Simulation results depicting physical conditions in a photoconductive semiconductor switch in the pulse charging state, prior to high power switching, are analyzed. Results show that surface conditions and EL2 traps in semi-insulating GaAs influence the conduction process, specifically at high bias. Formation of trap-filled regions renders the device inhomogeneous for stable conduction and premature breakdown occurs, due to a large extent on unstable current filamentation within the device. Unlike insulators, the breakdown of desorbed gas from the surface (surface flashover) does not contribute to premature breakdown. ©1998 American Institute of Physics.