A thin-film field-effect transistor with an organic semiconductor as the channel material was demonstrated. Spin-coated regioregular poly-3-hexylthiophene (P3HT) thin films show conducting characteristics with holes as carriers. The transistor showed a field-effect mobility of 1.1 cm(2)/Vs and an on/off ratio of 50 in the accumulation mode. No ideal device characteristics were obtained in the depletion mode. A change in drain current (I-d) was observed when device was exposed to small amounts of nitrous oxide (N2O) gas. The total variation in the Id was 0.35 mA. Using the transistor, it was possible to detect 1000 ppm of N2O gas with a response time of less than 1 min at 50 degreesC. A similar device response was not observed after oxygen exposure. N2O gas was found to affect the charge transport properties of the poly-3-hexylthiopbene film. The presence of polar molecules is known to change the rate of charge transport in organic materials by increasing the amount of energetic disorder through charge-dipole interactions. (C) 2004 Elsevier B.V. All rights reserved.