Contact effects have been investigated in fully printed p-channel organic thin film transistors with field effect mobility up to 2 cm2/Vs. Electrical characteristics of the organic thin film transistors, with channel length <200lm, are seriously influenced by contact effects with an anomalous increase of the contact resistance for increasing source-drain voltage. Assuming that contact effects are negligible in long channel transistors and using gradual channel approximation, we evaluated the current-voltage characteristics of the injection contact, showing that I-V characteristics can be modeled as a reverse biased Schottky diode, including barrier lowering induced by the Schottky effect

We have analyzed the characteristics of hydrogenated amorphous silicon source gated transistors (SGTs) by using numerical simulations and we found that the original SGT characteristics can be reproduced without introducing barrier lowering mechanisms at the Schottky contact. Output characteristics show reduced current increase when pinch-off of the source end of the channel is triggered by increasing Vds, while perfect saturation of the drain current is achieved when pinch-off at the drain occurs. According to our simulations, even in the saturation regime the current at metal-semiconductor interface does not reach the thermionic emission limit and remains diffusion limited. Gate bias dependence of the saturation current can be simply explained as a combination of increased saturation voltage and reduced output conductance, without invoking barrier lowering mechanisms. SGT contact effects were modeled by introducing a distributed diode equivalent circuit for the source contact, which reproduces very well the device characteristics and can be easily implemented in a circuit simulator.

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