The applicability of the transport-energy concept to various disordered materials

It is known that in disordered semiconductors with purely exponential energy distribution of localized band-tail states, as in amorphous semiconductors, all transport phenomena at low temperatures are determined by hopping of electrons in the vicinity of a particular energy level, called the transport energy. We analyse whether such a transport level exists also in materials with densities of localized states (DOSs) different from the purely exponential one. We consider two DOS functions g(epsilon) similar to exp{-(epsilon/epsilon(0))(lambda)} with lambda = 2, typical for polymers, heavily doped semiconductors, and, probably, liquid semiconductors and lambda = 1/2, typical for mixed crystals. It is shown that in both cases the transport energy exists, implying that it also exists for all intermediate forms of the DOS. Special attention is paid to the dependences of the transport level and of its width on the DOS parameters and temperature.