Reliability Study Of GaN-based High-electron-mobility Transistors

GaN-based high-electron-mobility transistors (HEMTs) are the most promising candidate for high-frequency and high-power microwave applications due to the intrinsic material advantanges of GaN-based semiconductors. However, imperfect material quality, strong polarization effect, high field operation enviroment and variation of processing technologies make the reliability problem of GaN-based HEMTs very complicated. As a result, GaN-based HEMTs have actual performance not up to their ideal level. These reliability challenges will hinder the extensive commercial applications of GaN-based HEMTs. In this work, we studied the reliability behaviors of AlGaN/GaN HEMTs from different perspectives.Firstly, we studied a carrier-trapping related anomalous kink effect in AlGaN/GaN HEMTs. The kink is found largely caused by trapping electrons from the gate leakage current by deep levels within the AlGaN barrier at high drain bias, while the carrier trapping process mainly occuring within the AlGaN barrier under the gate has to be induced by a high drain bias leakage current. The carrier detrapping process is caused by hot electrons injected into the AlGaN barrier which "knock off" the trapped electrons.Secondly, we observed a critical stress voltage in the step stress tests, beyond which the AlGaN/GaN HEMTs start to recover from drain current slump induced by the lower voltage stress. The recovery process is explained by a high field induced carrier detrapping mechanism, while the device degradation in low-to-medium stress voltage range is caused by carrier trapping in surface or bulk states.Thirdly, we comprehensively studied off-state breakdown and current transport process of GaN-based HEMTs. Premature hard-breakdown can be observed in three-terminal tests, illustrating that buffer layer plays an important role in off-state breakdown. Measurement using drain current injection technique proves the existence of drain-to-source buffer leakage. Based on detailed tests at different current injection levels and different temperatures, we find that the buffer leakage is determined by a combined effect of electric field induced energy barrier lowering, band bending and inherent defects in the buffer layer. This buffer leakage current can induce premature hard-breakdown when stress electric field is high enough.Fourthly, we fabricated and characteristized a metal-insulator-semiconductor (MIS) AlGaN/GaN HEMT with Al2O3gate insulating layer deposited by atomic-layer-deposition (ALD). It is found that although the insulating layer can effectively suppress gate leakage current, dielectric breakdown could happen at high bias voltage, which makes the gate leakage current recover to its original high level again.