Bipolar junction transistor - Types of BJT

TYPES OF BJT :

NPN :

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor between two N-doped layers. When there is a positive potential difference measured from the emitter of an NPN transistor to its base as well as positive potential difference measured from the base to the collector, the transistor becomes active. In this "on" state, current flows between the collector and emitter of the transistor.

PNP :

PNP consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base is amplified in the collector output. PNP transistor is "on" when its base is pulled low relative to the emitter.