Modeling of organic thin film transistors: Effect of contact resistances

Field effect transistors require an Ohmic source contact and an Ohmic drain contact for ideal operation. In many real situations, however, and specifically in organic devices, the injection of charge carriers from metals into semiconductors can be an inefficient process that is non-Ohmic. This has an adverse impact on the performance of thin film transistors and makes the analysis of electrical measurements a complex task because contact effects need to be disentangled from transistor properties. This paper deals with the effects of non-Ohmic contacts on the modeling of organic transistors and gives specific rules on how to extract the real transistor parameters (mobility, threshold voltage, and contact resistances) using only electrical measurements. The method consists of a differential analysis of the transfer characteristic curves (current versus gate voltage) and exploits the different functional dependences of current on gate voltage which is induced by the presence of contact resistances. This paper fully covers the situations from constant carrier mobility to power law gate-voltage-dependent mobility, from constant contact resistance to gate-voltage-dependent contact resistance, and in the linear and in the saturation regime of the operation of the transistor. It also gives important criteria for the validation of the extracted parameters to assess whether the conditions for the application of the method are fulfilled. Examples of application to organic transistors showing various behaviors are given and discussed.