3C-silicon carbide photoconductive switches

Abstract

Photoconductive semiconductor switches (PCSS) have unique advantages such as high breakdown field, high power, high speed, negligible time jitter and long lifetime. PCSS using conventional semiconductor materials such as silicon (Si) and gallium arsenide (GaAs) have limited application. At first, their breakdown fields are still not very high. For lateral Si and GaAs PCSS operating in linear mode, the breakdown electric field reported was only about 20 kV/cm/sup 2/. Secondly, owing to the thermal runaway effect as well as the lock-on effect, they can only operate in pulsed bias mode instead of continuous bias mode. Compared to Si and GaAs, silicon carbide (SiC), being a wide band gap material (2.2-3.3 eV), has a higher breakdown field (10 times higher than that of GaAs), higher thermal conductivity and higher electron saturated drift velocity. Furthermore, the high Si-C bonding energy of SiC makes it resistant to high temperature and radiation, and also results in high mechanical strength and chemical stability. All these properties of SiC are desired for PCSS devices. Therefore, better performances and more applications are expected for SiC PCSS. We report the results of PCSS devices fabricated on poly cubic SiC (3C-SiC), boron doped and unintentionally doped 3C-SiC materials. They were investigated by using the argon fluoride (ArF) excimer laser. The best results were obtained from the devices made of poly SiC materials. Their dark resistivity was as high as 10/sup 6/ /spl Omega/ cm. The highest breakdown field was at least 2.5/spl times/10/sup 5/ V/cm, which was limited by the surface flashover. The peak photocurrent density through the PCSS, when optically activated, was higher than 5 kA/cm/sup 2/. The ratio of the off-state resistance of the switches, i.e., without light shining on them, to the on-state resistance when illuminated with the pulsed laser of 1 MW (R/sub off//R/sub on/) was /spl sim/10/sup 5/. The lowest on-state resistance reached was 40 ohms. The width of the photocurrent pulse was /spl sim/15 ns, limited by the laser pulse width, indicating that the switches can operate at a frequency at least as high as 67 MHz. Neither lock-on nor thermal runaway effect was observed. The devices could operate in continuous bias mode at a voltage close to the breakdown voltage, which was 250 volts for 10 /spl mu/m gap devices.