To reduce off-current without lowering the on-current, related to a thin-film transistor used for a driver and the like of a liquid-crystal display device.

A thin-film transistor is provided, which comprises an active semiconductor thin-film layer 12, a gate insulating film 13, and a gate electrode 14 laminated on an insulating substrate 11 as well as a source electrode 15 and a drain electrode 16 connected to both sides of the active semiconductor thin-film layer 12. A metal layer 17 is formed at the lower part of the active semiconductor thin-film layer 12, over the insulating substrate 11 of the thin-film transistor. The metal layer 17 is applied with a reverse bias voltage, while the thin-film transistor is not conductive for controlling a void layer region occurring at the active semiconductor thin-film layer 12, resulting in change of a threshold voltage for a reduced off-current. Since the channel width and the channel length of the thin-film transistor are the same as before, on-current of the thin-film transistor is not lowered.