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

Wide band gap,high electron saturation velocity,and high critical break-down field are some of the advantages of GaN,which make it an excellent material for high-power,high-frequency,and high-temperature applications.Although the performance of GaN based high electron mobility transistors?HEMTs?can meets the requirements of practical application,it still has the problems of device reliability and the research of its failure mechanism.In this context,researches on the high electric-field and high temperature reliability of the AlGaN/GaN HEMTs are carried out in this paper.Firstly,the basic principles of AlGaN/GaN HEMTs are studied.The crystal structure of the GaN is described.The effects of spontaneous and piezoelectric polarization as well as the generation mechanism of two dimensional electron gas in the AlGaN/GaN heterostructure are analyzed,and then the analytical models are established.The DC characteristics of AlGaN/GaN HEMTs are studied,and then the DC parameters of experimental samples are described.Secondly,the reliability of AlGaN/GaN HEMTs under off-state electric stress are analyzed.The off-state drain step stress experiment reveals that there is a critical voltage(V_crit).When the gate-drain stress voltage(V_DGstress)is lower than V_crit,the electron capture machine plays a leading role,and when the V_DGstress is higher than V_crit,the inverse piezoelectric effect begins to emerge.The off-state constant stress experiment reveals that the inverse piezoelectric effect strongly depends on the stress time.The reliability analysis of the device under off-state electric stress shows that both the electron capture effect driven by hot electron and the inverse piezoelectric effect driven by electric-field lead to the degradation of device.The variations of gate leakage current can be used to determine the dominant degradation mechanism in the process of the stress.Finally,the DC and small signal characteristics of AlGaN/GaN HEMTs under high temperature are investigated.The DC characteristics and small signal characteristics under different temperatures are simulated.The simulation results clearly reveal that the device performance is severely degraded under higher temperature operation.When the ambient temperature is increased to 400K from 300K,the maximum drain current(I_Dmax)and maximum transconductance(G_mmax)are decreased by about 37.9%and 41%respectively,and the threshold voltage(V_th)has a slightly positive shift.When the ambient temperature increased to 425K from 300K,the cut-off frequency?f_t?and maximum oscillation frequency(f_max)is decreased by about 23.1%and 25.2%respectively.The results show that the degradation of device performance is mainly due to the decreasing of electron mobility in channel,which is mainly caused by the enhanced phonon scattering under the high temperature.But the channel electron concentration slightly decreases,so its impacts on the performance of device can be ignored.