Short-channel organic field effect transistors
Nanotechnology 15, 1023 (2004).

   Organic field effect transistors with submicron pentacene channels 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. Current-voltage characteristics show 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 drain, the structures display p-type behavior. 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. The figures show the device structure and a scanning electron micrograph obtained after pentacene deposition.

   Electron injection from Au into pentacene is difficult, because the Au Fermi level lies about 1.35 eV lower than the LUMO of pentacene. However, at a sufficiently strong electric field, electrons can tunnel into the LUMO, most likely by utilizing defect states within the band gap of pentacene. If the source-drain distance is sufficiently short, the injected electrons can survive this distance without recombining with holes in the channel, and electron transport then appears in the transistor characteristics. In a short-channel OFET, the gate voltage may also directly influence carrier injection. In a submicron silicon device, the effect of drain and gate voltages are mixed, due to the close proximity of the gate and drain (e.g. the threshold voltage is dependent on drain voltage in a submicron device). Similarly in submicron OFETs, the gate voltage will influence carrier injection at the source or drain contacts, leading to transistor action through a gate controlled drain current. Hence, whereas in a long-channel OFET the injection-limited current deleteriously impacts the transistor characteristics, in a short-channel OFET, contact effects can provide transistor action, similarly to recent observation in carbon nanotubes.