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Bipolar Junction Transistor
Introduction
Transistor is a three terminal semiconductor device made up of doped semiconductor material. The first transistor was developed in 1947 by the Bell laboratories which replaced the vacuum tubes previously used for amplification and switching. The advantages of transistor over the vacuum tubes were obvious as it was smaller in size, had rugged construction and consumes less power. The major use of the transistor includes amplification (voltage, current, power) and switching.
Structure of a BJT
A bipolar junction transistor (BJT) is made up of three semiconductor regions separated by two junctions. The device can have either two layers of p-type semiconductor material and one layer of n-type material or two layers of n-type materials and one layer of p-type material. The former is called p-type semiconductor and the latter is called an n-type semiconductor.
There are three terminals of a transistor indicated as emitter, base and collector terminal. The base terminal is sandwiched between emitter and collector terminal. The base is usually thin and lightly doped as compared to emitter and collector. The width of the base region is usually 150 times less than the width of a transistor. The low level of doping makes the base more reisitive as there are less charge carriers to carry the current. Emitter is heavily doped as compared to collector which is moderatly doped. The term bipolar refers to the current that is comprised of both holes and electrons hence the current has two polarities.
Transistor Operation
Regions of Operation
The transistor have three major regions of operation
o Saturation Region o Active Region o Cut-off Region
Region of Operation
Emitter Base Junction
Collector Base Junction
Use
Saturation Forward Forward As a switch
Active Forward Reverse As an amplifier
Cut-off Reverse Reverse As a switch
The behaviour of transistor is different in different regions of operation. To maintain a transistor in certain region external voltage sources are used and this process is called biasing of a transistor. To use a transistor as an amplifier it is biased to operate in active region i.e. the emitter base junction is forward biased and the collector base junction is reverse biased.
Operation of Transistor (Active Region)
The operation of a transistor in active region is explained using a pnp transistor. For npn transistor the operation will be similar but the direction of currents will be opposite and the polarities of bias voltage will reversed. VEE is the emitter bias voltage and VCC is the collector bias voltage. The emitter terminal is at higher potential than the base so the emitter base junction is forward biased and the depletion region on the emitter-base junction narrows. The collector is at lower potential than the base so the collector-base junction is reverse biased and the width of depletion region increases. We will consider both junctions separately to find the currents flowing through the junctions. First we consider the forward biased emitter-base junction shown in the figure below
As we have seen that the collector current is made up of two separate currents one due to majority carriers and other due to minority carriers (leakage current). So IC can be written as
IC IC(majority) ICO(minority)
Where ICO is the current due to minority carriers or it can also be mentioned as collector current with emitter terminal open. ICO is generally very small in microamperes but it is temperature dependent and increases with increase in temperature.
Common Base Configuration
In common-base configuration base terminal is common between input and output side of the transistor. In addition to that the base terminal is either at ground or the terminal closest to the ground. Emitter terminal is used as input and collector terminal is used as output for the common- base configuration. The direction of current mentioned in the figure represents the conventional flow and biasing voltage is maintained to ensure this direction of flow.
Input and Output Parameters
Input Terminal: Emitter terminal
Input Current: IE (Emitter Current)
Input Voltage: VBE (Voltage difference between base and emitter)
Output Terminal: Collector Terminal
Output Current: IC (Collector Current)
Output Voltage: VCB (Voltage difference between collector and base)
Input and Output Characteristics of Common Base Configuration
The behaviour of a transistor can be described using characteristics of the input parameters and output parameters
Input Characteristics
Input characteristic involves the relation of input current (IE) with input voltage (VBE) at different values of output voltage (VCB)
Fig: Input characteristic curves for common-base configuration
The input characteristic curves of common-base configuration reveal that the input current (IE) increases with the increase in output voltage (VCB). This increase is due to the increase in minority current flowing across the reverse biased collector-base junction.
Output Characteristics
The output characteristic curve will relate the output current (IC) with output voltage (VCB) at different values if input current (IE)
Cut-Off Region
Cut-off region is the region where the collector current is equal to 0 Amperes. In cut-off region both emitter-base junction and collector base junction are reverse biased
Saturation Region
The saturation region is the region left to VCB = 0. In this region both emitter-base junction and collector base junction are forward biased. There is an exponential rise in current as VCB increases towards 0
Input Approximation
The input characteristic curve reveals that for a fixed value of VCB as the input voltage is increased the input current resembles the forward biased diode characteristics which can be approximated by a constant voltage drop (practical model of a diode). The effect of VCB is negligible and can be ignored and we can safely assume that VBE remains constant across the forward biased junction.
V BE 0.7V
Alpha (α)
In dc mode the values of IC and IE are related by a value alpha (α)
C
dc E
I
I
α is the ratio of collector current to the emitter current. Typically the value of α lies between 0.9 to 0.998. α is also called common-base forward current gain. For high values of ICBO the collector current will be equal to
I C I E ICBO
For ac situations the operation point moves along the curves and α is defined as
VCB =constant
C
ac E
I
I
For most cases the values of alpha (ac) and alpha (dc) are close enough to permit the use of one in place of other.
Example:
a) Find IC if α = 0.998 and IE = 4mA b) Find α if IE = 2.8mA and IB=20μA c) Find IE if IB = 40μA and α = 0.
Amplifying Action of Transistor
Common base configuration can be used for voltage amplification. The output current in common base configuration is almost the same as the input current. Input resistance of common-base configuration is low and the output resistance of common-base configuration is high.
Using the common-base configuration we can find the amplification factor of the amplifier.
200 input current 10 20
i E i i
V mV I I mA R
using approximation I E IC
V L I (^) L R (^) L 10 mA 5 k 50 V
Voltage gain 250
L v i
V V A V mV
