Organic field-effect transistors (OFETs) with non-Ohmic contacts, e.g., pentacene with gold electrodes, exhibit a linearly growing threshold voltage with increased film thickness due to tunnel injection [R. Schroeder et al., Appl. Phys. Lett. 83, 3201 (2003)]. In this letter, we demonstrate gold/pentacene OFETs with a low threshold voltage independent of pentacene thickness. By doping the pentacene in the contact area with FeCl3 (iron-III-chloride), the metal-insulator-type tunneling barrier was changed to a metal-semiconductor Schottky barrier. Since the injection through a Schottky barrier depends on the potential and not on the electric field, the threshold voltage is no longer a function of the semiconductor thickness. Through selective doping of the area under the electrode, the channel remains undoped, and large on/off ratios are retained.

The threshold voltage and carrier mobilities were characterized in pentacene-based organic field-effect transistors with gold top-contact electrodes for different thickness of the pentacene film. The thickness of the semiconductor layer influences the values of the threshold voltage and, to a lesser extent, the saturation current. In this letter, we show that the thickness-dependent part of the threshold voltage results from the presence of an injection barrier at the gold–pentacene contact. We also show how the ratio between the gate insulator thickness and the semiconductor layer thickness alter the value for the saturation current, and therefore produces values for the field-effect mobility that are too low.