Self-aligned gate electrode for hydrogen-terminated diamond field-effect transistors with a hexagonal boron nitride gate insulator

Diamond electronic devices have attracted significant interest owing to their excellent semiconducting properties. We recently demonstrated that eliminating surface-transfer doping enhances carrier mobility and achieves normally off behavior in diamond field-effect transistors (FETs) with a hexagonal boron nitride (h-BN) gate insulator. In our previous study, the gate electrode was overlapped onto the source/drain electrodes to prevent the increase in access resistance caused by excluding surface-transfer doping. However, it is known that gate overlap increases parasitic capacitance and gate leakage current. In this study, we developed a technique for self-aligning the gate electrode with the edge of h-BN using oblique-angle deposition. The diamond FET with a self-aligned gate electrode exhibits optimal FET characteristics, including high mobility of ≈ 400 cm2V−1s−1, low sheet resistance of 2.4 kΩ, and output characteristics demonstrating pinch-off behavior. Furthermore, the capacitance-voltage characteristics clearly indicate distinct ON and OFF states, validating the efficacy of this technique. This method enables the fabrication of diamond/h-BN FETs with no gate overlap and without increasing access resistance, making it a promising approach for developing high-speed, low-loss diamond FETs with a wide range of applications.