The conductivity of organic semiconductors is measured in situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. The depletion layer thickness can be directly determined as a shift of the threshold thickness at which electric current began to flow. The in situ and continuous measurement can also determine qualitatively the accumulation layer thickness together with the distribution function of injected carriers. The accumulation layer thickness is a few monolayers and it does not depend on gate voltages. Rather it depends on the chemical species.

We present a simple but powerful method to determine the thicknesses of the accumulation and depletion layers and the distribution curve of injected carriers in organic field effect transistors. The conductivity of organic semiconductors in thin film transistors was measured in-situ and continuously with a bottom contact configuration, as a function of film thickness at various gate voltages. Using this method, the thicknesses of the accumulation and depletion layers of pentacene were determined to be 0.9 nm (VG=-15 V) and 5 nm (VG=15 V).

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Transient current excited by application of step gate voltages was investigated for C60 field effect transistors (FETs). The measurements were continued during film growth and the decay curves were obtained as a function of C60 film thickness. The decay curve contains two different relaxation times. The fast decay could be explained in terms of channel resistance and gate insulator capacitance, while the slower decay relates with trapping processes of carriers. The slower decay observed for the larger mobility film implies that mobility is not an absolute figure for evaluating the performance of organic FETs.

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