Impact ionization rate calculations in wide band gap semiconductors.

An algorithm for calculating impact ionization rates in the semiclassical Fermi’s Golden Rule approximation which is efficient close to threshold is presented. Electron and hole initiated rates are calculated for three semiconductors with particular band structure characteristics, as are the distributions of the generated carriers. Simple analytic expressions of the form R = A(E−E0)P are fitted to the calculated rates. The role of the matrix elements in influencing the distribution of final states is investigated. In the direct gap materials, they act to significantly enhance the low-q transitions, while in the indirect gap case they have a lesser effect on the distribution. Results for GaAs obtained here and by several other workers are compared and possible causes of the discrepancies examined, including differences in band structure and approximations made in evaluation of the matrix element. It is found that these differences do not influence the rate sufficiently to account for the wider variation between authors, and so it is concluded that differences in the implementation of the rate integration algorithm are the main cause