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 characterized. A method is developed to estimate the field-effect mobility mu corrected for the contact series resistance. The mobility is found to increase by a factor of nearly 100 from quaterthiophene (4T) to octithiophene (8T). More importantly, mu increases quasilinearly with gate voltage. The origin of this gate bias dependence is discussed. One explanation could be the presence of traps that limit charge transport. Alternatively, the gate-voltage dependence is tentatively attributed to a dependence of the mobility with the concentration of carriers in the accumulation layer. (C) 1999 American Institute of Physics.