Why do we assume complete ionization of donor energy level (any doping level) even if we know that when Ef will be closer to it, the probability of electron finding will increase?
Context- DISCUSSION: You may have noticed that as Na increases, E_f rises toward E_d and the probability of nonionization can become quite Targe. In reality, the impurity level broadens into an impurity band that merges with the conduction band in heavily doped semiconductor (i.e., when donors or acceptors are close to one another). This happens for the same reason energy levels broaden into bands when atoms are brought close to one another to form a crystal . The electrons in the impurity band are also in the conduction band Therefore, the assumption of n Na (or complete ionization is reasonable even at very high doping densities. The same holds true in P-type materials.