Organic field-effect transistors (OFETs) in which the active semiconductor
is made of polycrystalline octithiophene (8T) were fabricated and
characterized. Several methods have been used to extract the parameters of
the polycrystalline (8T) transistors as function of gate voltage at room and
at low temperatures. These parameters such as the mobility, the threshold
voltage, contact resistance and density of traps are extracted from the
current-voltage characteristics of OFETs. The first method consists of
deriving the drain current as function of gate voltage (transconductance),
leading to the so-colled field effect mobility. It appears that the data
must be corrected for the substantial source and drain contact resistance.
In the second method, the carrier mobility has been corrected for the
contact resistance. It is found to increase quasi linearly with gate
voltage. Therefore, the contact resistances have been directly extracted
from the experimental channel conductance. In the third method, data are
interpreted according the multiple thermal trapping and release model using
a power law dependence of the mobility with gate voltage together with a
constant resistance. Finally, using a model where charge transport is
limited by trapping and thermal release at localized states located at grain
boundaries, the barrier height at grain boundaries in polycrystalline
octhithiophene FETs has been estimated. It is shown that is gate bias
dependent.