Investigation Of The Short Circuit Capability Of SiC Power MOSFETs

Semiconductor devices are the basis of the development of power electronics.As an important representative of the third generation of semiconductor,silicon carbide(SiC)features high breakdown voltage,high saturation velocity and high thermal conductivity.SiC devices deliver superior performance in high voltage,high temperature,high-speed switching and high power applications.SiC power MOSFETs are considered as a promising candidate to displace traditional Si IGBTs for their advantages in blocking voltage,on-state resistance,operating temperature and switching losses.The adoption of SiC MOSFETs will greatly improve the total efficiency of power electronics appliance.However,the study of commercially available SiC MOSFETs performance under abnormal situation is still inadequate.At the same time,the interface states of SiC/SiO2 as well as the trench gate structure also lead to uncertainty to the device reliability.Among all the abnormal working conditions,short circuit is considered as the most severe one.When the power MOSFET operates in on-state mode,a short circuit across the load can result in a direct connection of the device to the high bus bar voltage.This challenges the device capability in handling high voltage,high current density as well as high temperature simultaneously.However,regarding the datasheets of commercially available MOSFETs from different corporations,seldom do they mention the ruggedness of device against short circuit operation.Previous work on short-circuit capability of SiC power MOSFETs mainly focus on the withstand capability before a severe destruction occurs,while the degradation of device before the destruction are not investigated,which results in the risk during application.Besides,with the continuous pursuit of high power density and integration level,the trench MOSFET are becoming more and more popular.However,there is no systematic comparison and discussion between the short circuit withstand capability of SiC planar MOSFET and trench MOSFET up till now.Considering the above limitations,a short circuit test board is built.The further study is carried on from following two aspects.First,the short circuit capability of SiC power MOSFETs are investigated through experiments.Based on the comparison of device performance under different test conditions as well as the static characterization of device before and after tests,the whole degradation and failure process of SiC MOSFET during single pulse short circuit tests are systematically investigated.The degradation and failure SOA(safe operating area)are precisely define.It is found that the degradation of SiC MOSFETs during short circuit tests are caused by various physical mechanisms including impact ionization,tunneling effects and hot carrier injection,etc.The failure of device is mainly caused by the short circuit energy limitation.The test results and analysis of repetitive short circuit tests are also provided as a complement to the study of degradation mechanisms.Moreover,the comparison of the differences between commercial SiC trench MOSFETs and SiC planar MOSFETs under short circuit tests are provided in this work.Besides the analysis of thermal effect during short circuit operation,the numerical device simulations are conducted with the aid of Silvaco simulation tool.We found that the high mobility as well as high current density of trench MOSFETs lead to the rapid temperature rise during short circuit tests,limiting its short circuit robustness.It can be inferred from the simulation results that the conduction current of asymmetric trench gate SiC MOSFET is more constrained in the high electric field region,leading to the non-uniform heat generation during short circuit tests,reducing the short circuit withstand capability of device.The test results as well as device simulations are particularly instructive for the optimization of device process and further application.