Motivation: Electronics based on organic thin-film transistors (OTFTs) enables a variety of attractive applications like active matrix displays, flexible sensor systems and, at a longer timescale, printable item-level RFID tags. As analogue circuit applications of OTFTs are being extensively researched, accurate OTFT models suitable for the simulation of these circuits become increasingly more relevant and urgent. In this work we propose a physically-based, analytical model for OTFTs, which takes into account the influence of both tail and deep localized states. The model is symmetric, considers channel modulation, and provides good accuracy both in sub-threshold and above-threshold regime. It also preserves the continuity of the small-signal parameters, as it is required for analogue simulations. Model, results and discussion: Charge transport in organic semiconductors is usually explained by means of the variable range hopping (VRH) theory, i.e., thermally activated tunnelling of carriers between localized states. From a percolation model of hopping between localized states, Vissenberg and Matters  developed an analytic expression of field-effect mobility used to describe transport in OTFTs. Their model is based on the assumption of an exponential DOS. From the direct determination of the DOS by high lateral resolution Kelvin probe measurements , it was shown that the ‘’real’’ DOS is well fitted by a Gaussian function in tail states and an exponential function in deep states. Therefore the Vissenberg and Matters work is a very good approximation of the Gaussian DOS only in tail states  whereas it neglects deep states. Based on these experimental evidences, we consider the influence of both tail and deep states on the OTFT current. We show that the former are important at high gate voltages when the transistor works in strong accumulation regime and the latter are relevant at low VGS, both in saturation and in sub-threshold regime (see Fig. 1). Channel length modulation is also accounted for in the model, ensuring both current and output-resistance continuity at the pinch-off voltage: this is very important when the model is used in a CAD environment and for analogue design purposes. In Figs. 1, 2 the proposed model (red continuous line) is compared with experimental data: accuracy is good both below- and above- threshold. The model is fully symmetrical and can be easily extended to take into account contact resistance effects. The model parameters are extracted in a simple and direct way from the experimental measurements (without any optimization method) using the standard procedure described in . From measurement in biasing conditions where the drain current is dominated by deep-states, one can determine the flat-band voltage VFB, the conductivity sd and the DOS characteristic temperature Td. It is worth noting that the flat-band condition is unambiguously determined from the deep-states current and, contrary to , no other threshold voltage needs to be introduced. Measurements where ID is dominated by tail-states current give st and Tt. Apart from geometric parameters, the model requires only five physical parameters related to the material properties.
|Title of host publication||proceeding of SAFE 2010 - Veldhoven, The Netherlands, 18-19 November 2010|
|Publication status||Published - 2010|