted by γ.

ΔIE
γ=
ΔIB

The current gain in CC configuration is same as in CE configuration.

The voltage gain in CC configuration is always less than 1.

Relation between γ and α

Let us try to draw some relation between γ and α

ΔIE
γ=
ΔIB

ΔIC
α =
ΔIE

IE = IB + IC

ΔIE = ΔIB + ΔIC

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 ΔI B = ΔI E − ΔI C

Substituting the value of IB, we get

ΔI E
γ=
ΔI E − ΔI C

Dividing by ΔIE

ΔIE /ΔIE
γ=
ΔIE
ΔIE − ΔΔII
C
E

1
=
1 −α

1
γ=
1 −α

Expression for collector current

We know

IC = αIE + ICBO

IE = IB + IC = IB + (αIE + ICBO )

IE (1 α) = IB + ICBO

IB ICBO
IE = +
1 −α 1 −α

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9
 IC ≅IE = ( β + 1)IB + (β + 1)I CBO

The above is the expression for collector current.

Characteristics of CC Configuration

This configuration provides current gain but no voltage gain.

In CC configuration, the input resistance is high and the output resistance is low.
The voltage gain provided by this circuit is less than 1.
The sum of collector current and base current equals emitter current.
The input and output signals are in phase.
This configuration works as non-inverting amplifier output.

This circuit is mostly used for impedance matching. That means, to drive a low
impedance load from a high impedance source.

Transistor Regions of
Operation

The DC supply is provided for the operation of a transistor. This DC supply is given to the two
PN junctions of a transistor which influences the actions of majority carriers in these emitter
and collector junctions.
The junctions are forward biased and reverse biased based on our requirement. Forward
biased is the condition where a positive voltage is applied to the p-type and negative voltage is
applied to the n-type material. Reverse biased is the condition where a positive voltage is
applied to the n-type and negative voltage is applied to the p-type material.

Transistor Biasing

The supply of suitable external dc voltage is called as biasing. Either forward or reverse
biasing is done to the emitter and collector junctions of the transistor.
These biasing methods make the transistor circuit to work in four kinds of regions such as
Active region, Saturation region, Cutoff region and Inverse active region (seldom used).
This is understood by having a look at the following table.
Emitter Junction Collector Junction Region of Operation

Forward biased Forward biased Saturation region

Forward biased Reverse biased Active region

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3
Reverse biased Forward biased Inverse active region

Reverse biased Reverse biased Cut off region

Among these regions, Inverse active region, which is just the inverse of active region, is not
suitable for any applications and hence not used.

Active Region

This is the region in which transistors have many applications. This is also called as linear
region. A transistor while in this region, acts better as an Amplifier.
The following circuit diagram shows a transistor working in active region.

This region lies between saturation and cutoff. The transistor operates in active region when the
emitter junction is forward biased and collector junction is reverse biased.

In the active state, collector current is β times the base current, i.e.

IC = βIB

Where IC = collector current, β = current amplification factor, and I B = base current.

Saturation Region

This is the region in which transistor tends to behave as a closed switch. The transistor has the
effect of its collector and emitter being shorted. The collector and emitter currents are maximum
in this mode of operation.
The following figure shows a transistor working in saturation region.

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3
The transistor operates in saturation region when both the emitter and collector junctions are
forward biased.
In saturation mode,

IC
β <
IB

As in the saturation region the transistor tends to behave as a closed switch,

IC = IE

Where IC = collector current and IE = emitter current.

Cutoff Region

This is the region in which transistor tends to behave as an open switch. The transistor has the
effect of its collector and base being opened. The collector, emitter and base currents are all
zero in this mode of operation.

The figure below shows a transistor working in cutoff region.

The transistor operates in cutoff region when both the emitter and collector junctions are

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3
reverse biased.

As in cutoff region, the collector current, emitter current and base currents are nil, we can write
as

IC = IE = IB = 0

Where IC = collector current, IE = emitter current, and IB = base current.

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3