Topic Bipolar Junction Transistor

Sub Topics CB, CE, CC Configurations and Characteristics

BIPOLAR JUNCTION TRANSISTORS (BJT)

Introduction:
 Transistor is a three terminal device: Base, emitter and collector, can be operated in three
configurations common base, common emitter and common collector.
 According to configuration it can be used for voltage as well as current amplification.
 The amplification in the transistor is achieved by passing input current signal from a region of
low
resistance to a region of high resistance. This concept of transfer of resistance has given the
name
TRANSfer-resISTOR (TRANSISTOR).
Types of Transistors
 There are two types of transistors: Unipolar junction transistor and bipolar junction
transistor.
 In unipolar transistor, the current conduction is only due to one type of charge carriers,
majority carriers.
 The current conduction in bipolar transistor is because of both the types of charge carriers,
holes and electrons. Hence this is called Bipolar Junction Transistor,
 In BJT output current is controlled by input current and hence it is a current controlled device.
 Hence the BJTs are of two types. They are
 N-P-N transistor
 P-N-P transistor
Advantages of BJT
1. Low operating voltage
2. Higher efficiency
3. Small size and ruggedness and
4. Does not require any filament power
Construction of npn and pnp Transistors and its symbol
When a transistor is formed by sandwiching a single p-region between two n-regions, as
shown in the Fig. 1 (a), it is an n-p-n type transistor. The p-n-p type transistor has a single
n-region between two p-regions, as shown in Fig. 1 (b).

Fig 1(a) n-p-n transistor Fig 1(b) p-n-p transistor

.
Transistor has 3 terminals.
 Emitter
 Base
 Collector
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 Transistor has two p-n junctions. They are:
 Emitter –Base junction
 Collector –Base junction
Emitter:
 Emitter is heavily doped because it is used to emit the charge carriers. The doping level in
emitter is slightly greater than that of collector.
Base:
 The charge carriers emitted by the emitter should reach collector passing through the base.
 Hence base should be very thin & to avoid recombination, & to provide more collector current
base is lightly doped.
Collector:
 Collector has to collect the most of charge carriers emitted by the emitter & it is moderately
doped.
 Hence the area of cross section of collector is more compared to emitter; it helps in better
power dissipation.
Transistor can be operated in 3 regions.
 Active region
 Saturation region
 Cut-off region
Active Region:
For the transistor to operate in active region base to emitter junction is forward biased and
collector to base junction is reverse biased. Transistor can be act as a amplifier in this region.
Saturation region:
Transistor to be operated in Saturation region if both the junctions i.e, collector to base
junction & base to emitter junction are forward biased. transistor can be act as a switch in this
region
Cut-off region:
For the transistor to operate in Cut-off region both the junctions i.e., base to emitter junction &
collector to base junction are reverse biased. transistor can be act as a switch in this region

Principle of Operation of Transistors

Unbiased Transistor
 An unbiased transistor means a transistor with no external voltage (biasing) is applied.
Obviously, there will be no current flowing from any of the transistor leads.
 Since transistor is like two p-n junction diodes connected back to back, there are depletion
regions at both the junctions, emitter junction and collector junction, as shown in the Fig. 2.
 During diffusion process, depletion region penetrates more deeply into the lightly doped side
in order to include an equal number of impurity atoms in the each side of the junction.
 As shown in the Figure, Depletion region at emitter junction penetrates less in the heavily
doped emitter and extends more in the base region.

 Similarly, depletion region at collector junction penetrates less in the heavily doped collector
and extends more in the base region.
 As collector is slightly less doped than the emitter, the depletion layer width at the collector
junction is slightly more than the depletion layer width at the emitter junction.

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 Barrier voltage is the voltage necessary to cause electrical conduction in a junction of
two dissimilar materials.

 Like diodes, a barrier voltage exists within the transistor.

 The barrier voltage at each junction is positive on the n-side and negative on p-side.
 The barrier voltage across the junction in a silicon transistor is about 0.7 volt approximately
0.3 volt in a germanium transistor.
Operation of npn Transistor
 The base to emitter junction is forward biased by the d.c. source Thus, depletion region at
this junction is reduced. The collectors to base junction reverse biased, increasing depletion
region at collector to base junction as show in Fig. 3.

 The forward biased EB junction causes the electrons in the n-type emitter to flow towards the
base. This constitutes the emitter current As these electrons flow through the p-type base,
they tend to combine with holes in p-region (base).
 Due to light doping, very few of the electrons injected into the base from the emitter
recombine with holes to constitute base current, and the remaining large number of electrons
cross the base region and move through the collectorre region to the positive terminal of the
external d.c. source.
 This constitutes collector current . Thus the electron flow constitutes the dominant current
in an npn transistor.
 Since, the most of the electrons from emitter flow in the collector circuit and very few
combine with holes in the base. Thus, the collector current is larger than the base current. The
relationship between these current is given by

 Since it is a bipolar device, the collector current comprises two components: majority and
minority.
 The minority current component is called the leakage current and is symbol ( current
with
emitter terminal open).
 The collector current, therefore, is determined in total by

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 :It is an injected collector current due to majority carriers crossing the Collector base
junction.
Operation of pnp Transistor
The pnp transistor has its bias voltages and reversed from those in the npn transistor.
This is necessary to forward-bias the emitter-base junction and reverse-bias the collector base
junction. as shown in figure.
 The forward biased EB junction causes the holes in the p-type emitter to flow towards the
base. This constitutes the emitter current
 As these holes flow through the n-type base, they tend to combine with electron in n-region
(base). As the base is very thin and lightly doped, very few of the holes injected into the base
from the emitter recombine with electrons to constitute base current,
 The remaining large number of holes crosses the depletion region and move through the
collector region to the negative terminal of the external d.c source. This constitutes collector
current . Thus the hole flow constitutes the domin at current in an pnp transistor.

