Impact ionization rate in GaAs

The impact ionization rate and its orientation dependence in k space is calculated for GaAs. The numerical results indicate a strong correlation to the band structure. The use of a q-dependent screening function for the Coulomb interaction between conduction and valence electrons is found to be essential. Comparison with recent results for Si, GaAs shows a harder threshold behavior. A simple fit formula is presented for easy calculation of the direction-dependent transition rate. With only the band structure as input the numerical results are closely reproduced in an arbitrary direction in the Brillouin zone.