Subnanosecond high-voltage gallium arsenide photoconductive switches are studied to understand how to improve their switching speed, efficiency, and durability. Two possible mechanisms for such switching are discussed: field-compression-induced ionization of valence states and field-dependent trapping of charge carriers. Analysis and computations suggest that field compression is limited to roughly a factor of two at low initial fields. This shows that one cannot achieve arbitrary amounts of field enhancement and so obtain avalanche-like performance at will. At initial field strengths of 10 to 50 kV/cm, Gunn instabilities produce large field compression, but carrier trapping and recombination quench intrinsic photoavalanching, according to the calculations presented. Observed avalanching may therefore be due to extrinsic effects related to deep levels. As an alternative to intrinsic impact ionization, it is shown that field-dependent trap filling can yield an avalanche-like rise in current and may account for two other experimental observations, the existence of a voltage threshold and a voltage-dependent delay between the start of illumination and the occurrence of switching.
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