Performance degradation of superlattice MOSFETs due to scattering in the contacts

TL;DR

This paper investigates how electron scattering in contacts degrades the ideal subthreshold swing performance of superlattice MOSFETs, using a multiscale quantum transport model to analyze and suggest design improvements.

Contribution

It introduces a multiscale quantum transport model to study contact scattering effects on superlattice MOSFETs and proposes design modifications to mitigate performance degradation.

Findings

Scattering increases electron transmission in the minigap, degrading subthreshold swing.

Shortening the spacer region improves subthreshold swing from 51mV/dec to 40mV/dec.

Degradation depends on scattering rate and region length.

Abstract

Ideal, completely coherent quantum transport calculations had predicted that superlattice MOSFETs may offer steep subthreshold swing performance below 60mV/dec to around 39mV/dec. However, the high carrier density in the superlattice source suggest that scattering may significantly degrade the ideal device performance. Such effects of electron scattering and decoherence in the contacts of superlattice MOSFETs are examined through a multiscale quantum transport model developed in NEMO5. This model couples NEGF-based quantum ballistic transport in the channel to a quantum mechanical density of states dominated reservoir, which is thermalized through strong scattering with local quasi-Fermi levels determined by drift-diffusion transport. The simulations show that scattering increases the electron transmission in the nominally forbidden minigap therefore degrading the subthreshold swing (S.S.) and the ON/OFF DC current ratio. This degradation varies with both the scattering rate and the length of the scattering dominated regions. Different superlattice MOSFET designs are explored to mitigate the effects of such deleterious scattering. Specifically, shortening the spacer region between the superlattice and the channel from 3.5 nm to 0 nm improves the simulated S.S. from 51mV/dec. to 40mV/dec. I. INTRODUCTION