Investigation of SiNx and AlN passivation for AlGaN/GaN high-electron-mobility transistors: Role of interface traps and polarization charges

Abstract

In this work, we studied the mechanisms and switching properties of AlGaN/GaN high-electron-mobility-transistors (HEMTs) passivated by amorphous-SiNx and monocrystal-like AlN. The effects of interface traps and polarization charges on current collapse are investigated by TCAD simulations and experimental characterizations. Surface/interface deep levels can be compensated by both shallow donor-like traps (SiNx passivation) and polarization charges (AlN passivation) at passivation/heterostructure interface, but with different levels of effectiveness under fast switching conditions. SiNx-passivation introduces shallow donor-like trap states with short time constant that favors a fast emission of trapped electrons in the access region and suppressed current collapse, but nevertheless exhibits more severe time-dependent recovery of dynamic on-resistance. For AlN passivation, interface traps are compensated by the fixed positive polarization charges and the off-state depletion region (in the 2DEG channel) is formed predominantly by electric-field effect, leading to an immediate accumulation of high channel electron concentration after switching the HEMT devices back to on-state and instant response of drain current to gate and drain bias. The field plate structure is necessary in SiNx-passivated devices for both current collapse suppression and electric field alleviation. With AlN passivation, the field plate can be solely designed for achieving more uniform electric field distribution for gate reliability concern without the concern of current collapse.