Structure Design And Characteristics Of High Voltage Silicon Carbide MOSFET Transistors

With the material properties of wide band gap semiconductor and the conduction mechanism of unipolar transport,SiC MOSFET has lower switching loss,smaller chip area,and higher operating temperature compared with Si IGBT of the same blocking voltage.With the above advantages,SiC MOSFET can reduce the volume of passive devices and weight of cooling modules in the system,reduce the overall size and cost of the system,realize the high efficiency,miniaturization and lightweight of the system.SiC MOSFET has been gradually applied to the new energy fields represented by electric vehicles,rail transit and wind power generation.However,compared with Si IGBT,SiC MOSFET suffer more concentrated electric and thermal effects during short-circuit due to the larger short-circuit peak current and smaller chip area,resulting in the lower short-circuit reliability,and the existing short-circuit reliability studies have not considered the switching stress of SiC MOSFET under actual working situation,and the influence of switching stress on short-circuit degradation behavior cannot be studied.Therefore,a SiC MOSFET with improved short-circuit performance and a repeated short-circuit testing method of SiC MOSFET with superimposed switching stress are proposed in this thesis,in order to extend the short-circuit withstand time of SiC MOSFET and improve the short-circuit reliability evaluation method of SiC MOSFET.1,a novel SiC fin-shaped MOSFET with improved short-circuit performanceIn order to improve the short-circuit capability of SiC MOSFET,this thesis proposes a 1200V SiC fin-shaped gate MOSFET with embedded dual P well（SiC DP-MOS）.By improving the channel length modulation effect and narrowing the current path,SiC DP-MOS reduces the saturation current and extends the short-circuit withstand time without increased ON-resistance.In addition,due to the the second P well shields the lower part of trench sidewall,the gate to drain capacitance C_GD,gate to drain charge Q_GD of SiC DP-MOS,and the switching loss is reduced.Compared to SiC shielded fin-shaped gate MOSFET（SiC SF-MOS）,SiC DP-MOS reduced the saturation current by 34%,reduced the C_GD by 19 times,reduced the Q_GD by 78%,reduced the switching loss by 13%,improved the device figure of merit（HD-FOM）by 2.7 times,and extended the short-circuit withstand time at 800V bus voltage by 50%.SiC DP-MOS provides a feasible scheme for short-circuit reinforcement of SiC MOSFET.2,a SiC MOSFET repetitive short-circuit test method considering switching stressIn order to solve the problem of the existing studies have not considered the impact of switching stress on short-circuit reliability of SiC MOSFET,this thesis proposes a repetitive short-circuit superimposed BTI reliability experiment to simulate the impact of switching stress on short-circuit reliability.The experimental results show that the threshold voltage of SiC MOSFET shift in the positive direction and negative direction under repetitive short-circuit condition with and without switching stress,respectively.Through the experimental results and TCAD simulation,this work proves that the reason for the change of threshold voltage shift direction of SiC MOSFET is that the switching（i.e.,BTI）stress causes an increase in the effective barrier height of electrons and a decrease in the effective barrier height of holes in gate oxide.Further,the change of potential barrier height results in the strength of two degradation mechanisms:（1）a positive shift of the threshold voltage caused by electron injection into the oxide layer at the corner of the gate trench in the short-circuit state,and（2）a negative shift of the threshold voltage caused by hole injection into the oxide layer at the corner of the gate trench and the oxide layer near the channel after the short-circuit state is changed,mechanism（1）is suppressed and mechanism（2）is enhanced,resulting in a change in the direction of the threshold voltage shift.The repetitive short-circuit superimposed BTI reliability experiment not only provides a short-circuit reliability test method which is closer to the actual situation,but also reveals the influence of switching operation on the short-circuit degradation behavior and mechanism of SiC MOSFET for the first time,deepening the understanding of the short-circuit reliability of device and provides the basis for reinforcement research for short-circuit reliability.