A field-effect transistor including an electrically conductive substrate; a first insulating film coating the electrically conductive substrate; a gate electrode disposed on the electrically conductive substrate with the first insulating film interposed therebetween; a source electrode; a drain electrode opposing the source electrode with the channel therebetween; a second insulating film covering the gate electrode; and a semiconductor layer having a width larger than a width of the gate electrode in the channel width direction and being partly provided on the gate electrode with the second insulating film interposed therebetween so that the gate electrode, the second insulating film, and the semiconductor layer are laminated in the channel.