Organic transistor model with nonlinear injection: Effects of uneven source contact on apparent mobility and threshold voltage

This work addresses nonlinear characteristics observed in organic field effect transistors due to uneven injection at the source contact. A self-consistent OFET model has been developed for bottom-contact organic transistor, and is described in detail. Most relevant contact parasitics are taken into account, such as source and drain series resistances, leakage source/drain resistance, and of course nonlinear charge injection at the source contact, being the focus of this work. The drain current calculation is fully self-consistent and implemented in an original manner. First, results are compared with literature and internal data on poly( 3-hexylthiophene) bottom contact transistors presenting nonlinear behavior. In both cases, a single constant parameter set is used to fit all the transistor characteristic with good agreement. All extracted parameters, such as mobility or internal source contact voltage drop, are coherent with the device physics, showing the model capability to be used as a characterization tool. Following, it is shown that, in presence of series resistances and nonlinear injection, the usual mobility extraction procedure may become highly unreliable, showing a pronounced apparent mobility dependence with gate bias. Also the effective threshold voltage is shown to shift towards higher values due to the potential drop at the source contact. This is quantitatively described. Finally, the link between nonlinear injection and instabilities and breakdown conditions is established. It is shown that for a source/channel barrier height greater than 0.5-0.6 eV, the 5-6 MV cm (1) breakdown limit is reached even at very low drain bias. (C) 2010 Elsevier B.V. All rights reserved.