The first solid‐state field‐effect transistor has been fabricated utilizing a film of an organic macromolecule, polythiophene, as a semiconductor. The device characteristics have been optimized by controlling the doping levels of the polymer. The device is a normally off type and the source (drain) current can be modulated by a factor of 102–103by varying the gate voltage. The carrier mobility and the transconductance have also been determined to be ∼10−5cm2/V s and 3 nS, respectively, by means of electrical measurements.