Threshold voltage control by gate oxide thickness in fluorinated GaN metal-oxide-semiconductor high-electron-mobility transistors

Abstract

This paper demonstrates the compensation of the intrinsic positive charges in Al₂O₃ gate dielectric by fluorine ions in GaN metal-oxide-semiconductor high-electron-mobility transistors (MOSHEMTs). Negatively-charged fluorine ions diffused into the oxide from the AlGaN barrier during the 250 °C atomic layer deposition compensate the intrinsic positive charge present in the Al₂O₃. This compensation is key to control the threshold voltage (Vth) of enhancement-mode (E-mode) transistors. A comprehensive analytical model for the V[subscript th] of fluorinated MOS-HEMTs was established and verified by experimental data. This model allows the calculation of the different charge components in order to optimize the transistor structure for E-mode operation. Using the proposed charge compensation, the V[subscript th] increases with gate dielectric thickness, exceeding 3.5V for gate dielectrics 25 nm thick.