Organic FETs are produced having high mobilities in the accumulation mode and in the depletion mode. Significantly higher mobility is obtained from FETs in which RR-P3HT film is applied by dip-coating to a thickness of only about 20 .ANG. to 1 .mu.m. It was found that the structural order of the semiconducting polymer at the interface between the semiconducting polymer and the SiO.sub.2 gate-insulator is important for achieving high carrier mobility. Heat-treatment under an inert atmosphere also was found to increase the on/off ratio of the FET.