  The conductivity of organic semiconductors is measured {\it 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 {\it 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 mono layers, and it does not depend on gate voltages, rather depends on
the chemical species.
