Research On Reverse Conduction Reliability Of P-GaN Power Devices

In recent ten years,GaN HEMTs have expanded in power conversion applications with the maturation of enhanced device structure and silicon substrate heteroepitaxy technology,as well as increased reliability requirements.In the application of synchronous Buck converter,the power device as a switch device not only needs the forward conduction to the load to transfer energy,but also needs the reverse conduction to sustain the inductor energy.Moreover,the threshold voltage instability of the Schottky P-GaN HEMT,as the main commercial device,has been one of the focus of reliability problems.The variation of threshold voltage will directly affect the dead time power loss in the process of reverse conduction.However,the existing researches mainly focus on the forward conduction mode,and the variation of the device characteristics during reverse conduction mode are less studied.Therefore,based on the Schottky P-GaN HEMT,this thesis will study the stability of the reverse conduction characteristics after off-state stress and long-time switch stress respectively,and analyze the influence of threshold voltage variation on the device power loss.The main contents are as follows:(1)The influence of drain off-state stress on device characteristics is studied.Through the quasi-static test,it is found that off-state stress will lead to the increase of the reverse threshold voltage and transconductance.Based on the variation law,combined with the distribution of device capacitance and the TCAD simulation of electric field distribution,it is proposed that two mechanisms,charge storage effect and trap effect,lead to the variation of threshold voltage.When the off voltage is low(VDS<50V),the charge storage effect is dominant.When the off voltage is high(VDS>50V),the collision ionization probability is increased,and the hole trap effect is more significant.The recovery characteristics of threshold voltage also indicate the existence of two mechanisms,in which the recovery speed of charge storage effect is faster than trap effect.(2)The influence of long-time switch stress on the device characteristics of deadtime conduction mode(VG=0V)and reverse conduction mode(VG=5V)is studied respectively.The deadtime conduction mode switch stress test reveals the process of the threshold voltage gradually increasing to stability over time,that is,the process of negative gatecharge accumulation.Through the comparison of the experimental result,it is found that the threshold voltage and the on-resistance have more significant degradation phenomenon after the reverse conduction mode switch stress.It is proposed that the gate voltage stress will enhance the trap effect of two-dimensional electron gas,and further aggravate the threshold voltage increase.Combined with the phenomenon of threshold voltage forward shift in the long-time switch stress test,LTSpice simulation was used to study the influence of threshold voltage variation on the device power loss.It was found that the threshold voltage forward shift would increase the overall power loss of the device,among which the dead-time power loss increase was the most significant.Finally,the influence of threshold voltage variation,output current,switching frequency and other working conditions on device power loss is studied,which has guiding significance for improving the accuracy of power consumption calculation of power conversion system.