Gate tunable electron injection in submicron pentacene transistors

We study submicron organic field effect transistors with a pentacene channel, and observe either p-type or n-type behaviour under different gate and drain voltage conditions. Transistor structures of 0.8 µm channel lengths were fabricated by evaporating Au on a tilted substrate, featuring an oxide step. When evaporating pentacene on the step structure, the edge of the oxide step is used as a shadow mask to ensure the gap between source and drain. Current–voltage characteristics reveal that positive gate voltages increase the drain current, when the lower Au contact is operated as drain electrode, indicating electron transport through the channel. When the upper Au contact is used as a drain, the devices display p-type behaviour. These ambipolar device characteristics are explained in the light of electron injection enhanced by the submicron geometry, and by electron transport in the presence of electron traps.