Study On Semi-insulating 4H-SiC PCSS

Photoconductive semiconductor switches(PCSS) are widely used in the pulse power fields. Semi-insulating 4H-Si C is an attractive material for the PCSS due to their wide bandgap, high critical electric field strength and high electron saturation velocity. The domestic research on Si C PCSS starts relatively late, and the researches are most about simulation. However, problems about the process and test of PCSS needs to be solve.This paper introduces the photoelectronic properties of semiconductors, which is the basic theory of the PCSS. Based on the deeply analysis to instrinsic, exstrinsic absorption and electric conductivity, high purity semi-insulating 4H-Si C PCSS were made in lateral structure, the performance of which were studied. The extrinsic excitation is limited by the concentration of deep imperfection levels in high purity semi-insulating Si C material, so the PCSS reported here is triggered by the laser of 355 nm. The result shows that the lateral PCSS of 5 mm electrode gap has a withstand voltage of 11 k V and the on-state resistance is about 17 W.In order to reduce the on-state resistor and increase the lifetime of the electrodes of the PCSS, working with large current, the ohmic contact was designed. The Ni contacts with a thickness of 80 nm deposited on the ion implantation zone simulated by trim, the specific contact resistance of which is 3.82?10-6W×cm2 with the testing TLM structures. The flashover occurred on the surface is the main reason of breakdown for the lateral PCSS. The details of 4H-Si C PCSS process are given, in which the Si O2 were used as the passivation layer to improve the breakdown field. The Si O2 passivation layer was composed of 20 nm by dry oxidation and 1 mm by PECVD. The performance of the PCSS was measured at Shanghai Institute of Ceramics. Overshoot phenomena in the experiment were analyzed and the microstrip line was designed to improve the test board, which has a good impendence match. In the test system, CVR(current viewing resistor) is satisfied with needs of load resistor that should be working normally under impulse voltage and high frequency.Step phenomenon was observed on the rising edge of the photocurrent, which is related to the power of the laser and voltage. It was analyzed by extrinsic excitation and a model of resistor and capacitor in parallel separately. There is no step exits when the extrinsic and intrinsic excitation linear were superimposed directly. The step would be existed when they were superimposed of a FWHM difference on time axis, which goes against the working way of the laser signal. There is a linear relationship between carriers and photon and the variation between on-state resistor and laser keeps consistent, which is Gaussian shape. The concentration of the carriers in PCSS changes rapidly during the illumination time, which leads to the capacitor change. The step is the result of charge and discharge of the changing capacitor, which means that the rapidly change of inner capacitor should be avoid when designing the structure of the PCSS.The future research directions is pointed out based on the current experiment at last. The PCSS would be widely used if it is made of vanadium compensated Si C and excited by the low power laser energy of wavelength of 532 nm. The PCSS, the vertical structure, made of the {1120} crystal orientation would be a good future research direction, which avoid the leakage current due to the microtubules. There is a great significance that the ohmic contact is made of transparent Zn O, which contributes to reducing the size of the PCSS.