A transistor has a 3 sandwich layers of doped semiconductors. Basically Transistors are 2 types. 1 . NPN Transistor 2\. PNP Transistor Transistor is the very important components in electronics system. This is the basic electronic component that uses vastly in the making  of electronic circuit. Mainly transistors are used for the switching purpose. another uses of transistor is amplifier. Transistor is made by using of 2 pn junction diode .                       [For more circuit diagram click on ](http://www.circuitspedia.com/) NPN= negative -positive-negative.  [For more Details click here](http://www.circuitspedia.com/transistor-as-switch-working-how-transistor-works/)