A Bipolar junction Transistor has three semiconductor regions that are doped at different levels. Out of three regions two of them are doped with either donor or acceptor atoms and the third region is doped with other atom.
In case of NPN transistor, two regions are doped with donor impurity and the other regions is doped with acceptor impurity.
In case of PNP transistor, two regions are doped with acceptor impurity and one regions is doped with donor impurity.
Actual a BJT comprise of two P-N junction, which are connected in series. The word bipolar indicates the role of both charge carriers (electrons and holes). In this article, a Bipolar Junction Transistor is discussed elaborately.
Construction of Bipolar Junction Transistor
A Bipolar Junction Transistor ( BJT ) is constructed using Silicon or Germanium Crystal. It either has P-type semiconductor sandwiched between two N- type semiconductor or N-type semi conductor sandwiched between two P-type semiconductor .The first type forms NPN transistor and the second type forms PNP transistor.
A Bipolar Junction Transistor ( BJT ) has three terminals named Emitter ,base and collector.
- Emitter ( E )
The Emitter region is placed on one side of BJT. This region provides charge carriers ( either electrons or holes ) to the collector & base region. The emitter regions is always highly doped with either donor or acceptor atoms.
Doping concentration = 1019 per cm3
- Base ( B )
The base region is constructed in the middle of two N or P regions. The base of transistor is very thin, when composed to emitter and collector regions. The doping concentration of base region is very less ( lightly doped ).
Doping Concentration = 1016 per cm3
- Collector
The collector region is constructor on the opposite side of emitter and as the name indicates, it collects the charge carrier. ( either electrons or holes ). The collector region has larger area than base and emitter. The doping concentration of collector is slightly less than emitter but more than base.
Doping Concentration = 1017 per cm3
Construction of PNP Transistor
Construction of NPN Transistor
Here,
E = Emitter
B = Base
C = Collector
N = N-region
P = P-region
Je = Emitter base junction
Jc = Collector base function
Current in Bipolar Junction Transistor
In NPN Transistor, the direction of flow of current is opposite to the direction of flow of electrons.
Current flow in NPN Transistor
In a PNP Transistor, the direction of flow of current and holes are similar.
Current flow in PNP Transistor
Here,
IE = Emitter current
IB = Base current
IC = Collector current
According to Kirchhoff current law, the emitter current is equal to sum of base and collection currents.
IE = IB + IC
Circuit configuration of Bipolar Junction Transistor
A BJT has three terminals known as Emitter, Base ( B ) & Collector and two junctions named as emitter base junction ( Je ) and collector base junction ( Jc ).
A BJT is used in three different configurations
- Common Emitter ( CE ) configuration
- Common Base ( CB ) configuration
- Common Collector ( CC ) configuration
Common Emitter ( CE ) configuration
In this configuration, the emitter of transistor is taken common between input and output. As shown in figure below
The input signal is applied between base and emitter and the output signal is obtained from collector and emitter. This configuration is widely used in circuits.
Common Base ( CB ) configuration
In this configuration, the base of transistor is taken common between input and output. As shown in figure below.
The input signal is applied between emitter and base terminals and the output is obtained from collector and base terminals.
Common Collector ( CC ) configuration
In this configuration, the base of transistor is taken common between input and output. As shown in figure below.
The input signal is applied between base and collector and the output is obtained from emitter and collector.
Transistor configuration characteristics are given below
CB | CE | CC | |
Input Impedance | Low | Medium | High |
Output Impedance | Very High | Low | Low |
Phase Angle | 0° | 180° | 0° |
Voltage Gain | High | Medium | Low |
Current Gain | Low | Medium | High |
Power Gain | Low | Very high | Medium |
Advantages of BJT
- It can be operated at high frequency.
- High bandwidth gain.
- Good performance at higher frequency.
- Used in both low and high power applications.
- Low forward voltage drop
Disadvantages of BJT
- Less thermal stability.
- Complex base control, needs better circuit.
Application of BJT
- Digital switch
- Clippers
- Modulator and Demodulator
- Amplifiers
- Electronic switches
Author
Zener diode is a special PN Junction diode that is designed to operate in reverse biased condition. It conducts the flow of current in both forward and reverse biased condition.