JUNCTION TRANSISTOR OR BIPOLAR JUNCTION TRANSISTOR (BJT):

It is a 3 terminal semiconductor device formed by either growing a very thin

layer of n-type SC crystal between very thick layer of p-type crystal or a thin
layer of p-type crystal between very thick layer of n- type crystal. The first type
is called p-n-p transistor and second type is called n-p-n transistor.

BIPOLAR JUNCTION TRANSISTOR (BJT) IS A CURRENT CONTROLLED DEVICE.

The central layer or segment is called base (B) or wafer. It is very thin and
lightly doped. Other two layers are called emitter (E) and Collector (C).

Emitter is heavily dopped and moderate in size. It supplies large number of

majority charge carrier.

Collector is moderately doped and larger in size as compared to emitter.

Collector collects the major portion of the majority charge carrier coming from
the emitter.

Principle: Emitter base junction is forward biased and collector base junction is
reverse biased.

Note: 1. (Doping)Emitter> (Doping)Collector>(Doping)Base

2. (Size)Collector > (Size)Emitter >(Size)Base

3. Bi - polar transistor is a current controlled device. In simple terms,
This means that the main current in a bipolar transistor (collector
current) is controlled by the base current.

Classification of Transistor:
Modes of operation of Transistor:
1. Common Base (CB) mode
2. Common Emitter (CE) mode
3. Common Collector (CC) mode

All three types of modes are shown below

characteristics COMMON BASE COMMON COMMON
(CB) EMITTER (CE) COLLECTOR(CC)

Input resistance Low(about 100Ω) Low (about 750Ω) Very high (about
750KΩ)
Output Very high(about High (about 45KΩ) Low(about 50Ω)
resistance 450KΩ)
Current I IC IE
= C = =
Amplification IE IB IB
factor
Current gain <1 Greater than 1 Greater than 1
Voltage gain About 150 About 500 Less than 1
Phase relation In phase 1800 shift In phase
between input
and output
applications For high frequency For audio- For impedance
application frequency matching
applications
Collector current  I  I
IC = I B + CBO IC = I B + CBO
1− 1− 1− 1−
=  I B + I C EO
I CBO =Collector base current when emitter open

I CEO =Collector emitter current when base open

RELATION BETWEEN α, β and γ:

 =  +1
1
=
1−
 WORKING OF A TRANSISTOR:

(1) An N—P-N TRANSISTOR:

It works on a principle that

Emitter-base junction is forward
biased and collector-base junction
is reverse biased.

The forward bias of the emitter-

base circuit repels the electrons
of the emitter towards the base,
setting up emitter current (IE). As
the base is very thin and lightly
doped, a very few electrons (< 5%)
from the emitter combine with the
holes of base giving rise to base
current (IB) and remaining
electrons (>95%) are pulled by the collector Fig.: WORKING OF N-P-N TRANSISTOR
AND ITS BIASING
which is a high positive potential. The electrons are finally collected by the
positive terminal of the battery (VCC or VCB), giving rise to collector current (IC).

Thus, IE=IB+IC
Note: IB<<IC

(1) An P-N-P TRANSISTOR:

It works on a principle that

Emitter-base junction is
forward biased and collector-
base junction is reverse biased.

The forward bias of the emitter-

base circuit repels the HOLES
of the emitter towards the
base, setting up emitter current
(IE). As the base is very thin and
lightly doped, a very few
HOLES (< 5%) from the emitter
combine with the ELECTRONS of base giving Fig.: WORKING OF P-N-P TRANSISTOR
AND ITS BIASING
rise to base current (IB) and remaining HOLES
(>95%) are pulled by the collector. The holes are finally collected by the
negative terminal of the battery (VCC or VCB), giving rise to collector current (IC).

Thus, IE=IB+IC

Ex. 1. A transistor with α=0.99 is operated in common base circuit. What will be
the current gain of the same transistor in common emitter configuration?

