GaAs Photoconductive Semiconductor Switch Fabrication for Improved Reliability

Summary form only given. Pulsed power systems designed with many switches or many different switch trigger times are heavily dependent upon the triggering systems. Conceptual pulsed power systems for fusion and isentropic compression are being designed with hundreds to hundreds of thousands of switches. Some have to be triggered simultaneously and some will need to be programmable and staggered in time. Trigger system cost, performance, and lifetime will be driving factors in these systems. For example, designers of repetitive pulsed power systems considered for fusion are seeking switch lifetimes >10⁷ shots. A solution that addresses the performance requirements is the use of optically triggered photoconductive semiconductor switches (PCSS), which excel at delivering low jitter, fast-rise-time pulses. Lifetimes for these devices have been demonstrated to be in the >> 106 shot range at lower currents (tens of amps), but drop to the 10~ shot range when the current per filament is hundreds of amps. To increase the switch reliability at higher current levels necessitates further investigation of device reliability and failure mechanisms. Results from lifetime studies on GaAs PCSS devices indicate that the majority of failures occur near the anode. However, switch fabrication variables can shift this failure mechanism to a short-life, cathode-limited condition. A statistical sample population of PCSS switches in a circuit designed to deliver 500V/100A, 10ns pulses is shown to have a single log-normal distribution of shot life over a range of testing temperatures and voltages. Switches for higher operating voltage (70 kV) were fabricated in multiple split lots to identify the influence of design and processing parameters on the switch lifetime. It was found that these larger-gap 70kV switches exhibit the same anode-limited lifetime as the 500V devices. This paper discuss results from the lifetime studies, including variables in the fabrication proc- ess (annealing temperature, pad-metal thickness, length of contact-metal extension beyond the pad-metal, and inclusion of a silicon nitride passivation/anti-reflection coating) that affect reliability, and provide strategies for improving shot life.