Introduction
The transistor consists of three-terminal electronic device. Both electrons and holes are responsible for electrical conduction and hence it is called bipolar. Among the trio of original transistor inventors, William Shockley in 1951 invented the first junction transistor consisting of two back-to-back p-n junctions. It is made up of doped semi-conductor material which is mainly used in switching or amplifying applications. It has wide spread application in computers, satellites and other modern communication systems.
STRUCTURE:
A transistor is made up of 3 doped regions that form two p-n junctions between them. So, there are two types of transistors, as shown below:
(i) p-n-p transistor: Here two segments of p-type semiconductor (known as emitter and collector) are separated by a segment of n-type semiconductor (known as base).
(ii) n-p-n transistor: Here two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type semiconductor (base).
The direction of the arrow tells the direction of current.
The three segments that comprise a BJT are:
(a)Emitter: It is of moderate size and is heavily doped. It supplies a large quantity of majority carriers for the current to flow through the transistor.
(b)Collector: It is moderately doped and larger in size than the emitter.It consists of the major portion of the majority carriers supplied by the emitter.
(c)Base:It is a very thin central portion and is doped lightly.
NPN TRANSISTOR WORKING:
Consider the diagrams shown below:
The emitter base junction is forward biased and the collector-base junction has been rigged to be in reverse bias. The base is common to both the collector and emitter so it is called common base configuration.
As the emitter region is n type hence electrons are the majority carriers and as emitter base junction is forward biased large number of electrons enters into the p type base region. As it is lightly doped so it has less number of holes. A small portion of the free floating electrons combine with these holes. When the base-collector junction is set to have a reverse biased the electrons appears as minority carriers and cross over the base-collector junction making the current Ic. A small current IB is also constituted by the movement of holes of p-type base region to the emitter region. As the base is lightly doped so it has less majority carriers i.e. holes, hence IB<<Ic
And as evident the current IE entering the emitter is equal to-
TRANSISTOR CHARACTERISTICS:
Since a transistor has three terminals it can be connected in three different ways, in each configuration one terminal is common to the input and the output.
a) Common emitter
b) Common base
c) Common collector
The common emitter configuration is generally used and is discussed below:
Common emitter transistor characteristics
When a transistor is used in CE configuration, the input is between the base and the emitter and the output is between the collector and the emitter.
Circuit diagram for studying input output characteristics of n-p-n transistor in CE configuration.
Input characteristics Output characteristics
The input characteristics are determined by plotting base current (IB) vs base –emitter voltage VBE while keeping the collector emitter voltage VCE constant.the output characteristics are determined by plotting collector current vs collector-emitter(IC) vs collector emitter(VCE) voltage keeping IB fixed.
APPLICATIONS OF TRANSISTORS:
- Transistor is used in amplifier circuits to amplify weak signals with the signal amplification of the order of 10-1000.
- It is used as switch in digital circuits.
- It is used in oscillator circuits.