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Transistors consist of three terminal semiconductor devices. If p-type region is sandwiched between two n-type regions, it is called an npn transistor. If n-type region is sandwiched between two p-type regions, it is called pnp transistor. The middle region is called the base and the two end regions are called the emitter and the collector.


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Fig. NPN Transistor and its Symbol



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Fig. PNP Transistor and its Symbol

The emitter is heavily doped than the collector region and the base is lightly doped. The emitter is always forward biased, and the collector is always reverse biased. The emitter injects electrons into the base. The base controls the flow of charge carried into the collector.

Applications of Transistors
Transistors are used as
  1. amplifiers,
  2. oscillators and
  3. switching circuits.

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