Lucky-drift model for impact ionization in amorphous semiconductors

A lucky-drift model for impact ionization has been recently successfully used to account for avalanche phenomenon in amorphous selenium (a-Se). We extend the calculations in order to compare the effect in a-Se with possible impact ionization phenomenon in another prototype amorphous semiconductor: hydrogenated amorphous silicon (a-Si:H). The results suggest that the higher phonon energy in a-Si:H as compared to a-Se shifts the threshold field for impact ionization in a-Si:H to essentially higher fields than those needed for avalanche multiplication in a-Se. Furthermore, it has been recently suggested that impact ionization is a precursor of the switching effect in the phase-change-memory materials (Ge2Sb2Te5). We apply the lucky-drift model to Ge2Sb2Te5 and show that it is capable to account for the magnitude of the electric field necessary to launch the electronic switching in this material.