Ab Initio Transfer Length Method Simulations of Tunneling Limits in 2D Semiconductors

TL;DR

This paper develops a first-principles computational framework to analyze quantum tunneling limits and contact resistance in 2D semiconductor devices, providing insights into contact engineering for future nanoscale transistors.

Contribution

It introduces an ab initio transmission line model for 2D contacts, revealing the tunneling-to-thermionic transition and guiding contact material selection.

Findings

Universal transition from tunneling to thermionic emission at ~10 nm

Optimal contact strategies depend on doping type and metal work function

Provides a metric for the tunneling limit in 2D transistors

Abstract

As semiconductor devices approach the sub-2 nm technology node, identifying the quantum-mechanical limits of contact-resistance scaling becomes imperative; however, the transition from thermionic emission to direct tunneling in this deep nanoscale regime remains experimentally inaccessible and theoretically undefined. Herein, we present a systematic first-principles framework to characterize metal/2D-semiconductor interfaces at the atomic scale and identify their intrinsic contact resistance and tunneling limits. Based on large-scale multi-space density functional theory calculations, we perform ab initio transmission line model (TLM) analyses for monolayer MoS2 contacted by Sc, Ag, Au, and Pd electrodes in both top-contact and edge-contact geometries. This computational procedure reveals a universal transition in resistance scaling from metal-induced gap states-mediated direct tunneling in the sub-10 nm regime to thermionic emission at longer channel lengths. The resulting transition length provides a rigorous first-principles measure of the critical tunneling length, establishing a physically grounded metric for assessing contact quality and the source-to-drain tunneling limit of 2D ballistic transistors. Using the ab initio TLM method, we further identify optimal contact strategies-top contact with low-work-function metals for n-type operation and edge contact with high-work-function metals for p-type operation. Our study introduces a general computational framework for evaluating and comparing 2D semiconductor contacts and offers practical guidelines for engineering low-resistance, scalable contact technologies for next-generation 2D transistors.