Surface field reduction in hydrogen-terminated diamond MESFET for high-voltage applications

Diamond metal-semiconductor field-effect transistor (MESFET) offers the advantages of room-temperature gate fabrication and fewer trap states in the Schottky metal, making it a promising candidate for improving gate control capability and enabling normally-off operation. However, its low Schottky barrier height may cause the device to operate in a normally-on mode and exhibit a large leakage current, which increases the failure risk and limits power capacity and efficiency. In this work, Al-gated MESFETs with weakly oxidized hydrogen-terminated channel were fabricated utilizing the ultraviolent ozone process. Benefiting from the Schottky junction of Al/diamond, the holes under the gate were depleted, thereby resulting in a normally-off operation. Although the oxidation generates relatively low output density, the higher barrier height of the weakly oxidized hydrogen-terminated diamond/Al was realized with the increased oxidation time (OT), which lowered the channel leakage current (∼10−9 mA/mm) when the OT was over 3 min, and the gate leakage was also improved. All weakly oxidized MESFETs showed very low source-drain leakage (∼10−8 A), demonstrating the excellent ability of Schottky junction to block the hole transport. As the OT increased, the gate leakage was well-suppressed, significantly increasing the breakdown voltage. When the OT was 5 min, the device with a gate-drain length of 19 μm reached the best breakdown of 1112 V and a small subthreshold swing of 146 mV/dec. The weak oxidation structure was analyzed using Silvaco TCAD. The electric field profiles demonstrate that weak oxidation is an effective approach to reduce the surface electric field under the gate and thereby improving the breakdown voltage. This work demonstrates the great potential of hydrogen-terminated diamond MESFETs for high-power applications.