Gate voltage dependent mobility of oligothiophene field-effect transistors

Organic field-effect transistors, in which the active semiconductor is made of oligothiophenes of various lengths, have been fabricated and characteri zed. A method is developed to estimate the field-effect mobility mu correct ed for the contact series resistance. The mobility is found to increase by a factor of nearly 100 from quaterthiophene (4T) to octithiophene (8T). Mor e importantly, mu increases quasilinearly with gate voltage. The origin of this gate bias dependence is discussed. One explanation could be the presen ce of traps that limit charge transport. Alternatively, the gate-voltage de pendence is tentatively attributed to a dependence of the mobility with the concentration of carriers in the accumulation layer. (C) 1999 American Ins titute of Physics.