Research On Ruggedness And Reliability Of Silicon Carbide MOSFET

In recent years,with the rapid development of emerging applications such as new energy power generation,electric vehicles,communication power supplies,and smart grid applications,more stringent requirements are proposed for the efficiency and application environment of power electronics equipment.Wide bandgap(WBG)semiconductor devices represented by SiC MOSFETs break through the material performance limitations of traditional Si devices.SiC MOSFETs have hightemperature operation capability,high switching frequency,low on-state resistance,which have a wide application requirement to meet the demand for power devices in new-generation power electronics applications.However,the reliability of SiC MOSFETs has not yet been fully solved,which restricts its large-scale application.Especially in some critical power electronics applications with high reliability requirements,such as rail transportation,electric vehicles,and aerospace,the stable and reliable operation of power electronics devices is crucial.Once the power device fails,it may cause serious safety problems.In addition,the degradation of power devices may affect the normal operation of converters and increase the maintenance cost of power electronics applications,resulting in huge economic losses.There is an urgent need to investigate the ruggedness and reliability of SiC MOSFETs and reveal its failure mechanism and degradation mechanism.Moreover,it is necessary to explore methods to enhance the device's ruggedness and laying the theoretical foundation for the application of SiC MOSFETs in critical power electronics applications.In this thesis,the ruggedness and reliability of SiC MOSFETs are investigated.The main contents of this paper include the following aspects: First,the ruggedness and failure mechanism of SiC MOSFETs under extreme operating conditions with high current shocks when operating in the first quadrant and third quadrant are investigated,respectively.Second,the degradation mechanism of SiC MOSFETs under power cycling stress and compound stress conditions are investigated,respectively.Finally,an online junction temperature measurement method based on dynamic threshold voltage is proposed.(1)The short-circuit ruggedness of SiC MOSFETs and its failure mechanism are investigated.The effects of gate drive voltage,DC bus voltage,and case temperature on the SiC MOSFET short-circuit ruggedness and short-circuit failure modes are experimentally analyzed.Two main short circuit failure modes of SiC MOSFET are found.A TCAD electro-thermal simulation model of the SiC MOSFET is developed to analyze the internal temperature distribution of SiC MOSFET's cell under short-circuit conditions.The mechanism of gate leakage current and drain leakage current of SiC MOSFET under short circuit stress are theoretically analyzed,and the failure mechanism of gate interlayer dielectric layer breakdown failure under short-circuit stress is revealed by means of physical analysis.Finally,a method to enhance the ruggedness of the gate dielectric layer of SiC MOSFETs under short-circuit is proposed.(2)The surge current ruggedness and failure mechanism of SiC MOSFET's body diode are investigated.The surge current ruggedness of SiC MOSFET's body diode,SiC junction barrier Schottky diode,and SiC JMOS's body diode are experimentally analyzed and compared.Moreover,the influence of gate turn-off voltage on surge current ruggedness of SiC MOSFET's body diode are investigated.The internal junction temperature distribution of SiC MOSFET's cells under surge current stress is analyzed,and the failure mechanism of the three types of SiC diode under surge current stress is compared.A method to enhance the surge ruggedness of SiC MOSFET body diodes is proposed.(3)The reliability of SiC MOSFET's body diode under repetitive surge current stress and its degradation mechanism are revealed.The influence of surge current amplitudes,surge current shock numbers,and gate turn-off voltage on the degradation of SiC MOSFET are analyzed.The electric field stress on the gate oxide of SiC MOSFETs under surge current stress is simulated and analyzed,and the effects of gate turn-off voltage and surge current on the degradation of SiC MOSFETs are investigated.A method to suppress the gate oxide degradation of SiC MOSFETs under repetitive surge current stress is proposed.(4)An online junction temperature monitoring method based on the dynamic threshold voltage of SiC MOSFETs is proposed.An analytical model of the dynamic threshold voltage is established,and the influence of the gate loop parasitic inductance,gate drive resistance,and power loop parasitic inductance on the temperature sensitivity of the dynamic threshold voltage are investigated.A dynamic threshold voltage monitoring circuit is designed,and the effectiveness of the proposed dynamic threshold voltage monitoring method is verified by double-pulse experiments.(5)The reliability of SiC MOSFETs under compound stress conditions and its degradation mechanism are revealed.Considering that the conventional aging tests are predominantly performed in a simplified and controlled testing condition,which might not fully reveal the degradation of SiC MOSFETs under actual stress conditions.In this paper,the PFC circuit is selected as a typical application scenario,and the effects of temperature and electric field stress on the degradation of SiC MOSFET are considered.The degradation mechanism of dynamic electrical parameters and static electrical parameters under compound stress is revealed.The relationship between the gate oxide degradation of the SiC MOSFET and the internal electric field stress of the device under high voltage switching stress is analyzed through simulation.The hole capture phenomenon within gate oxide degradation is verified.Finally,the differences in the degradation mechanism of SiC MOSFETs under power cycling stress and under compound stress conditions are discussed.