Doping allows us to control the majority and minority charge carrier concentration in organic field-effect transistors. However, the precise mechanism of minority charge carrier generation and transport in organic semiconductors is largely unknown. Here, the injection of minority charge carriers into n-doped organic field-effect transistors is studied. It is shown that holes can be efficiently injected into the transistor channel via Zener tunneling inside the intrinsic pentacene layer underneath the drain electrode. Moreover, it is shown that the onset of minority (hole) conduction is shifted by lightly n-doping the channel region of the transistor. This behavior can be explained by a large voltage that has to be applied to the gate in order to fully deplete the n-doped layer as well as an increase in hole trapping by inactive dopants.
Keywords: Zener tunneling; ambipolar transistor; minority currents; n-channel transistor; n-type molecular doping; pentacene field-effect transistor.