AVALANCHE PHENOMENON IN AMORPHOUS SELENIUM

Avalanche multiplication characteristics in amorphous Se (a-Se) are in vestigated by using sandwich-type photocells with an a-Se layer thickn ess ranging from 0.5 mum to 4 mum. The impact ionization rates are der ived from the multiplication factors, and empirical formulas for the e lectric field dependences of the impact ionization rates for holes and electrons have been obtained. The effective quantum efficiencies are calculated with the impact ionization rates and quantum efficiencies, which are evaluated by taking geminate recombination into account. Mea sured electric field dependences of the effective quantum efficiency f or wavelengths of illuminated light ranging from 400 nm to 600 nm are well fitted by these calculated values. The impact ionization rates fo r holes and electrons have an exponential dependence on the reciprocal electric field, as is the case for a variety of crystalline materials . The observed positive temperature dependence of the multiplication f actor, however, is opposite to that in crystalline avalanche photodiod es. This indicates that there are characteristic temperature-dependent factors in the impact ionization process in a-Se.