 0.99
Answer: = = = 99
1 −  1 − 0.99

Ex.2. The current gain of a transistor in common emitter (CE) configuration is 49.
What will be the current gain of the same transistor in common base
configuration?
Answer: given,  = 49

  49
Therefore,  =  = = = .98
1−  + 1 49 + 1
Common emitter transistor characteristics:

Common Emitter Connection (or CE

Configuration)
Definition: The configuration in which the emitter is connected
between the collector and base is known as a common emitter
configuration. The input circuit is connected between emitter and
base, and the output circuit is taken from the collector and emitter.
Thus, the emitter is common to both the input and the output circuit,
and hence the name is the common emitter configuration. The
common emitter arrangement for NPN and PNP transistor is shown
in the figure below.

Base Current Amplification Factor (β)

The base current amplification factor is defined as the ratio of the
output and input current in a common emitter configuration. In
common emitter amplification, the output current is the collector current IC, and the
input current is the base current IB.

In other words, the ratio of change in collector current with respect to base current is
known as the base amplification factor. It is represented by β (beta).
Relation Between Current Amplification Factor (α) & Base Amplification Factor (β)

The relation between Β and α can be derived as

We Known,

Now,

Substituting the value of ΔIE in equation (1), we get,

The above equation shows that the when the α reaches to

unity, then the β reaches to infinity. In other words, the current gain in a common
emitter configuration is very high, and because of this reason, the common emitter
arrangement circuit is used in all the transistor applications.

Collector Current
In CE configuration, the input current IB and the output current IC are related by the
equation shown below.

If the base current is open (i.e., IB = 0). The collector current is current to the emitter,
and this current is abbreviated as ICEO that means collector- emitter current with the
base open.

Substitute the value ΔIB in equations (1), we get,

Characteristics of Common emitter (CE) Configuration

The characteristic of the common emitter transistor circuit is shown in the figure
below. The base to emitter voltage varies by adjusting the potentiometer R1. And the
collector to emitter voltage varied by adjusting the potentiometer R2. For the various
setting, the current and voltage are taken from the milliammeters and voltmeter. On
the basis of these readings, the input and output curve plotted on the curve.
Input Characteristic Curve
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 below.

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.
Input Resistance: The ratio of change in base-emitter voltage VBE to the change in
base current ∆IB at constant collector-emitter voltage VCE is known as input
resistance, i.e.,

Output Characteristic
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 below.

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
In the active region IC = βIB, a small current IC is not zero, and it is equal to reverse
leakage current ICEO.

Output Resistance: The ratio of the variation in collector-emitter voltage to the

collector-emitter current is known at collector currents at a constant base current
IB is called output resistance ro.

The value of output resistance of CE configuration is more than that of CB

Field effect transistors (FET):
• The field-effect transistor (FET) is a type
of transistor that uses an electric field to control the
flow of current in a semiconductor.
• It comes in two types: Junction field-effect transistor
(JFET) and metal-oxide-semiconductor FET (MOSFET).
• FETs are also known as unipolar transistors since they involve
single-carrier-type operation.
• The main difference between a bi polar transistors and a field effect transistor is that, Bi - polar transistor is a
current controlled device. This means that the main current in a bipolar transistor (collector current) is
controlled by the base current.
o Filed effect transistor is a voltage controlled device. This means that the voltage at the gate (similar to
base of a bipolar transistor) controls the main current.
Junction Field effect Transistor(JFET):

It is a three terminal device and looks similar to a bi-polar

transistor. The standard circuit symbols of N-channel and P-channel
type FETs are shown in Fig 2.

There are types of JFET: N-channel FET and P- channel FET

Field-Effect Transistor (FET) is a semiconductor device that consists of a channel made of a
semiconductor material, with two electrodes connected at either end, namely the drain and the
source. The flow of current between the source and the drain terminals is controlled by a third
electrode, known as the gate, which is placed in close proximity to the channel.

