Impact of Al2O3 Passivation on AlGaN/GaN Nanoribbon High-Electron-Mobility Transistors

Passivation films are used in III-nitride (III-N) based devices to suppress current collapse and improve frequency performance. Several passivation films and deposition methods have the added effects of increasing the dc ON- and OFF-state currents in devices. In this paper, the physical mechanisms behind this current increase have been studied in both nanoribbon and planar devices with atomic-layer deposited Al2O3 passivation. Increased tensile stress in the AlGaN layer due to passivation leads to an increase in the charge density in nanoribbon devices. Simultaneously, the mobility in nanoribbons increases after Al2O3 passivation. These effects lead to a large (similar to 118%) increase in the saturation drain current in nanoribbon devices. In contrast, fixed positive charge at the Al2O3-AlGaN interface leads to a small (similar to 6%) saturation drain current increase in planar devices. In addition, the mechanisms behind the increase in the OFF-state drain current in the passivated devices are investigated. Schottky barrier lowering and the increase in surface and buffer conduction are found to be the major causes for the OFF-state current increase with passivation.