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

Abstract 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 Al 2 O 3 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 Al 2 O 3 . 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 × 10 12 cm −2 and increased hole mobility of 110 cm 2 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.