Extreme Enhancement‐Mode Operation Accumulation Channel Hydrogen‐Terminated Diamond FETs with Vth < −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 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.