Abstract

The thin-film transistor (TFT) is a popular tool for determining the charge-carrier mobility in semiconductors, as the mobility (and other transistor parameters, such as the contact resistances) can be conveniently extracted from its measured current-voltage characteristics. However, the accuracy of the extracted parameters is quite limited, because their values depend on the extraction technique and on the validity of the underlying transistor model. We propose here a new approach for validating to what extent a chosen transistor model is able to predict correctly the transistor operation. In the two-step fitting approach we have developed, we analyze the measured current-voltage characteristics of a series of TFTs with different channel lengths. In the first step, the transistor parameters are extracted from each individual transistor by fitting the output and transfer characteristics to the transistor model. In the second step, we check whether the channel-length dependence of the extracted parameters is consistent with the underlying model. We present results obtained from organic TFTs fabricated in two different laboratories using two different device architectures, three different organic semiconductors and five different materials combinations for the source and drain contacts. For each set of TFTs, our approach reveals that the state-of-the-art transistor models fail to reproduce correctly the channel-length-dependence of the transistor parameters. Our approach suggests that conventional transistor models require improvements in terms of the charge-carrier-density dependence of the mobility and/or in terms of the consideration of uncompensated charges in the carrier-accumulation channel.

Highlights

  • The fabrication of organic thin-film transistors (TFTs) has reached a level at which devices with excellent performance, small device-to-device variations, and smooth electrical characteristics with small hysteresis can routinely be provided [1,2,3,4]

  • Before applying our two-step fitting approach (TSFA), we analyze the data measured on the bottom-contact DNTT TFTs using the popular transmission line method (TLM)

  • We have proposed a two-step fitting approach (TSFA) to check whether or not a transistor model is capable of describing the experimentally-obtained current-voltage characteristic of organic TFTs

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Summary

Introduction

The fabrication of organic thin-film transistors (TFTs) has reached a level at which devices with excellent performance, small device-to-device variations, and smooth electrical characteristics with small hysteresis can routinely be provided [1,2,3,4] These technological advances are significantly ahead of our current ability to reliably extract crucial transistor parameters. While materials-related transistor parameters comprise, for example, the charge-carrier mobility and the permittivity of the gate dielectric, the most prominent geometry parameters are the channel length, the channel width, and the gate-dielectric thickness Such theoretical models hold much promise of being able to associate correctly any changes in the current-voltage characteristics to changes in these transistor parameters. In the presumably most prominent field-effect-transistor model, the gradual channel approximation, it is assumed that all mobile charges are confined to the interface between the semiconductor layer and the gate dielectric

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