Increase the threshold voltage of high voltage GaN transistors by low temperature atomic hydrogen treatment

High-electron-mobility transistors (HEMTs) based on AlGaN/GaN epitaxial heterostructures are a promising element base for the fabrication of high voltage electronic devices of the next generation. This is caused by both the high mobility of charge carriers in the transistor channel and the high electric strength of the material, which makes it possible to attain high breakdown voltages. For use in high-power switches, normally off-mode GaN transistors operating under enhancement conditions are required. To fabricate normally off GaN transistors, one most frequently uses a subgate region based on magnesium-doped $p$-GaN. However, optimization of the $p$-GaN epitaxial-layer thickness and the doping level makes it possible to attain a threshold voltage of GaN transistors close to $V_{\operatorname{th}}$ = +2 V. In this study, it is shown that the use of low temperature treatment in an atomic hydrogen flow for the $p$-GaN-based subgate region before the deposition of gate-metallization layers makes it possible to increase the transistor threshold voltage to $V_{\operatorname{th}}$ = +3.5 V. The effects under observation can be caused by the formation of a dipole layer on the $p$-GaN surface induced by the effect of atomic hydrogen. The heat treatment of hydrogen-treated GaN transistors in a nitrogen environment at a temperature of $T$ = 250$^\circ$C for 12 h reveals no degradation of the transistor’s electrical parameters, which can be caused by the formation of a thermally stable dipole layer at the metal/$p$-GaN interface as a result of hydrogenation.