Transistor is a semiconductor device used to amplify or switch electronic signals. Transistors are broadly divided into three types: bipolar transistors (bipolar junction transistors: BJTs), field-effect transistors (FETs), and insulated-gate bipolar transistors (IGBTs).
Transistors are one of the basic building blocks of modern electronics. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit.
Their are two types of transistor.
The main difference between the NPN and PNP transistor is, an NPN transistor turns on when the current flows through the base of the transistor. A PNP transistor turns ON, when there is no current at the base of the transistor. In this transistor, the current flows from the emitter (E) to the collector (C).