Transistor is a active electronic component and a semiconductor device. Transistors are made from semiconductor materials and have three or more terminals. In general, NPN and PNP are two types of transistors. P and N refer P-type (holes are majority carriers) and N-type(electrons are majority carriers) semiconductor materials formed from silicon or germanium.

Three terminals of a transistor are Base (b), Emitter (e) and Collector (c). The doping concentration or impurities added in terminal of base is very less and in collector is high. The impurities or doping in emitter is slightly greater than collector. The three regions of Base,Emitter and Collector are separated by junctions. Since the transistor have two junctions, It is known as Bipolar Junction Transistor (BJT).

For the analysis of NPN transistor, it can be thought of two diodes with common anode. As positive voltage is given between emitter-base junction, it forward biases that junction and Reverse bias is applied to collector-base junction.The current flows from collector to emitter.

To make the transistor in conduction state, voltage between base and emitter (VBE) should be in minimum value and that minimum possible voltage is known as cut in voltage. The total transistor current, that is nothing but emitter current, is sum of other two terminals currrent.

IE = IB + IC

The symbol of a NPN transistor :

In PNP transistors,base-collector junction is forward biased and base-emitter junction is reverse biased.Current flow takes place from emitter to collector. Transistors, especially Bipolar Junction Transistor (BJT), depend on both electrons and holes to current flow.

The symbol of a PNP transistor :

A transistor circuit can be connected in three ways. Common base (CB), Common emitter (CE) and Common collector (CC) configurations are three types of connections. Most of the transistor circuits are connected in Common emitter configuration (CE) where ground terminals of power source are connected to emitter.

Transistors are used for various functions like switching and amplifying. In switching, base and emitter voltage is same and current is either off or on depending on base voltage.It is used for high power application like switched mode power supply and low power application like logic gates.