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Transistor

The name of this electronic device is derived from its fundamental action transfer resistor.
  • Transistor does not need any heater or hot filament; transistor is small in size and light in weight.
  • Transistor in general is known as bipolar junction transistor.
  • Transistor is a current-operated device.
  • It consists of three main regions:
    • Emitter (E): It provides majority charge carriers by which current flows in the transistor. Therefore the emitter semiconductor is heavily doped.
    • Base (B): The based region is lightly doped and thin.
    • Collector (C): The size of collector region is larger than the two other regions.
Junction transistors are of two types:

NPN transistor

It is formed by sandwiching a thin layer of P-type semiconductor between two N-type semiconductors (Fig. 23).
 
108644.png
Fig. 23
 
In NPN transistor-electrons are majority charge carriers and flow from emitter to base.

PNP transistor

It is formed by sandwiching a thin layer of N-type semiconductor between two P-type semiconductor (Fig. 24).
 
108653.png
Fig. 24
 
In PNP transistor, holes are majority charge carriers and flow from emitter to base.
 
In the symbols of both NPN and PNP transistors, arrow indicates the direction of conventional current.




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