Current spreading effects in fully printed p-channel organic thin film transistors with Schottky source-drain contacts

Contact effects have been analyzed, by using numerical simulations, in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source/drain contacts. Considering source-drain Schottky contacts, with a barrier height of 0.46 eV, device characteristics can be perfectly reproduced. From the detailed analysis of the current density we have shown that current spreading occurs at the source contact, thus influencing the effective contact resistance. At low V-ds and for a given V-gs, the current is mainly injected from an extended source contact region and current spreading remains basically constant for increasing V-ds. However, by increasing V-ds the depletion layer of the Schottky contact expands and reaches the insulator-semiconductor interface, causing the pinch-off of the channel at the source end (V-dsat1). For V-ds > V-dsat1 the current injected from the edge of the source contact rapidly increases while the current injected from the remaining part of the source contact basically saturates. Current spreading shows a V-gs-dependence, since the contact injection area depends on the channel resistance and also barrier lowering of the Schottky source contact depends upon V-gs. The injected current from the edge of the source contact can be reproduced using the conventional diode current expression, assuming a constant value for the zero barrier lowering saturation current and considering a V-gs-dependent barrier lowering. The presented analysis clarifies the V-gs-dependence of the contact current-voltage characteristics and points out that the I-V contact characteristics cannot directly be related to a single diode characteristics. Indeed, the contact characteristics result from the combination of two rather different regimes: at low V-ds the current is injected from an extended source contact region with a current spreading related to V-gs, while for V-ds above the pinch-off of the channel at source end, the current is injected primarily from the edge of the source contact and is strongly enhanced by the barrier lowering. (C) 2012 Elsevier B.V. All rights reserved.