AlGaN/GaN HEMTにおける電流コラプスの機構と抑制

摘要

An AlGaN/GaN-based high-electron-mobility transistor (HEMT) is considered as an excellent candidate for future power devices due to its high breakdown voltage, high saturation drain current and low on-resistance (Ron). However, current collapse, i.e., dispersion of drain current or increased dynamic Ron, is regarded as one of the most critical issues to be solved for actual power-switching applications. Even though there have been remarkable improvements in growth and device technologies, it is still essential to understand mechanism of current collapse and ways to get rid of it. In this work, I have investigated the effect of passivation on current collapse in AlGaN/GaN HEMTs. A detailed analysis on trapping effects and localization of traps has been made. In order to suppress current collapse in AlGaN/GaN HEMTs, field plate (FP) structured devices have been studied.Drain current dispersion measurements have been performed for AlGaN/GaN HEMTs having different passivation films such as SiN and Al2O3. Passivated devices exhibited dispersion between static and pulsed current-voltage characteristics. The maximum drain current for SiN passivated devices was improved by around 25% at pulsed measurements as compared to its static value, whereas Al2O3 passivated devices exhibited that of around 56% deterioration. The drain current was decreased with increasing on-state duration time for SiN passivated devices, while it was increased for Al2O3 passivated devices. The mechanism responsible for the increase of drain current with on-state duration time by AlGaN surface traps is proposed, while GaN buffer traps reasonably govern the decrease of drain current.Effects of SiN passivation on current collapse have been studied by monitoring the dynamic change in Ron. The normalized dynamic Ron was decreased with increasing SiN-deposition temperature and became high with increasing off-state drain bias voltages. The normalized dynamic Ron was also decreased with increasing annealingtemperature. These results indicate that the SiN/AlGaN interface trap density is reduced with increasing both SiN-deposition and annealing temperatures.Current collapse measurements have been performed for AlGaN/GaN HEMTs having identical breakdown voltages but with different FP lengths. The results indicated that applying more positive on-state gate biases resulted in pronounced recovery in the dynamic Ron for the FP device, whereas no gate-bias effects were observed for the device without FP. The mechanism responsible for the reduced current collapse by FP is proposed, in which the key role is played during on-state by the quick field-effect recovery of partial channel depletion caused by electron trapping at AlGaN surface states between gate and drain.The above studies indicate that the current collapse is suppressed by good passivation film and longer FP with more positive on-state gate biases in AlGaN/GaN HEMTs for future high-power applications.