Bipolar transistors are mainly formed of two layers of semiconductor material of the opposite type, connected back to back. The type of impurity added to silicon or germanium decides the polarity when it is formed.

NPN Transistor

An NPN transistor is composed of two N type material separated by a thin layer of P type semiconductor material. The crystal structure and schematic symbol of the NPN transistor are shown in the above figure.

There are three leads taken out from each type of material recognized as the emitter, base, and collector. In the symbol, when the arrowhead of the emitter is directed outwards from the base, it indicates that the device is of the NPN type.

PNP Transistor

A PNP transistor is composed of two P type material separated by a thin layer of N type semiconductor material. The crystal structure and schematic symbol of a PNP transistor is shown below.

In the symbol, when the arrowhead of the emitter is directed inwards towards the base, it indicates that the device is of the PNP type.