An opposed-contact photoconductive semiconductor switch, with a n+ region next to the cathode electrode has been simulated. Physical conditions during the pulse charging state, prior to high power switching, are analyzed in order to explain the increased hold-off characteristic of such devices. Results show that the introduction of the n+ region near the cathode inhibits the flow of electrons at the n+/semi-insulating interface until very high fields are reached. The formation of trap-filled regions near the contacts and the resultant inhomogeneous device characteristics that lead to breakdown are thereby shifted to higher voltages. Thus, for switches with a n+ region next to the cathode, the breakdown voltage due to unstable filamentary conduction is also increased beyond those achieved previously, allowing for higher power operation. © 1999 American Institute of Physics.