Carrier Propagation Dependence on Applied Potentials in OFET Investigated by Impedance Spectroscopy

Based on our reported technique for evaluation of the transit time by the phase response in impedance spectroscopy (IS), in this report, we further apply this technique to show the dependence of carrier propagation on the applied potentials, including gatesource potential (Vgs) and drain-source potential (Vds), and also the charging effect of the device. The IS phase response shows that the critical frequency, which represents the reciprocal transit time, moves to a higher frequency systematically with increasing applied gate bias. This experimental result suggests that the transit time becomes shorter for higher applied gate bias. Interestingly, given a fixed gate bias, the dependence on the source-drain bias shows the opposite trend. With increasing Vds, the critical frequency becomes lower. This indicates slower propagation across the channel despite higher potential difference between source and drain electrodes, which contradicts with the commonly used assumption based on the two-electrode system (metal-insulator-metal) that the transit time is inversely proportional to the potential difference. We further propose a new model based on the Maxwell-Wagner Model for carrier propagation on the pentacene-insulator interface, and evaluate the transient mobility. We argue that the contradictory behavior of the OFET originates from the carrier propagation on the organic-insulator interface, as opposed to bulk transport in an MIM structure.