Formation of Recessed-Oxide Gate for Normally-Off AlGaN/GaN High Electron Mobility Transistors Using Selective Electrochemical Oxidation

A selective electrochemical oxidation has been applied to the AlGaN surface to fabricate a recessed-oxide-gate structure for normally-off AlGaN/GaN high-electron-mobility transistors (HEMTs). We observed bias-dependent oxidation current characteristics peculiar to the AlGaN/GaN heterostructure. A flat interface between the oxide and AlGaN was confirmed by cross-sectional transmission electron microscopy. The selective formation of the recessed oxide allowed the local depletion of two-dimensional electron gas at the AlGaN/GaN interface and thus the achievement of normally-off operation. The recessed-oxide-gate HEMT with the oxide thickness of 20 nm showed good gate control of drain current with the threshold voltage of +1.2 V.