Extreme Enhancement-Mode Operation Accumulation Channel Hydrogen-Terminated Diamond FETs with V_th < −6 V and High on-Current

In this work, a new Field Effect Transistor device concept based on hydrogen-terminated diamond (H-diamond) is demonstrated that operates in an Accumulation Channel rather than a Transfer Doping regime. The FET devices demonstrate both extreme enhancement-mode operation and high on-current with improved channel charge mobility compared to Transfer-Doped equivalents. Electron-beam evaporated Al₂O₃ is used on H-diamond to suppress the Transfer Doping mechanism and produce an extremely high ungated channel resistance. A high-quality H-diamond surface with an unpinned Fermi level is crucially achieved, allowing for the formation of a high-density hole accumulation layer by gating the entire device channel which is encapsulated in dual-stacks of Al₂O₃. Completed devices with gate/channel length of 1 µm demonstrate record threshold voltage < −6 V with on-current > 80 mA mm⁻¹. Carrier density and mobility figures extracted by CV analysis indicate a high 2D charge density of ≈ 2 × 10¹² cm⁻² and increased hole mobility of 110 cm² V⁻¹ s⁻¹ in comparison with more traditional Transfer-Doped H-diamond FETs. These results demonstrate the most negative threshold voltage yet reported for H-diamond FETs and highlight a powerful new strategy to greatly improve carrier mobility and enable enhanced high power and high frequency diamond transistor performance.