Ordinarily, junction transistor is a three terminals Semiconductor device. A transistor is formed when a thin layer of one type of the intrinsic semiconductor is sandwiched between two thick layers of other types of extrinsic semiconductors.
1. When a thin layer of p-type semiconductor is sandwiched between two thick layers of n-type semiconductor we get a junction transistor which is known as NPN transistor.
Symbolic representation of NPN transistor is shown in the figure.
2. When a thin layer of n-type semiconductor is sandwiched between two thick layers of p-type semiconductors, we get a junction transistor which is known as PNP transistor.
Symbolic representation of PNP transistor is shown in the figure.
Meaning of Junction transistor
A transistor has two junctions namely emitter-base junction and collector-base junction. The former is forward biased and the later is Reverse Biased. Forward biased junction has low resistance while reverse Biased Junction has high resistance.
A signal is fed to the low resistance circuit and the output is taken from the high resistance circuit. Thus transistor transfers a signal from low to high resistance.
Therefore, Transistor = Transfer + Resistor
The terminology used in transistor
The thin layer or middle part of the transistor is known as a base (B) which is sandwiched by emitter (E) and collector (C).
The emitter is a reason left to the base which supplies the change carriers I.e Electrons or holes. The emitter is always forward biased w.r.t the base so that it may supply a large number of majority carriers through the emitter-base junction.
The middle region which forms two PN junctions between ammeter and collector is called a base. The base is lightly doped and very thin so that the majority carriers may pass easily through the Base collector junction. The base-emitter junction is forward biased to have low resistance for the emitter circuit. The base-collector junction is reverse biased to have high resistance in the collector circuit.
The collector is a region right to the base which receives or collects the change Carriers (electrons or holes) from the base. The collector is always reverse Biased with respect to the base. The collector is larger than that of emitter and base.
The junction between emitter and base is called emitter-base junction or emitter junction while the junction between the collector and the base is called a collector-base junction or collector Junction.
In a junction transistor, the current is carried by two types of charge carriers that are holes and electrons, and hence it is known as bipolar junction transistor (BJT).