Transistor is an active electronic component composed of semiconductor material with three terminals that can function as electronic switches or as signal amplifiers.
Modern transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Application of current in BJTs and voltage in FETs between the input and common terminals increases the conductivity between the common and output terminals, there by controlling current flow between them. The transistor characteristics depend on their type.
Transistors are made from the n-type and p-type semiconductor materials as diodes and employ the same principles. Transistors have two PN junctions instead of just one like a diode has. The two PN junctions allow a transistor to perform more functions than a diode, such as acting as a witch or an amplifier.
The bipolar transistor is made up of three parts: the emitter, the base and the collector. There are two types of bipolar transistors: the PNP transistor and the NPN transistor.
In the PNP transistor the emitter made from p-type semiconductor material, the base is made from n-type semiconductor material and the collector is p-type semiconductor material. For the PNP transistor to operate, the emitter must connect to positive, the base to negative and the collector to negative.
In the NPN transistor the emitter made from n-type semiconductor material, the base is made from p-type semiconductor material and the collector is n-type semiconductor material. For the NPN transistor to operate, the emitter must connect to negative, the base to positive and the collector to positive.