ELECTRONIC-PROPERTIES IN DISORDERED SOLID S .3. HOPPING TRANSPORT MECHANISMS ASSOCIATED TO PAIR MODELS AND POLARON CONDUCTION

In two previous papers, we presented the general band scheme for amorp hous semiconductors and the related transport phenomena. With an alter native way, two hopping models can be used in the case of high disorde r: the pair model linked to hopping over the barrier of potential and the polaronic model. The first model leads to a frequency dependence s igma(omega) proportional to omega(s), where s is temperature dependent , contrary to the Austin-Mott law (tunnelling effect). The second mode l is independent of the band scheme and takes into account the carrier -lattice coupling: when the distortion tends to lower the energy of th e carrier, a polaron is formed; in this case, we will describe the evo lution of the d.c. conductivity, of the thermopower and of the alterna tive conductivity with temperature.