Study On High Electric-field And High Temperature Reliability Of AlGaN/GaN HEMTs

GaN has become a strong competitor in the field of high-temperature,high-frequency and high-power device fabrication,because of its wide band gap,high break-down field and high electron saturation velocity.Although great improvement has been done with GaN based high electron mobility transistors?HEMTs?in recent years,different device reliabilities caused by different work environments still exist,the degradation mechanism and failure mechanism under different stress should be found.And this paper is written in this background to research the high electric-field and high temperature reliability of the AlGaN/GaN HEMTs.Firstly,main research contents of the high electric-field and high temperature reliability of the AlGaN/GaN HEMTs are introduced briefly.The basic structure of AlGaN/GaN HEMTs is described.The effects of spontaneous and piezoelectric polarization as well as the generation mechanism of two dimensional electron gas?2DEG?in the AlGaN/GaN heterostructure are analyzed.Next,we extract some devices randomly and measured their DC characteristics to make sure that the test would be carried out correctly.Then we researches the high electric-field reliability of AlGaN/GaN HEMTs through experiments.The experiments include the changes of device's DC characteristics and main parameters under off-state constant stress,on-state constant stress,off-state gate step stress,off-state drain step stress and on-state step stress.The result of experiments show that both the on-state stress and the off-state stress have the hot electron effect and the inverse piezoelectric effect,and both play a different role in the degradation of the device characteristics.The trapping of electrons by the devices'inherent traps under electrical stress caused the degradation of the DC characteristics of the device.After the stress is removed,the traps release electrons to restore the DC characteristics.The inverse piezoelectric effect causes the lattice relaxation of the AlGaN barrier layer,creating an irreversible leakage path from the channel to the gate.Moreover,the inverse piezoelectric effect has different critical conditions under different stresses to cause the lattice relaxation of the AlGaN barrier layer and generate new traps.The off-state drain step experiments and the off-state gate step experiments reveal that there is a critical voltage(V_crit=21V)for inverse piezoelectric effect to increase gate leakage current.When the gate-drain stress voltage(V_DGstress)is lower than 21V,the inverse piezoelectric effect does not introduce new traps;and when the V_DGstressGstress is higher than 21V,the inverse piezoelectric effect begins to introduce new traps and form an unrecoverable gate leakage channel.The off-state constant experiment shows that there is a critical time(t_crit=1000s),when the stress application time is 1000s,the inverse piezoelectric effect begins to introduce new traps,resulting in an increase of the gate leakage current.Finally,we have simulated the high-temperature working environment of the AlGaN/GaN HEMTs by the incubator,studied the high temperature reliability of the device and came up with the degradation mechanism in the high temperature.We have measured the devices'DC characteristics and AC characteristics in different temperatures,output current under different pulse stresses and gate delay characteristics in different temperatures.It is found that the increase in device gate leakage current at high temperatures is mainly due to:1.The high temperature increases the ideal factor and activates the defects introduced by the screw dislocation,which assists the tunneling of electrons to the gate;2.The Schottky barrier height decreases.In addition,the surface density of 2DEG increases,but at the same time,the mobility of the channel 2DEG decreases,and the increase in the output current caused by the former is smaller than the decrease in the output current caused by the latter.This results in a reduction in the output current of the device at high temperatures.