What is Transistor

The device which consists of two p-n junctions is called transistors. These are of two types. Namely p-n-p transistor n-p-n transistor. The three sections of the  are Emitter, Base and Collector.

Emitter

The section at one end of the transistor and is heavily doped is called emitter. It supplies large number of charge carriers for current flow.

Base

The middle part of the transistor which is lightly doped and very thin is called base. It allows most of the charge carriers into collector without getting neutralized.

Collector

The section at the other end of the transistor and is moderately doped is called collector. It is the largest of the three regions and collects charge carriers.

Working of p-n-p transistor

A p-n-p transistor is formed by sandwitching n-type semiconductor between a pair of p-type semiconductors. p-n-p transistor is connected in common base configuration i.e., base terminal is common to both the input and output.

The emitter base junction is forward biased by connecting the emitter to positive terminal battery and base to the negative terminal.

The collector base junction is reverse biased by connecting negative terminal to the collector and positive terminal of battery to base.

The forward bias of emitter base junction causes the holes in the p-type. Emitter flows forward base. This constitute emitter current.

When these holes enter into n-type base, they tend to combine with the electrons. As the base is lightly doped only few holes recombine with electrons. This constitutes base current. The remaining holes enter into the collector region. This constitutes collector current. In p-n-p transistor current conduction is mainly due to the holes, but in the external circuit the current is still by electrons.

Working of n-p-n transistor

The emitter-base junction is forward biased and base collector junction is reverse biased.

The forward bias of emitter-base junction causes the electrons n the n-type emitter to flow towards base. This constitute emitter current. When these electrons enter into p-type base they tend to combine with the holes. As the base is lightly doped, only few electrons recombine with holes. This constitutes base current. The remaining electrons enter into the collector region.

This constitutes collector current. In n-p-n transistor, current conduction with the circuit as well as in the external circuit takes place by electrons.