Ambipolar pentacene field-effect transistors with calcium source-drain electrodes

Field-effect transistors consisted of vacuum-sublimed polycrystalline pentacene films and calcium source-drain electrodes were prepared and device characteristics were evaluated in an oxygen-free condition. The field-effect transistor showed typical ambipolar characteristics and field-effect hole mobility of $4.5\times {10}^{-4}{\mathrm{cm}}^2/\mathrm{Vs}$ and field-effect electron mobility of $2.7\times {10}^{-5}{\mathrm{cm}}^2∕\mathrm{Vs}$ were estimated from saturation currents. Appearance of an electron enhancement mode in pentacene field-effect transistors was ascribed to the lowering of barrier for electron injection at source-drain electrodes. Effective elimination of electron traps using an oxygen-free condition was found to be another requirement for the observation of ambipolar behavior in pentacene.