Reliability-limiting defects in GaN/AlGaN high electron mobility transistors
The reliability of GaN/AlGaN HEMTs, fabricated using MOCVD, and MBE under Ga-rich, N-rich and ammonia-rich conditions, is studied using high field stress experiments and low frequency 1/f noise measurements. Hot electron stress results in positive shifts in pinch-off voltage for Ga-rich and N-rich devices, and negative shifts for devices grown using MOCVD and ammonia-rich MBE. Density functional theory (DFT) calculations suggest that dehydrogenation of Ga-vacancies is responsible for the positive shift in pinch-off, while the dehydrogenation of N-antisites causes the negative shift. Low frequency 1/f noise measurements show that dehydrogenation of N-antisites causes noise to increase in devices after stress, while dehydrogenation of carbon impurities causes noise to decrease after stress. Noise in GaN devices, when measured at high current conditions, obeys the Dutta-Horn model. Temperature-dependent noise measurements show a peak at 0.2 eV in all devices. The peak corresponds to the reconfiguration of an oxygen DX center in AlGaN. A second peak at high temperatures is seen in N-rich devices, which corresponds to the reconfiguration of N-antisites in AlGaN. Irradiation using 1.8 MeV protons causes positive shift in pinch-off voltage and increase in noise in all devices; these shifts are caused by the creation of Ga-N divacancies during proton irradiation.