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We present analytical physics-based compact models for the Schottky barriers at the interfaces between the organic semiconductor and the source and drain contacts in organic thin-film transistors (TFTs) fabricated in the coplanar and the staggered device architecture, and we illustrate the effect of these Schottky barriers on the current-voltage characteristics of the TFTs. The model for the source barrier explicitly considers the field-dependent barrier lowering due to image charges. Potential solutions have been derived by applying the Schwarz-Christoffel transformation, leading to expressions for the electric field at the source barrier and for the contact resistance at the source contact. With regard to the drain barrier, a generic compact-modeling scheme based on the current-voltage characteristics of a barrier-less TFT is introduced that can be applied to any compact dc model. Finally, both models are incorporated into an existing charge-based compact dc model and verified against the results of measurements performed on coplanar and staggered organic TFTs with channel lengths ranging from 0.5 to 10.5 mu m.

The development of organic thin-film transistors (TFTs) for high-frequency applications requires a detailed understanding of the intrinsic and extrinsic factors that influence their dynamic performance. This includes a wide range of properties, such as the device architecture, the contact resistance, parasitic capacitances, and intentional or unintentional asymmetries of the gate-to-contact overlaps. Here, we present a comprehensive analysis of the dynamic characteristics of the highest-performing flexible organic TFTs reported to date. For this purpose, we have developed the first compact model that provides a complete and accurate closed-form description of the frequency-dependent small-signal gain of organic field-effect transistors. The model properly accounts for all relevant secondary effects, such as the contact resistance, fringe capacitances, the subthreshold regime, charge traps, and non-quasistatic effects. We have analyzed the frequency behavior of low-voltage organic transistors fabricated in both coplanar and staggered device architectures on flexible plastic substrates. We show through S-parameter measurements that coplanar transistors yield more ideal small-signal characteristics with only a weak dependence on the overlap asymmetry. In contrast, the high-frequency behavior of staggered transistors suffers from a more pronounced dependence on the asymmetry. Using our advanced compact model, we elucidate the factors influencing the frequency-dependent small-signal gain and find that even though coplanar transistors have larger capacitances than staggered transistors, they benefit from substantially larger transconductances, which is the main reason for their superior dynamic performance.