NPN transistors are bipolar junction transistors. The functions of any bipolar transistor, either NPN or PNP type, are to amplify the electric signals or to switch circuits digitally on or off.
An NPN transistor is composed of semiconductor material with three terminals for connecting with an external circuit. The three terminals are named the collector, the base and the emitter. The collector terminal is normally connected to a positive electric supply and the emitter to a negative electric supply. Both NPN and PNP transistors are the same in their function but have opposite current flows. For any given state of operation, the voltage polarities and current directions for each kind of transistor are exactly opposite to each other. In an NPN transistor, current flows from the collector to the emitter, and in PNP transistor, current flows from the emitter to the collector. NPN transistor is the mostly used type transistor because of its high ‘electron’ mobility compared to the ‘hole’ mobility in semiconductors. As a result, an NPN transistor operates faster and allows greater amount of current. Moreover, NPN transistors are easy to build from silicon. Transistors are very common in modern electronic system and considered as a fundamental building block of any circuit.