BC140, NPN transistor

An NPN transistor is a bipolar junction transistor (BJT). It is operated with an electric current and is used for switching and amplifying signals without using mechanically moving parts.

There are two types of BJTs: NPN and PNP. The letters indicate the order and doping type of the layers. Essentially, a BJT constitutes two p-n junctions connected together in series (like that of a p-n diode.) The three terminals are called collector (C), base (B) and emitter (E).