Capacitance-voltage characteristics of organic thin-film transistors

We have fabricated pentacene-based thin film field-effect transistors and analyzed quasi-static current and capacitance measurements as a function of gate bias. The latter provides an independent and accurate estimation of the threshold voltage, an important device parameter that cannot be extracted unambiguously from the I-V measurements. The C-V characteristics of the transistors were furthermore characterized using impedance spectroscopy as a function of frequency and gate bias for the zero drain bias case. We model the impedance data with a simple transmission line equivalent circuit and find that the frequency dependence of the capacitance and phase can be described adequately from the channel conductance, as determined from quasi static current-voltage measurements, and the geometrical values of the channel and source/drain to gate electrode overlap capacitances.