Low-leakage-current and improved thermal stability of normally-OFF hydrogen-terminated diamond FET via boron nitride passivation

Abstract

Normally-OFF hydrogen-terminated diamond (H-diamond) field-effect transistor (FET) with low leakage current is highly desirable for energy-saving switching applications. Metal/H-diamond FET with Schottky gate exhibits potential application advantages, owing to its excellent switching speed and small subthreshold swing. However, a lower interface barrier height (Schottky junction) typically results in high leakage current and poor thermal stability. In this work, a normally-OFF H-diamond metal/insulator/semiconductor-metal/semiconductor hybrid gate FET (MIS-MES HGFET) was achieved by nano‑boron nitride (nano-BN) passivation at the Al/H-diamond interface to reduce leakage currents. The results achieved a low OFF-state drain-source current density of ∼10⁻⁹ mA/mm. And the gate leakage current was suppressed by up to 6 orders of magnitude compared to FETs without nano-BN passivation. An ON/OFF ratio of 3.3 × 10⁹ was also obtained for devices with a gate length of 4 μm, the subthreshold swing is 102 mV/dec, and the maximum drain current density is −8.6 mA/mm. Encouragingly, this FET still exhibited improved thermal stability at high temperatures. At 450 K, the device maintained an ON/OFF ratio of ∼1 × 10⁹ and low leakage currents of ∼10⁻⁹ mA/mm for both gate and OFF-state drain-source. In contrast, the counterpart device without nano-BN passivation failed at only 425 K. Such improvements can be attributed to the excellent passivation and insulating properties of BN, which not only enhance the Schottky barrier height at the Al/H-diamond interface but also serve as a gate insulator to effectively suppress tunneling-induced leakage current. Results of this work can promote the development of diamond FETs for energy-saving and high-temperature applications.