Construction:
As shown in Fig 3a, a n-Channel JFET has a narrow
bar of n-type. To this, two p-type junctions are
diffused on opposite sides of its middle part Fig 3a.
These diffused junctions form two P-N diodes or
gates. The n-type semiconductor area between these junctions/gates is called channel. The diffused P regions on
opposite sides of the channel are internally connected and a single lead is brought out which is called gate lead or
terminal. Direct electrical connections are made at the two ends of the bar. One of which is called source terminal, S
and the other drain terminal, D. A p-channel FET will be very similar to the n-channel FET in construction except that it
uses P-type bar and two N type junctions as shown in Fig 3b.

1. Source terminal: It is the terminal through which majority carriers

enter the bar (N or P bar depending upon the type of FET).

2. Drain terminal: It is the terminal through which majority carriers

come out of the bar.

3. Gate terminal: These are two internally connected heavily doped

regions which form two P-N junctions.

4. Channel: It is the space between the two gates through which

majority carriers pass from source to drain when FET is working(on).
Working of FET: Similar to Biploar transistors, the working point of
adjustment and stabilization are also required for FETs.

Biasing a JFET

– Gates are always reverse biased. Therefore the gate current IG is

practically zero.

– The source terminal is always connected to the end of the supply

which provides the necessary charge carriers. For instance, in an N-
channel JFET source terminal S is connected to the negative of the
DC power supply. And, the positive of the DC power supply is
connected to the drain terminal of the JFET.

Whereas in a P channel JFET, Source is connected to the positive

end of the power supply and the drain is connected o the negative
end of the power supply.

Let us now consider an N channel JFET, the drain is made positive

with respect to source by voltage VDS as shown in Fig 4a. When
gate to source voltage VGS is zero, there is no control voltage and
maximum electron current flows from source(S) - through the
channel - to the drain(D). This electron current from source to drain
is referred to as Drain current, ID.
When gate is reverse biased with a negative voltage(VGS negative) as
shown in Fig 4b, the static field established at the gate causes
depletion region to occur in the channel as shown in Fig 4b.

This depletion region decreases the width of the channel

causing the drain current to decrease.

If VGS is made more and more negative, the channel width decreases
further resulting in further decrease in drain current. When the
negative gate voltage is sufficiently high, the two depletion layers
meet and block the channelcutting off the flow of drain current as
shown in Fig 4c. This voltage at which this effect occurs is referred
to as the Pinch off voltage, VP .

Thus, by varying the reverse bias voltage between gate and

source(-VGS ), the drain current can be varied between maximum
current (with -VGS =0) and zero current(with VGS =pinch off voltage).
So, JFET can be referred as a voltage controlled devices.

P channel JFET operates in the same way as explained above

except that bias voltages are reversed and the majority carrier of
channel are holes.

Typical applications of JFET:

One very important characteristic of JFET is its very high input

impedence of the order of 109 ohms. This characteristic of FET has
made it very popular at the input stage of a majority of electronic
circuits.

As discrete components FETS are chiefly used in,

– DC voltage amplifiers

– AC voltage amplifiers(input stage amplifiers in HF and LF ranges)

– Constant current sources

– Integrated circuits of both analog and Digital technology.

Difference between JFET and BJT:

JFET BJT
1 In a JFET there is only one type of carrier ie., In BJT both electrons
holes in p type channel and electrons in n-type and holes play role in
channel. For this reason, it is called unipolar conduction. It is called
transistor. as bipolar transistors.
2 As the input circuit of a JFET is reverse biased The input circuit of a
and therefore it as a input independence. BJT is forward based
and hence has low input
independence.
 3 No current enters the gate & JFET. In typical BJT base
current might be a few
ampere
4 JFET uses voltage on the gate terminal to the BJT uses the current
control current between drain and source. No into its base to control a
junction in JFET so noise level is very small large current between
collector and emitter.
5

Metal–oxide–semiconductor field-effect transistor

(MOSFET): Insulated-gate FET (IFET or IGFET),
• It is a three-terminal device with gate (G), drain (D) and source (S)
terminals.
• MOSFETs are also referred to as Insulated-gate FET, for which the abbreviation used are IFET or IGFET.
• MOSFETs are electronic devices used to switch or amplify voltages in circuits. It is a
voltage controlled device and is constructed by three terminals.