Semiconductors are those substances whose electrical conductivity lies between conductors and insulators. In terms of energy band, the valance band is almost filled and conduction band is almost empty. In a semiconductor, the outermost orbit will be partially filled. For example, the number of valance electrons is 4 in Germanium and Silicon. A comparatively smaller electric field is required to push the electrons from the valance band to conduction band. At low temperature, the valance band is completely filled and the conduction band is completely empty. Therefore a semiconductor virtually behaves as an insulator at low temperature. However even at room temperature some electrons crossover to the conduction band giving conductivity to the semiconductors. As temperature increases, the number of electrons crossing over to the conduction band increases and hence electrically increases. Hence a semiconductor has negative temperature coefficient of resistance. The types of semiconductor are intrinsic semiconductor, extrinsic semiconductor, N-type semiconductor and P-Type semiconductor. Pure semiconductor is called intrinsic semiconductor. A small amount of impurity is added to intrinsic semiconductor is referred as extrinsic semiconductor. The amount of impurity added is extremely small say 1 to 2 atoms of impurities for 106 intrinsic atoms. A small amount of pentavalent impurities such as arsenic, antimony or phosphorus is added to the pure semiconductor to get N-Type semiconductor. A small amount of trivalent impurities such as aluminum or boron is added into the pure semiconductor to get the P-Type semiconductor.