Semiconductors are a class of materials whose electrical conductivity lies between that of a Conductor and of an insulator. Semiconductor Devices plays pivotal role in electronics. There are a lot of Semiconductor devices viz : P-N Junction diode transistor LED Photo diode FET MOSFET UJT SCR LDR Logic gate amplifier flip - flop tunnel diode PIN diode triacs diacs rectifier etc. Without Semiconductor we can not imagine electronics. When a third doped element is added to a crystal diode in such a way that two P-N Junctions are formed the resulting device is known as a transistor. Semiconductor devices are used in IC as well as digital world. BJT transistor is very practical transistor. Transistor acts as an amplifier.