Formation of inversion layers in organic field-effect transistors

An inversion current in unipolar organic field-effect transistors is not observed, which can be due to trapping of electrons or to negligible electron injection. Here, we distinguish between both cases by studying the depletion current of unipolar p-type transistors based on a deliberately doped organic semiconductor. For each doping level, the current can be completely pinched off, which unambiguously shows that no inversion layer is formed. Numerical calculations show that for electron injection barriers >1 eV, the transistor is thermodynamically not in equilibrium, such that a steady state is not reached in the time frame of the experiment.