In a manufacturing process of a transistor including an oxide semiconductor film, oxygen doping treatment is performed on the oxide semiconductor film, and then heat treatment is performed on the oxide semiconductor film and an aluminum oxide film provided over the oxide semiconductor film. Consequently, an oxide semiconductor film which includes a region containing more oxygen than a stoichiometric composition is formed. The transistor formed using the oxide semiconductor film can have high reliability because the amount of change in the threshold voltage of the transistor by a bias-temperature stress test (BT test) is reduced.

The present invention provides a thin-film transistor in which transistor characteristics such as drain current and threshold voltage are improved, and a method of manufacturing the same. The present invention provides a thin-film transistor provided with a source electrode (108), a drain electrode (109), a semiconductor layer (105), a gate electrode (103), and an insulating layer (104); wherein the semiconductor layer (105) contains a composite metal oxide obtained by adding to a first metal oxide an oxide having an oxygen dissociation energy that is at least 200 kJ/mol greater than the oxygen dissociation energy of the first metal oxide, whereby the amount of oxygen vacancy is controlled; and the insulating layer (104) is provided with an SiO.sub.2 layer, a high-permittivity first layer, and a high-permittivity second layer, whereby the dipoles generated at the boundary between the SiO.sub.2 layer and the high-permittivity layers are used to control the threshold voltage.

The present invention provides a thin-film transistor in which transistor characteristics such as drain current and threshold voltage are improved, and a method of manufacturing the same. The present invention provides a thin-film transistor provided with a source electrode (108), a drain electrode (109), a semiconductor layer (105), a gate electrode (103), and an insulating layer (104); wherein the semiconductor layer (105) contains a composite metal oxide obtained by adding to a first metal oxide an oxide having an oxygen dissociation energy that is at least 200 kJ/mol greater than the oxygen dissociation energy of the first metal oxide, whereby the amount of oxygen vacancy is controlled; and the insulating layer (104) is provided with an SiO.sub.2 layer, a high-permittivity first layer, and a high-permittivity second layer, whereby the dipoles generated at the boundary between the SiO.sub.2 layer and the high-permittivity layers are used to control the threshold voltage.

A display device includes: a substrate; a transistor disposed on the substrate; a first electrode connected to the transistor; an emission layer disposed on the first electrode; a second electrode disposed on the emission layer; a common voltage line connected to the second electrode; and a third electrode and a fourth electrode disposed between the common voltage line and the second electrode.