Semiconductor is that class of materials which have their energy band gap in between conductors and insulators, typically ranging from 0.7 to 3.6 eV. Electrical conductivity varies in the range of 103 to 10-8 seimens per cm. Semiconductor conductivity can be readily modified by introducing impurities into the crystal lattice and this particular feature makes them most useful in electronic devices. Semiconductors in their native form without impurities are referred to as ‘Intrinsic’ and while the ones with added impurities are called ‘Extrinsic’ semiconductors. Silicon and Germanium elemental semiconductors, while Gallium Arsenide is a very important compound semiconductors.