Impact ionization of deep traps in semi-insulating GaAs substrates

The behavior of the leakage current in the semi-insulating substrate of a GaAs device is more complicated than previously recognized. The voltage dependence of this current seen in a conventional voltage-controlled experiment has hysteresis, which arises from an S-type negative differential conductivity (S-NDC). This is incompatible with the conventional trap-fill-limited model, and we propose an alternative explanation based on the impact ionization of deep-level traps. We show how this simple model can account qualitatively for the S-NDC and the associated current instability, and how it can be extended when the deep traps are photoexcitable.