RECOVERY OF HIGH-FIELD GAAS PHOTOCONDUCTIVE SEMICONDUCTOR SWITCHES

GaAs photoconductive semiconductor switches (PCSS) are useful as fast recovery switches at electric field stresses below 3.5-8 kV/cm (a threshold field which depends upon the specific type of GaAs). Following a short activating light pulse in this low-field regime, the switch resistance increases roughly exponentially with time constants ranging from 0.1 to 1000 ns, depending on the type of GaAs. At higher fields, however, the resistive recovery depends not only on material properties, but also on the minimum resistance reached during the activating light pulse (trigger resistance), the wavelength of the light pulse, and the field remaining across the switch during recovery. In particular, if the switch is activated with a significantly intense light pulse, it will stay on and maintain the circuit-dependent impedance which is required to hold a constant, material-dependent field across the switch (lock-on). Since the circuit-dependent impedance achieved during lock-on can be several orders of magnitude less that the resistance required for triggering, a substantial savings can be made in the optical energy required to activate the switch. The lock-on switching mode shows gain relative to linear (low-field) PCSS, but recovery is inhibited as long as high-field stresses remain across the switch. Fast recovery of GaAs switches after high-field switching is of particular interest for high repetition rate applications where it is difficult to provide the large optical trigger energy required for switches operating at low fields. We have explored three categories of circuits to induce fast recovery after lock-on by temporarily reducing the field across the switch. Measurements of recovery times from 35 to 80 ns, multiple monopolar and bipolar bursts at 5-40 MHz, and hold-off fields ranging from 5 to 44 kV/cm (corresponding to 15-66 kV across individual switches), will be presented. A comparison will be made of the different circuits used to induce recovery from lock-on and the various factors that influence the recovery will be discussed.