Reliability Of GaN-based Hemts Under On-state And Off-state Electrical Stress

The research of gallium nitride(GaN)has been very popular in recent years.Compared with traditional semiconductor materials,it has higher saturation drift speed,wider forbidden band width,bigger thermal conductivity and greater radiation resistance.Therefore,GaN materials are typical third-generation semiconductor materials.Due to its excellent characteristics,it is widely used in high frequency and high power field.With the development of science and technology,the working voltage requirements for the device are getting higher and higher.Under working in such high voltage,the electrical characteristics of the device will be greatly degraded,which severely limits the development of GaN HEMT devices.Therefore,the research on GaN HEMT devices under high field is of great significance.And in this paper,the degradation mechanism of GaN HEMT devices under on-state stress and off-state stress will be studied separately,by combining DC test and low-frequency noise test methods.Firstly,we carried out the on-state stress to D-mode HEMT device.Under the on-state stress,as the stress time increases,the V_th of the device will move forward,the g_mand i_dsat will be greatly reduced,the R_on will increase,the forward and reverse Schottky current will decrease.The hot electron effect is considered to be the main cause of device degradation under on-state stress.Therefore,this paper focuses on the influencing factors of the hot electron effect from the three aspects,the energy of hot electrons,the number of hot electrons,and the structure of devices.Through these on-state stress experiments under different conditions,it is proved that the hot electron energy,the number of hot electrons,and the device structure will have a significant impact on the hot electron effect.Finally,the low-frequency noise characteristics of D-mode HEMT device during the on-state stress are tested.Combined with the Mc Whorter model,the total density of traps N_t in the barrier layer of the device is quantitatively extracted.Secondly,we carried out the off-state stress to D-mode HEMT device.The paper designs a comparative experiment of symmetrical off-state stress and unsymmetrical off-state stress.The experimental results show that the characteristics of the device will be greatly degraded under symmetrical off-state stress.The low-frequency noise characteristics of D-mode HEMT devices under off-state stress are also tested.The total density of traps N_tin the barrier layer of the device decreases after the off-state stress,which may be related to the leakage channel introduced by the inverse piezoelectric effect.In the end,the reliability of fluorine plasma treatment E-mode devices under off-state stress is studied.It is found that the threshold voltage of the E-mode device under the off-state high field stress will shift negatively in parallel.Three possible degradation mechanisms were analyzed,respectively.Based on the experimental investigation,F ions impact ionization is considered as the main reason for the threshold voltage instability in E-mode HEMTs under off-state stress.Finally,the temperature characteristics of E-mode devices after off-state stress are studied.Through the annealing experiment in the stressed device,it is found that the threshold voltage of the device will partially recover.For this phenomenon,this paper proposes a neutral trap theory to explain,this is because a large number of recoverable neutral traps generated at the AlGaN/GaN interface during off-state stress.These traps can trap channel electrons during annealing which cause the recovery of V_th.