Variation of Fermi level with temperature and donor concentration

To start with, the increase in temperature increases the Fermi level as per equation (29). Further increase in temperature ionizes more and more donor atoms. For a particular temperature all the donor atoms are ionized. If the temperature increased beyond this electron hole pairs will be produced due to breaking of covalent bonds and the material tends to behave like an intrinsic manner. The Fermi level gradually moves towards the intrinsic Fermi level Ei as shown in Fig. 8

If we compare the behavior of a n-type semiconductor of higher donor concentration with a lower one, we find that lowering of Fermi level to intrinsic Fermi level with temperature is slow in the case of higher donor concentration than the lower one. The highly doped semiconductor will behave in intrinsic manner only after all the donor atoms are ionized. The behaviour of Fermi level for two different donor concentrations is given in Fig. 8.