Electron irradiation effects on photoconductive semiconductor switches (PCSSs) used in sub-nanosecond transient generators
Radiation-induced damage occurs in GaAs photoconductive semiconductor switches used in sub-nanosecond transient generators when subjected to 600 keV and 6 MeV electron irradiation. These switches are made from semi-insulating (SI) compensated material through a EL2/carbon compensation mechanism, and the liquid encapsulated Czochralski process. New defect levels are formed as a result of the non-ionizing energy loss (NIEL) process. The formation of new defect levels in the device alters the compensating balance between the existing deep level EL2 trap/donors and carbon acceptors, and changes the material properties. As a result, two important parameters of the device are adversely affected-the hold-off voltage of the switch at the pulse-charging (off) state, and the rise time during the conduction (on) state. The hold-off voltage shifts to a lower value since there are more trap-filled regions available that can fill up and alter the homogenous nature of the device material. Unstable filamentary conduction then occurs at a lower voltage and leads to premature breakdown. As with EL2 trap levels, new defect states induced by electron irradiation will further contribute to the delay in the rise time of the switch. The rise time determines the maximum energy transferred to the load. The electron damage mechanism and its effects on the switch characteristics depend on the material properties. Intrinsic material, or material made through compensation other than through the deep donor and shallow acceptor balancing process are not expected to behave similarly. Simulation results at higher bias show a marked degradation of material properties. The switch current-voltage (I-V) characteristic when the bias increases to the kilovolt range is similar to trap-dominated semiconductors. An initial sublinear current regime at low bias is followed by a super-linear regime of current flow at higher bias, and is in agreement with earlier observations.