Electrical conductivity of semiconductor changes appreciably with temperature variations. At absolute zero, it behaves as an insulator. At room temperature, because of thermal energy, some of covalent bonds of the semiconductor break. The breaking of bonds sets those electrons free, which are engaged in the formation of these bonds. This results in few free electrons. These electrons constitute a small current if potential is applied across the semiconductor crystal. This shows the conductivity for intrinsic semiconductor increases with increase in temperature as given by η = Aexp (− E g / 2kt ) where η is the carrier
concentration, Eg is the band gap, T is the temperature and A is constant. In case of extrinsic semiconductors, addition of small amount of impurities produces a large number of charge carriers. This number is so large that the conductivity of an extrinsic semiconductor is many times more than that of an intrinsic semiconductor at room temperature. In n - type semiconductor all the donors have donated their free electrons, at room temperature. The additional thermal energy only serves to increase the thermally generated carriers. This increases the minority carrier concentration. A temperature is reached when number of covalent bonds that are broken is large, so that number of holes is approximately equal to number of electrons. The extrinsic semiconductor then behaves like intrinsic semiconductor.