Gate-Localized Fluorination Enables Enhancement-Mode AlGaN/GaN High-Electron Mobility Transistors

Gallium nitride (GaN)-based high-electron-mobility transistors (HEMTs) are key to high-power and high-frequency electronics owing to their wide bandgap, high breakdown field, and ability to form a high-density two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. For power-switching systems, enhancement-mode (E-mode) operation, where devices remain normally off at zero gate bias, is preferred for intrinsic failsafe behavior and reduced standby power. However, conventional E-mode strategies, such as deep gate recessing or p-type gate insertion, often introduce fabrication complexity, surface damage, and long-term instability. Here, we demonstrate a gate-localized CHF₃ plasma process that simultaneously produces a self-limiting recess with a fluorine-terminated surface, enabling a normally off AlGaN/GaN HEMT. Fluorine incorporation compensates polarization-induced charges and drives a positive shift in threshold voltage (V _th), whereas hydrogen species generated during plasma exposure passivate etch-induced Ga-related defects and suppress interface-trap formation. By confining plasma exposure to the gate region, this method mitigates surface degradation and charge trapping typically observed with CF₄ processing, achieving precise and stable V _th control without deep gate recessing. The fabricated devices exhibit normally off operation while maintaining low gate leakage under bias stress. This single step, lithographically confined approach offers a practical route toward E-mode GaN HEMTs for energy-efficient, high-frequency, and high-power electronic systems.