Investigation of the environmental influence on C(V) characteristics of gallium nitride metal–insulator–semiconductor high-electron-mobility transistors

This study examines the impact of humidity and temperature on the reliability of silicon nitride (SiN) passivated 80 μm gallium nitride (GaN) metal–insulator–semiconductor high-electron mobility transistor (HEMT) test structures. Degradation is determined by threshold voltage shifts in the capacitance–voltage C(V) characteristics. High temperature and humidity, while maintaining a constant gate and drain bias, are deleterious to reliability, leading to pronounced positive threshold voltage shifts (ΔVth). In contrast to previous humidity studies on Schottky-gate GaN HEMTs, which reported negligible humidity-induced threshold shifts, the present work shows that GaN MISHEMTs exhibit a strong sensitivity of ΔVth to humidity. A lifetime analysis based on Peck’s model was conducted to quantify these effects, yielding a humidity acceleration factor B = 0.021 at 85 °C and an activation energy Ea = 0.45 eV in ambient air. It has been shown that increasing the relative humidity (RH) from 0% to 40% reduces the time to failure by approximately one order of magnitude. A comparison of different SiN passivation thicknesses reveals that thick gate passivation substantially mitigates humidity-induced degradation. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis showed that degradation mainly occurs in the access region rather than the intrinsic region. The presence of oxidized SiN passivation and cracks was revealed by both ToF-SIMS and scanning electron microscopy/element-dispersive x-ray analysis of these regions. The epitaxial layers and the SiN passivation are identical to those of short-channel (100 nm) GaN HEMTs, meaning that these findings can be transferred.