  The field-effect mobility in an organic thin-film transistor is studied
theoretically. From a percolation model of hopping between localized states and
a transistor model an analytic expression for the field-effect mobility is
obtained. The theory is applied to describe the experiments by Brown et al.
[Synth. Met. 88, 37 (1997)] on solution-processed amorphous organic
transistors, made from a polymer (polythienylene vinylene) and from a small
molecule (pentacene). Good agreement is obtained, both with respect to the
gate-voltage and the temperature dependence of the mobility.
