Research On Degradation Mechanism Of P-GaN Enhancement-mode GaN HEMTs

GaN-based High Electron Mobility Transistor(HEMT)has high breakdown electric field,low on-resistance and high temperature resistance.Its application in power converter as a switching device can greatly improve the power density and efficiency of the system.Therefore,it has broad prospects in the application of power electronics.Considering the safety of the system and simplifying the gate drive circuit,the switching device should be normally-off.The p-GaN gate HEMT has become the mainstream technology for enhancement-mode GaN devices,with a good balance between performance,reliability,and cost.However,there are still reliability problems when the device works under the combined action of high temperature and electrical stress.So,it is necessary to study the degradation mechanism of the device under high temperature and high voltage stress.This paper focuses on the degradation mechanism of p-GaN enhancement-mode GaN HEMT under high temperature forward gate stress and high temperature reverse bias stress.The traps inside the device are quantitatively characterized by low-frequency noise and deep-level transient spectroscopy(DLTS).The evolution law of the traps with stress time is obtained,and then the instability of the threshold voltage(VTH)of the device under typical stress is explained.The main research contents and conclusions of the paper are as follows:The degradation law and mechanism of the device under high temperature forward gate stress have been studied.The research results show that after 100ks of high temperature stress at 200℃,VTH does not change significantly,and the reverse gate leakage decreases by about one order of magnitude.The change of reverse gate leakage can be attributed to the high temperature improving the interface between the passivation layer and the AlGaN barrier layer.Under the forward gate stress at 25℃,VTH has good stability.Under 7V gate stress at 25℃,the off state leakage current of the device increases by 5 orders of magnitude.The reason is that there is a stronger electric field in the p-GaN layer,which accelerates the holes injected from the gate metal,and the high-energy holes will destroy the lattice structure of the GaN channel layer.Under the coupling effect of high temperature of 200℃ and 5V or 6V forward gate stress,VTH shifts significantly.Using 1/f noise to characterize the traps inside the device,by extracting the change of trap concentration during stress,it is found that the decrease and increase of the trap concentration correspond to the positive and negative shift of VTH,respectively.The shift of the VTH under the high temperature forward gate stress can be attributed to the trapping and releasing of carriers by the traps in the AlGaN barrier layer.The effects of temperature and drain bias on device degradation in high temperature reverse bias(HTRB)stress were investigated respectively.The results show that under the HTRB stress with the temperature higher than 150℃,VTH of the device shows an unrecoverable positive shift.Increasing the temperature will aggravate the positive shift of VTH.However,the change of VTH is small when the temperature is lower than 150℃.Combined with the analysis of the energy band below the gate,it is concluded that the degradation of the device caused by HTRB stress can be attributed to the trapping of electrons.The high temperature in the HTRB stress causes electrons to be injected into the p-GaN layer by thermionic emission.The electrons are accelerated under the high field in the p-GaN layer and generate traps.The trapped electrons result in an increase of the net negative charge,and thus the VTH shifts positively.Increasing the temperature will activate more electrons into the p-GaN layer,creating more electron traps,so it shows that the high temperature exacerbates the positive shift of VTH.In addition,under the same temperature of 200℃ and different drain biases,the degradation of the electrical parameters of the device is basically the same,and the positive shift of VTH is about 0.75V Based on the theoretical analysis of simulation,it is believed that with the increase of drain voltage,most of the change in the electric field drops at the edge of the source field plate,and the change of the electric field in the gate region is not obvious,so the degradation of the device is roughly same.The bulk traps inside the p-GaN layer and near the AlGaN/GaN interface are extracted innovatively by DLTS.There are two kinds of electron traps El(～Ec-0.6eV)and E2(～EC-0.8eV)near the AlGaN/GaN interface,and two kinds of electron traps E3(～E-EC-0.7eV)and E4(～EC-0.8eV)in the p-GaN layer.Under HTRB stress(1ks-100ks stress time range)at 200℃&(VGS=-2V,VDS=80V),the positive shift of VTH shows a good linear relationship with the increase of E3 and E4 trap concentration.Otherwise,it is concluded that the degradation of the device under HTRB stress is closely related to the electron traps of 0.7eV and 0.8eV in the p-GaN layer.The research results on the degradation mechanism of p-GaN enhancement-mode GaN HEMT devices in this paper are helpful to determine the safe working conditions of the devices,and provide an important reference for the evaluation and improvement of device reliability.