  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).
