Characteristics of impact ionization rates in direct and indirect gap semiconductors

Abstract

Impact ionization rates were analyzed for the direct gap (DG) GaAs and In0.53Ga0.47As and the indirect gap (IG) Si0.5Ge0.5. In SiGe, the constant matrix element approximation agreed with the full expression for the matrix element accurately at all energies, whereas softer rates were predicted for DGs. This difference is explained in terms of the mean momentum transfer occurring in transitions. The role of the Γ valley in influencing the rate behavior in the DGs was greater than its small density of states would suggest. While threshold anisotropy acted to soften the thresholds in all materials, the effect was small and dependent on the variation in the rate increase of availability of final states as the impacting carrier energy increases above threshold.