Modeling and Parameter Extraction on Pentacene TFTs

We report on an empirically based physical model developed for small-molecule organic thin film transistors (OTFTs). The model is an extension of an adapted MOSFET model for hydrogenated amorphous silicon TFTs accounting for an arbitrary energy distribution of mobile and trap states and allows the extraction of the parameters from the measured device characteristics. Ideally all parameters can be derived from the material properties of the organic semiconductor, but often those are masked by extrinsic effects. To provide model input and validation data sets we fabricated top contact pentacene TFTs on heavily doped and thermally oxidized wafers. The device structure allows the systematic study of the influence of the source and drain contacts and the properties of the semiconductor/metal interface on the device characteristics by varying the contact metal, deposition parameters, or the silane coupling agent used to treat the gate dielectric prior to deposition. From this a functional dependence of the contact and interface effects is incorporated into the model. The subthreshold regime is mainly used to test the charge trapping and charge-configuration model because charge-configuration related effects are usually more exposed in this regime. Currently the model routinely exhibits > 90% accuracy for most devices. Further insight into the actual physical mechanisms is expected from comparing the extracted trap state distributions with those extracted with other methods.