CE, CB, CC Configurations:

Transistor Configuration:
 Transistor has three terminals namely emitter (E), base (B), collector(C).
 However, when a transistor is connected in a circuit, we require four terminals (ie) two
terminals for input and two terminals for output.
 This difficulty is overcome by using one of the terminals as common terminal.
 Depending upon the terminals which are used as a common terminal to the input and
output terminals, the transistors can be connected in the following three different
configuration.
1. Common base configuration
2. Common emitter configuration
3. Common collector configuration

Common base configuration:

 In this configuration base terminal is connected as a common terminal.
 The input is applied between the emitter and base terminals. The output is taken between
the collector and base terminals.

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 .

Common emitter configuration:

 In this configuration emitter terminal is connected as a common terminal.
 The input is applied between the base and emitter terminals. The output is taken between
the collector and base terminals.

Common collector configuration:

 In this configuration collector terminal is connected as a common terminal.
 The input is applied between the base and collector terminals. The output is taken between
the emitter and collector terminals

CB, CE,CC Characteristics :

Common Base Characteristics
Input Characteristics
For p - n - p transistor, the input current is the emitter current (IE) and the input voltage is
the collector base voltage (VCB).

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 As the emitter - base junction is forward biased, therefore the graph of I E Vs VEB is similar to
the forward characteristics of a p - n diode. IE increases for fixed VEB when VCB increases.
Output Characteristics
The output characteristics shows the relation between output voltage and output current IC
is the output current and collector – base voltage and the emitter current IE is the input current
and works as the parameters. The figure below shows the output characteristics for a p - n - p
transistor in CB mode.

As we know for p - n - p transistors IE and VEB are positive and IC, IB, VCB are negative. These
are three regions in the curve, active region saturation region and the cut off region. The active
region is the region where the transistor operates normally. Here the emitter junction is reverse
biased. Now the saturation region is the region where both the emitter collector junctions are
forward biased. And finally the cut off region is the region where both emitter and the collector
junctions are reverse biased.

Common Emitter Characteristics

Input characteristics
IB (Base Current) is the input current, VBE (Base - Emitter Voltage) is the input voltage for CE
(Common Emitter) mode. So, the input characteristics for CE mode will be the relation between I B
and VBE with VCE as parameter. The characteristics are shown below

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The typical CE input characteristics are similar to that of a forward biased of p - n diode. But as V CB
increases the base width decreases.
Output characteristics
Output characteristics for CE mode is the curve or graph between collector current (IC) and
collector - emitter voltage (VCE) when the base current IB is the parameter. The characteristics are
shown below in the figure.

Like the output characteristics of common - base transistor CE mode has also three regions
named (i) Active region, (ii) cut-off regions, (iii) saturation region. The active region has collector
region reverse biased and the emitter junction forward biased. For cut-off region the emitter
junction is slightly reverse biased and the collector current is not totally cut-off. And finally for
saturation region both the collector and the emitter junction are forward biased.
Common Collector Characteristics
Input characteristics
 It is a curve which shows the relationship between the base current, IB and the
collector base voltage VCB at constant VCE This method of determining the characteristic is as
follows.
 First, a suitable voltage is applied between the emitter and the collector. Next the input
voltage VCB is increased in a number of steps and corresponding values of IE are noted.
 The base current is taken on the y-axis, and the input voltage is taken on the x-axis. Fig.
shows the family of the input characteristic at different collector- emitter voltages.
 The following points may be noted from the family of characteristic curves. 1.Its
characteristic is quite different from those of common base and common emitter circuits.

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Output Characteristics
 It is a curve which shows the relationship between the emitter current l and collector-emitter
voltage, the method of determining the output characteristic is as follows.
 First, by adjusting the input a suitable current IB is maintained. Next VCB increased in a
number of steps from zero and corresponding values of IE are noted.
 The above whole procedure is repeated for different values of IB. The emitter current is taken
on the Y-axis and the collector-emitter voltage is taken on the X-axis.
 Fig shows the family of output characteristics at different base current values. The following
points are noted from the family of characteristic curves.

Application of BJT
BJT's are used in discrete circuit designed due to availability of many types, and obviously
because of its high transconductane and output resistance which is better than MOSFET. BJT's are
suitable for high frequency application also. That’s why they are used in radio frequency for
wireless systems. Another application of BJT can be stated as small signal amplifier, metal
proximity photocell, etc.