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TRANSISTOR: The transistor is a three terminal electronic device formed by the combination of two PN junctions in a specific manner. It was developed by Dr. shockley along with Bell laboratories in 1951. It is a main building block of modern electronic systems. In communication system it is used as the primary component in the amplifier. In digital electronics it is used as a high speed electronic switch. Basically transistor is a crystal with three doped regions. They are called emitter, base and collector. Extreme regions are emitter and collector, the middle region is base. All the three regions are provided with terminals which are known as E for emitter, B for base and C for collector.  These all three terms are discussed below:

Emitter: it is a region situated in one side of transistor. It is heavily doped region. It is thinner than collector but much thicker than base. Its main function is to supply majority charge carriers to other regions. In circuit it is denoted by E ore.

Base: it is middle region. It is lightly dopped region. Its main function is to pass the majority charge carriers injected from the emitter on the collector. The base is made thin and its doping level is light to minimize the recombination of electrons and holes. In circuit it is denoted by B or b.

Collector: it is situated on the other side of transistor. It is thickest region. It is moderately dropped region. Its main function is to collect the majority charge carrier supplied by emitter and passed by the base. Collector is made thick to dissipate the heat produced in the collector circuit. In circuit it is denoted by C.

On the basis of what type of semiconductor material from emitter base and collector of transistor there are two types of transistors. They are 1) NPN transistor and 2) PNP transistor.

NPN transistor: NPN transistor is composed of two N regions i.e. emitter and collector of NPN transistor are N type and base is P type semiconductor.

PNP transistor: PNP transistor is composed of two P regions separated by N region i.e. emitter and collector of PNP transistor are P type and base is n type semiconductor.