Simulation Of Vanadium Doped 6H-SiC Photoconductive Switches

As the third-generation wide bandgap semiconductor,silicon carbide(SiC)has the advantages of wide band gap,high electron saturation drift velocity and high thermal conductivity,and therefore it is the preferred material for fabrication of photoconductive semiconductor switches(PCSS).In this thesis,the simulation of vanadium doped semi-insulating 6H-SiC(V: SiC)PCSS by TCAD was carried out.The unintended doped impurities of V:SiC are nitrogen(N)and boron(B).The size of Ohmic contact electrode is 10 mm ?10 mm,and the thickness is 1 mm.After simulation,it is found that the resistivity of V:SiC under dark state is increased by 7-8 orders of magnitude compared to the intrinsic semiconductor.Therefore,the vanadium compensation can effectively increase the resistivity of 6H-SiC and realize its semi-insulating property.Based on the drift-diffusion model,the influence of the N doping concentration on the performance of the PCSS is discussed.Increasing the N concentration,the photocurrent is increased accompanied by serious trailing of current.The photocurrent is 0.26 A/mm when the N concentration is 1.42?1017 cm-3 and the power density of laser pulses is 107 W/cm2.From the viewpoint of flow capacity of the PCSS,the high nitrogen doping concentration is beneficial to improve the photocurrent of PCSS.However,the concentration of nitrogen should be less than that of vanadium when V:SiC is used to fabricate PCSS.Under this conditions,the semi-insulating property of V:SiC is guaranteed and the life-time of the carrier is constant.The PCSS is triggered by the different power densities of the laser pulses.The simulation results show that the rise time of the light current(tr)is much shorter than that of the fall time(td).The theoretical analysis shows that there is a certain relationship between these two time constants,which can be used to calculate the photoionization cross section(si)of the vanadium acceptor impurity.si is in 1016 s order of magnitude.The further investigation also shows that the quantum efficiency of light excitation decreases with the increase of the laser power density.