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Pictures are presented of the time evolution of current filaments during optically triggered, high gain switching in GaAs. Two filaments are triggered with two laser diode arrays and the time delay between them is varied. When the filament that is triggered first crosses the switch the voltage drops and the other filament ceases to grow. By varying the delay between the lasers, the tip velocity is measured to be 2±1×109 cm/s, 100 times larger than the peak drift velocity of carriers in GaAs. This observation supports switching models that rely on carrier generation at the tip of the filament.

Photoconductive semiconductor switches (PCSS's) are being developed at SNL for pulsed power applications that cannot be built with traditional high-power switching technology. For a variety of projects, we have constructed silicon or GaAs switches which have switched (not all at one time) over 120 kV and 4 kA, demonstrated less than 270-ps rise times, contributed a resistance of less than 1 R and a stray inductance of less than 10 nH, produced a burst of 20 bipolar pulses at 20 MHz, been triggered in a high-gain mode (called lock-on) with less than 100 FJ using laser diode arrays operated at 10 Hz, and demonstrated recovery from lock-on in 35 ns. This paper describes the highlights of this research effort and our approach in applying photoconductivity to pulse power switching.

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The longevity of high gain GaAs photoconductive semiconductor switches (PCSS) has been extended to well over 10 million pulses by reducing the density of carriers at the semiconductp to metal interface. This was achieved by reducing the density in the vertical and lateral directions. The first was achieved by varying the spatial distribution of the trigger light thereby widening the current filaments that are characteristic of the high gain switches. We reduced the carrier density in the vertical direction by using ion implantation. These results were obtained for currents of about 10 A, current duration of 3.5 ns, and switched voltage of -2 kV. At currents of -70 A, the switches last for 0.6 million pulses. In order to improve the performance at high currents new processes such as deep diffusion and epitaxial growth of contacts are being pursued. To guide this effort we measured a carrier density of 6 x 10 l8 electrons (or holes)/ cm in filaments that carry a current of 5 A.