TRANSISTORS Extreme enhancement-mode Diamond FETs push high-power electronics forward

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Researchers have achieved a record +8.3 V threshold voltage in hydrogen-terminated diamond accumulation-channel FETs, marking a breakthrough for normally-off, high-power device design.

Hydrogen-terminated diamond field-effect transistors (FETs) have long been recognized as promising devices for extreme power and high-frequency applications, thanks to diamond’s exceptional thermal conductivity, wide bandgap, and breakdown field. Until now, however, achieving enhancement-mode operation with high threshold voltages in accumulation-channel designs has remained elusive, limiting the devices’ voltage-handling and safety margins.

A study from researchers at the RMIT University, Melbourne, and the University of Glasgow demonstrates an extreme enhancement-mode hydrogen-terminated diamond FET with a threshold voltage exceeding +8 V, a record-high for this device class, and capable of high-performance operation under demanding electrical stress conditions. This advance not only overcomes a major design challenge but also positions diamond FETs for deployment in next-generation high-power, high-temperature electronics.

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Pushing threshold voltages beyond limits

Traditional hydrogen-terminated diamond FETs operate in depletion mode or at modest enhancement-mode thresholds, typically under +3 V, leaving little margin for safe gate biasing in high-voltage circuits. The research team engineered their devices with a re-optimized gate dielectric stack, integrating high-quality aluminum oxide (Al₂O₃) grown by atomic layer deposition on top of the hydrogenated diamond surface. Careful interface engineering minimized charge trapping and leakage pathways, enabling the accumulation channel to remain stable at significantly higher positive gate voltages.

Electrical characterization revealed a threshold voltage of +8.3 V, far exceeding prior records for hydrogen-terminated diamond accumulation-channel FETs. This allows the device to operate safely in circuits with higher supply voltages, expanding its application range from radio-frequency power amplifiers to pulsed-power switching in harsh environments. Importantly, this threshold shift was achieved without sacrificing channel mobility, preserving high transconductance and maintaining competitive on-state current densities.

Stable operation in extreme conditions

One of the hallmarks of diamond electronics is resilience in extreme thermal and electrical environments, and the demonstrated FETs continued this tradition. The devices maintained stable transfer characteristics at elevated temperatures, with negligible hysteresis and minimal drift even under prolonged high-field stress. The hydrogen-terminated surface, when combined with the optimized dielectric interface, suppressed the formation of interface states that typically degrade performance during temperature cycling.

Reliability testing under continuous bias stress confirmed long-term stability, with no significant threshold voltage shift or transconductance degradation after extended operation. These results underscore the viability of enhancement-mode hydrogen-terminated diamond FETs for mission-critical systems where failure is not an option, such as aerospace power conversion, radar transmitters, and high-voltage electric vehicleinverters.

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Toward practical high-power diamond electronics

By pushing threshold voltages to unprecedented levels while maintaining high mobility and stability, this work addresses one of the key roadblocks to mainstream adoption of hydrogen-terminated diamond FETs. Enhancement-mode operation is particularly valued in power electronics because it offers fail-safe “normally-off” behavior, reducing standby losses and enhancing system safety. The combination of high breakdown field, wide bandgap, and extreme thermal tolerance means that these devices could outperform both silicon carbide (SiC) and gallium nitride (GaN) transistors in the most demanding high-power roles.

The fabrication process, which relies on atomic layer deposition and surface treatment steps compatible with existing semiconductor manufacturing, also suggests a feasible path toward scaling production. As research continues to refine gate dielectrics, contact resistance, and thermal management, hydrogen-terminated diamond FETs could move from research labs to power modules in advanced industrial, defense, and renewable energy systems.

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