In accordance with the invention, the width of a gate electrode is smaller than the width of the semiconductor film. A sub gate electrode connected to the gate electrode is disposed, at the gate electrode side of the semiconductor film, away from the semiconductor film more than gate electrode. The width of the sub gate electrodes is larger than the width of the semiconductor film. Ends of the semiconductor film have regions formed of an intrinsic semiconductor which is not doped with dopant. In a semiconductor device, this structure is suitable to reduce degradation over time which is caused by an increase of the electric field strength or the carrier concentration at the ends of the semiconductor film.