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