Class 12 Physics Semiconductor Electronics Junction Transistor

Junction Transistor

A junction transistor is a semiconductor device having two junctions and three terminals.   The transistor has three doped regions forming two p-n junctions.

There are two types of transistors, namely, (a) n-p-n transistor (b) p-n-p transistor. The schematic representation and symbols of the two transistors are given below:

  • Emitter and collector can be differentiated depending upon whether the arrow is present or not.
  • Emitter has an arrow either pointing inwards or outwards
  • The arrow direction for npn transistor is indicated outwards in the emitter

  • The arrow head indicates the direction of the conventional current in the transistor
  • The arrow direction for pnp transistor is indicated inwards in the emitter
  • The p-n-p transistor is obtained by growing a thin layer of n-type semiconductor in between two relatively thick layers of p type semiconductor
  • The n-p-n transistor is obtained by growing a thin layer of p-type semiconductor in between two relatively thick layers of n type semiconductor
  • The 3 layers are – Emitter, Base and Collector
    • Emitter – It is the left hand side thick layer of transistor which is heavily doped. This supplies a large number of majority carriers for the current flow through the transistor
    • Base – It is the thin, central segment which is lightly doped
    • Collector –It is the right hand side thick layer of transistor which is moderately doped. This segment collects a major portion of the majority carriers supplied by the emitter
  • Depletion region –The depletion regions are formed at the emitter-base junction and the base-collector junction

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