Dear MANTOSH KUMAR,
A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit.
A transistor works on the principle of a semiconductor. It has two types, PNP and NPN transistor and they both work on the same principle. 
The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water. 

Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier.
 
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Godfrey Classic Prince
IIT-M
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transistor works on simply diode and its characterstics.and some effect like early effect are taken into account.