Research On Reliability Test And Degradation Mechanism Of GaN Power Device Under Multi-Stress Conditions

As the third generation semiconductor material?i.e.wide band gap semiconductor materials?,the gallium nitride?GaN?possesses the inherent advantages as a power electronic device and radio frequency?RF?device,because compared with Si and Ga As,it has wider forbidden band width,higher electron mobility,greater breakdown field strength and higher operating temperature.In recent years,GaN based high electron mobility transistor?HEMT?shows huge application potential in the field of power devices and RF devices due to its high frequency,high power,fast switching speed and other excellent performance.And GaN based schottky barrier diodes?SBD?also become popular in power devices benefiting from the excellent characteristics of GaN materials.Reliability testing and degradation mechanism analysis of GaN power devices have great significance to its popularization and application,and the reliability research of GaN power devices has been a research hotspot in related fields.Although the reliability of GaN-based power devices under high temperature and high electron field has been studied in recent years,the reliability of devices under multi-stress conditions?especially thermal and electrical?is still a major technical problem which puzzles researchers.Aimed at three mature types of GaN power devices,which are GaN MIS-HEMT?Metal-Insulator-Semiconductor HEMT?,p-GaN gate HEMT and GaN-based Schottky diode?SBD?,reliability research including stress conditions such as bias stress,temperature stress,etc.was conducted,and a comprehensive study and systematic analysis of the degradation behavior and mechanism of the device under multiple stress conditions were carried out in this dissertation.The followings are the main research contents:?1?The time-dependence of normally-on GaN MIS-HEMT degradation under different working states was studied,and different working states of device are controlled by setting different gate drain bias.In order to analyze the effect of bias stress on the performance of the device and explain the degradation mechanism of the device in different working states,we monitored the variation of V_th and max transconductance?g_m?of the device under three bias conditions experiments?off-state,semi-on state,on-state?.In the OFF-state,with the high reverse bias stress,the main mechanism is the charge-trapping in the gate dielectric layer,leading to the recoverable negative shift of the V_th.In the SEMI-ON-state,the main degradation mechanism is hot electron effect.The injection of high energy electrons will cause the decrease of saturation drain current and an unrecoverable V_th negative shift.In the ON-state,due to the attenuation of hot electrons effect,V_th hardly changes,while Idsat has a dramatic decrease caused by the self-heating effect.?2?The degradation mechanisms of p-type gate Al GaN/GaN HEMT under high temperature reverse bias?HTRB?stress and negative bias temperature instability?NBTI?are studied.Through real-time monitoring of the V_th and R_on during the long-range stress experiment of the device,combined with the variations in electrical properties before and after the experiment,the mechanism of the action of reverse bias stress and temperature on the device was analyzed.The research results show that high reverse bias stress could lead to hole emission in the p-GaN layer and the formation of a virtual gate effect in the gate-drain region,making threshold voltage(V_th)positively shift and on-resistance(R_on)degradation.And negative gate bias can cause charge trapping on the Al GaN layer and Al GaN/p-GaN surface,resulting in negative shift of the V_th.We also found that temperature increase can also accelerate the trapping process of defect states in Al GaN layer/interface layer.?3?A vertical structure GaN-based Schottky diode?SBD on GaN?was prepared,and the degradation behavior of GaN-based Schottky diode under temperature stress was studied.The electrical characteristics of SBD under a temperature gradient were tested to analyze the temperature dependence of device performance and the mechanism of reverse leakage at high temperature.In order to study the reverse leakage mechanism of SBD on GaN devices,the device was subjected to short-pulse stress test of bias voltage gradient,and the I-V characteristics of the device before and after short-pulse stress were analyzed.Combining the characterization of surface morphology and surface charge distribution with AFM and c-AFM?Conductive Atomic Force Microscope?,it is proved that more defects will be activated in the high-temperature environment to form a leakage channel and the reverse leakage current will increase.