Study On High Electric-field Reliability And High Temperature Characteristics Of GaN-based Double-Heterostructure MOS HEMT

Due to the high breakdown field and high saturation velocity, Galliumnitride-based high-electron-mobility transistors(HEMT) have become excellentcandidates for microwave and power electronics applications. Double-heterostructureHEMT have also gained much attention because of the excellent channel confinement.In addition, the utilization of gate dielectric can significantly reduce gate leakagecurrent, and increase breakdown voltage and power added efficiency (PAE), thusimproving device reliability. However, some reliability problems of GaN HEMT stillexist, hindering the further application. On this basis, this paper makes a detailedanalysis of high electric-field degradation and high temperature characteristics of MOSHEMT.High electric-field reliability of MOS HEMT is investigated by applying constantstress-recovery and step stress-recovery tests. By analysing the device characteristics ofDC output, transfer characteristics, and the gate leakage current through the stress tests,we draw the conclusions below. In the on-state stress test, AlGaN buffer layerdramatically enhances the channel confinement and alleviates the hot-electron effect.Off-state stress test shows that the degradation of gate leakage current is mainly causedby electrons trapping in Al2O3gate dielectric layer and AlGaN barrier layer, and that thedecrease of Vthis mainly caused by the electrons released from traps in AlGaN barrierlayer near2DEG channel. Through Off-step stress test, we find different degradationwhen the electric field increases. It may be the oxygen from device surface and Al2O3gate dielectic that diffuse into AlGaN barrier layer, increasing the trap assistanttunneling, so that the gate leakage current increases and Vthmoves backward, causingdevices degrade.In addition, high-temperature characteristics of MOS HEMT including DC, ohmiccontact and gate leakage current characteristics are investigated, and some possiblemechanisms are presented to give insight into the degradation phenomena. Astemperature increases, the maximum output current and the peak transconductancedecreases, which is mainly due to the decrease of carrier mobility. The increase ofchannel sheet resistence and ohmic contact resistence indicates a certain degradation ofohmic characteristics under high temperature environment. The gate leakage currentalso increases with temperature raising. In high temperature region, the forward leakage current is maily caused by Thermionic emission. After comparing with several leakagecurrent mechanisms, we find that the reverse leakage current meets Frenkel-Pooleemission model very well, and that the barrier height for the electron emission from thetrap state is0.25eV.