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High power photoconductive switch (PCSS) is an attractive device in pulse power systems. For the wide bandgap (3.26 eV), high critical field strength (3 MV/cm), and high thermal conductivity [4.9 W/(cm∙K)], semi-insulating 4H-SiC was considered an ideal material for PCSS. The on-state switch resistance should be as lower as better for high power output, but only about 1 Ohm on-state resistance was achieved so far, which may due to the low carrier mobility of SiC. So, it is important to check the limit of SiC material as substrate for PCSSs. This work provided a concise method for measurement of transient resistance of semi-insulating SiC under laser pulse.
The vertical PCSS with a transparent Al doped ZnO anode, semi-insulating 4H-SiC substrate, and high reflection silver cathode was introduced. A circular aperture was used to change the area of laser illuminating to the transparent anode. Under a 2kV bias and the 532nm laser with power density of 18.2 MW/cm^2, the on-sate resistance of the PCSS using different area of aperture was tested. The on-sate resistance was made of the resistance of SiC under laser excitation (Ron_SiC), and the inductive impedance (RL) in the switch and test circuit. The expression is given as follows, Ron_test = Ron_SiC + RL = ρH/S+ RL [Formula (1)], where ρ is the resistivity of SiC under laser excitation, H is the thickness of SiC substrate, S is the area of the aperture.
The measurement data and the fitting result by Formula (1) show a good linear relationship between Ron-test and 1/S. The resistivity of SiC excitated by 18.2 MW/cm^2 laser was 0.96 Ω∙cm by linear fitting, result in a resistance of 0.34 Ω for 1mm thick SiC under a circular aperture with diameter of 6mm. So, sub-Ohm on-state resistance is possible for SiC PCSS.
