Thin film organic field-effect transistors were grown
with vapor-deposited polycrystalline-octithiophene on silicon oxide
insulating layers. This component requires an ohmic source and drain
contacts for ideal operation. The performance of organic electronic-devices
is often limited by injection. In many real situations, however and
specifically in organic devices, the injection of charge carriers from
metals into semiconductors is non-linear. This has an adverse impact on the
performance of thin film transistors, and makes the analysis of electrical
measurements a complex task because contact effects need to be disentangled
from transistor properties. This paper deals with the effects of non-ohmic
contacts on the modeling of organic transistors and gives specific rules on
how to extract the real transistor parameters using only electrical
measurements. Several methods are used in order to study the influence of
the contact resistance on the performance of organic transistors. This
influence appears especially on the current-voltage characteristics of
organic field effect transistor. We present a first method used to extract
the key parameters of OFET such as; mobility, threshold voltage and contact
resistances using the fit of the transfer characteristic of the devices. The
second method has been used to exploit the different functional dependences
of current on gate voltage which is induced by the presence of contact
resistances in the linear and in the saturation regimes. All electrical key
parameters of OFETs based on octithiophene have been extracted and we
demonstrate that both mobility and contact resistance depend on gate voltage
and temperature.