Extreme Enhancement-Mode Operation Accumulation Channel Hydrogen-Terminated Diamond FETs with $V_{th} < -6V$ and High On-Current

TL;DR

This paper introduces a novel hydrogen-terminated diamond FET operating in accumulation mode with record negative threshold voltage and high on-current, utilizing a new device concept and surface engineering techniques.

Contribution

The work presents a new accumulation-channel FET design on hydrogen-terminated diamond with enhanced performance and a record threshold voltage, differing from traditional transfer doping methods.

Findings

Achieved threshold voltage less than -6 V with high on-current over 80 mA/mm.

Demonstrated increased hole mobility of 110 cm²/V·s compared to transfer-doped counterparts.

Produced high-density hole accumulation layer using dual-stacked Al₂O₃ on H-diamond.

Abstract

In this work we demonstrate a new Field Effect Transistor device concept based on hydrogen-terminated diamond (H-diamond) that operates in an Accumulation Channel rather than Transfer Doping regime. Our 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 $Al_2O_3$ 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 formation of a high-density hole accumulation layer by gating the entire device channel which is encapsulated in dual-stacks of $Al_2O_3$. Completed devices with gate/channel length of $1 \mu m$ demonstrate record threshold voltage $< -6 V$ with on-current $> 80 mA/mm$. Carrier density and mobility figures extracted by CV analysis indicate high 2D charge density of $~ 2 \times 10^{12} cm^{-2}$ and increased hole mobility of $110 cm^2 /V \cdot s$ 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 new strategy for the development of high-performance power devices that better exploit diamond's intrinsic dielectric properties and high hole mobility.