Threshold voltage and space charge in organic transistors

TL;DR

This study systematically measures and models the threshold voltage shift in rubrene single-crystal transistors caused by space charge transfer from contacts, using Poisson's equation and semiconductor density of states.

Contribution

It provides a quantitative model of threshold voltage shift in organic transistors without free parameters, linking experimental data to microscopic parameters.

Findings

Threshold voltage shift is due to space charge from contacts.

Model accurately predicts voltage shift using Poisson's equation.

FET measurements can determine microscopic charge carrier parameters.

Abstract

We investigate rubrene single-crystal field-effect transistors, whose stability and reproducibility are sufficient to measure systematically the shift in threshold voltage as a function of channel length and source-drain voltage. The shift is due to space-charge transferred from the contacts, and can be modeled quantitatively without free fitting parameters, using Poisson's equation, and by assuming that the density of states in rubrene is that of a conventional inorganic semiconductor. Our results demonstrate the consistency, at the quantitative level, of a variety of recent experiments on rubrene crystals, and show how the use of FET measurements can enable the determination of microscopic parameters (e.g., the effective mass of charge carriers).