Current-voltage Characteristics of Molecular Conductors: Two versus Three Terminal

TL;DR

This study compares the electrostatic and switching characteristics of molecular conductors in three-terminal devices to traditional silicon MOSFETs, revealing similar electrostatic constraints and the impact of contact types on subthreshold behavior.

Contribution

It demonstrates that molecular FETs face similar electrostatic limitations as silicon MOSFETs and shows how contact material influences their switching performance.

Findings

Gate control requires thin oxide relative to channel length.

Metallic contacts lead to temperature-independent subthreshold slope > 60 mV/decade.

Semiconductor contacts can improve off-state switching by band-limiting states.

Abstract

This paper addresses the question of whether a ``rigid molecule'' (one which does not deform in an external field) used as the conducting channel in a standard three-terminal MOSFET configuration can offer any performance advantage relative to a standard silicon MOSFET. A self-consistent solution of coupled quantum transport and Poisson's equations shows that even for extremely small channel lengths (about 1 nm), a ``well-tempered'' molecular FET demands much the same electrostatic considerations as a ``well-tempered'' conventional MOSFET. In other words, we show that just as in a conventional MOSFET, the gate oxide thickness needs to be much smaller than the channel length (length of the molecule) for the gate control to be effective. Furthermore, we show that a rigid molecule with metallic source and drain contacts has a temperature independent subthreshold slope much larger than 60 mV/decade, because the metal-induced gap states in the channel prevent it from turning off abruptly. However, this disadvantage can be overcome by using semiconductor contacts because of their band-limited nature.