Avalanche Injection and Lock-On in Photoconductive Semiconductor Switches

Optically-triggered, high-power photoconductive semiconductor switches (PCSS´s) composed of semi-insulating GaAs carry current in high carrier-density filaments after the optical trigger is discontinued. This highly conductive mode of operation is called lock-on. The properties of these filaments can be explained by collective impact ionization theory in which energy redistribution by carrier-carrier scattering within the filament enhances the impact ionization rate. This allows these filaments to be sustained by fields which are relatively low compared to the bulk breakdown fields. For GaAs, the sustaining lock-on field is approximately 4.5 kV/cm. For this talk, a hydrodynamic implementation of the collective impact ionization theory is used to compute the temporal evolution of these filaments following optical triggering. These continuum calculations are based on previous calculations in which the steady-state properties of filaments are computed using a Monte Carlo method to solve the Boltzmann equation. The same method will be used to calculated avalanche injection effects in a GaAs avalanche photodiode. The two modes of operation, lock-on and avalanche, will be compared and contrasted. In addition, the effects of carrier recombination at defects will also be discussed.