Influence of Pinch Effect on the Lifetime of a 2MW Silicon Carbide Photoconductive Semiconductor Switch

The lifetime of an opposed-contact vanadium-compensation semi-insulating 4H-silicon carbide (VCSI 4H-SiC) photoconductive semiconductor switch (PCSS) was investigated. The experimental setup involved a laser wavelength of 532 nm, a bias voltage of 19 kV, and a maximum photocurrent of 200 A. After 678 high power switching cycles, the 4H-SiC PCSS was fragmented along two crystal planes. The simulation of electric fields near the electrodes is on the order of the theoretical critical breakdown field strength of SiC. Due to the distribution of the electric field, the ionization rate at the edge of the electrodes is comparatively high. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) were used to analyze the cross-sectional morphology and elemental characteristics of the fragments. It was found that there was only carbon phase but no silicon phase near the electrodes. The simulation results show that the maximum temperature of the device is 1865 K, which is lower than the gas phase temperature of silicon atoms in silicon carbide. During the pinch process, the carrier density increases due to the shrinking of the current channel radius, which increases the temperature of the device. When the current density is $2.73\times {10}^{{5}}\, \text {A}/\text {cm}^{{2}}$ , the device temperature increment can reach 2546 K. Furthermore, the corresponding parameters are ${r}/{r}_{{0}}$ = 0.86 and ${t}/{t}_{{0}}$ = 0.94 in the pinch effect process. The difference in thermal expansion coefficient and elastic modulus between the metal electrode and SiC substrate, as well as the difference in temperature between materials, lead to the greatest thermal stress near the electrodes, which is confirmed by the cracks at the edge of electrodes.