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If two diodes connected in this way that their same terminals either P or N at a junction point or connected back to back then it is called transistor or, it is a three terminal devise which transfer the resistance signal i.e. Low impedance to high impedance. That's why it is called Transfer-Resistor or Transistor.
Transistor was invented by J.Bardeen and W.H.Barttain and modern transistor was invented by William Shockley.
Construction:
It consist of pure semi-conductor like silicon or germanium with impurities in such a way that either donor type or acceptor type formed at both ends and in centre just opposite to the ends there are two types of transistors :-
1. N.P.N. transistor :
If the transistor is formed in such a way that it's both ends. Have N. Type and at centre P. Type then it is called N.P.N. transistor. Or. P. Type semiconductor is sandwiched between two layers of N. Type semi-conductors. The flow of current is due to electrons as these are majority carriers and holes are being small in number are called minority carriers.
2. P.N.P. transistor:
It the transistor is formed having both ends of P- type and at centre N-type it is called P.N.P. transistor or N-type semi- conductor is sandwiched between layers of P-type semi- conductors. The flow of current if due to movement of holes as these are majority carriers and electrons are minority carriers. Every transistor has three terminals.
Working of a transistor
In normal mode of operation of a transistor, emitter base junction is forward biased and collector base junction is reversed biased.In the above circuit shows the action of n-p-n transistor with base as a common terminal for both input and output. Since the emitter base junction is forward biased, electrons being the majority charge carries in the emitter of the n-p-n transistor move towards the base (p region). If applied voltage VEB overcomes the barrier potential of the emitter base junction, the electron moves to the base region. This constitutes an emitter current IE.
Base:
The portion of transistor which is medium doped and having width of 00025 to0.005mm is called Base. Its function is to control the electrons and the symbols is "B".
Emitter:
The portion of transistor which is highly doped is called emitter and its function is to emit the electrons. Its symbol is "E".
Collector:
The portion of transistor which is lightly doped and collects the electrons called collector .Its symbol is "C".
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