What is transistor? Transistor is an electronic device. It is made with p and n type semiconductor. When a semiconductor is placed in middle between same type semiconductors the configuration is called transistor. We can say that a transistor is the combination of two diodes connected back to back.
Bardeen W.H. Brattain of Bell telephone laboratories, USA first invented transistor in 1948. After the invention of transistor revolution occurred in electronic world. Before the invention of transistor big size circuits were used. But transistor turned them into a tiny size.
Diagrammatic form of n-p-n and p-n-p transistor is shown. In circuit connection diagrammatic form is used. Arrow symbol defines the emitter current. In the n-p-n connection we know electrons flow into the emitter. This means that conventional current flows out of the emitter as indicated by the outgoing arrow. Similarly it can be seen that for p-n-p connection, the conventional current flows into the emitter as shown by the inward arrow in the figure.
Transistor is made by a very pure peace of semiconductor using special technique. Two types of transistors are produced viz. junction transistor and point contact transistor. Junction transistors are used now a days mostly.
Two kinds of transistors are i) n-p-n transistor, ii) p-n-p transistor.
A transistor has three elements emitter, base and collector. Block diagram and symbol of transistor is shown in figure for n-p-n and p-n-p transistor.
N-type semiconductor sandwiched between two layers of p-type semiconductor in a p-n-p transistor. In n-p-n transistor p-type semiconductor sandwiched between two layers of n-type semiconductor. Suitable impurities added in the three regions so the combination formed a single crystal of semiconductor. Successive addition of impurities in a pure semiconductor crystal is done in a very controlled way.
The middle region is called base in a transistor. Left side region is emitter and right side is collector. Base region is very thin than other region so that charge carries from the emitter travel small distance to reach the collector and while passing through the base too many neutral atoms are not formed by recombining with opposite charges in the base.
Emitter region is wider than base and collector region is much wider than two region as the heat is dissipated by the collector region. The collector doped heavily. A transistor have two junctions made are emitter-base junction and the other one is base-collector junction. One junction is made forward biased and another is reverse biased. The resistance of forward biased junction is very small than reverse biased junction. At low resistance point weak input signal is applied and the output is taken from the high resistance junction in the circuit. Applications of forward bias and reverse is shown in figure for p-n-p and n-p-p transistor. Transistor transfers a signal from a low resistance to high resistance. The prefix ”trans” mean the signal transfer properly of the device while “istor” classifies it as a solid element in the same general family with resistors. Since it transfers current through resistance hence its nomenclature is transfer resistor or in short transistor.
Working principle of transistor
I am discussing working principle of an n-p-n and p-n-p transistor. In upper figure an n-p-n transistor is shown. In the left side the emitter base junction is forward biased. P-region is more positive than n-region. So electrons from n-region can move easily towards the p-region. It is meant that electrons come to the base from the emitter. On the right side collector-base junction is reverse biased so the collector region is more positive. Hence electrons travelling from the emitter towards the base are attracted strongly by the collector i.e. collector collects those electrons. While coming through the base some electrons and holes recombine in the base but about 95% electrons are speeded towards the collector and current flows from base to collector. Base is made very thin and lightly doped so that only a few number of electrons recombine with holes in the base and passes quickly through the base region. For p-n-p transistor the biasing is done in a different way. Holes from the p-region enter into the base region and since collector is more negatively biased these holes speedily move towards the collector. In this way a strong current is produced.
Current flowing in the emitter region due to the flow of charges is called emitter current (IE), current produced for the recombination of electrons and holes is called base current (IB) and the current generated in the collector region due to the flow of charges is called collector current (IC). The base current does not flow in the collector region. This current comes out through the base terminal. So it is seen that total emitter current does not flow to the collector region i.e. the collector current is less than the emitter current. The relation between IE, IB and IC is
IE = IB + IC