It is an object to manufacture a semiconductor device in which a transistor including an oxide semiconductor has normally-off characteristics, small fluctuation in electric characteristics, and high reliability. First, first heat treatment is performed on a substrate, a base insulating layer is formed over the substrate, an oxide semiconductor layer is formed over the base insulating layer, and the step of performing the first heat treatment to the step of forming the oxide semiconductor layer are performed without exposure to the air. Next, after the oxide semiconductor layer is formed, second heat treatment is performed. An insulating layer from which oxygen is released by heating is used as the base insulating layer.

To provide a semiconductor device that includes an oxide semiconductor and is miniaturized while keeping good electrical properties. In the semiconductor device, an oxide semiconductor layer is surrounded by an insulating layer including an aluminum oxide film containing excess oxygen. Excess oxygen in the aluminum oxide film is supplied to the oxide semiconductor layer including a channel by heat treatment in a manufacturing process of the semiconductor device. Furthermore, the aluminum oxide film forms a barrier against oxygen and hydrogen. It is thus possible to suppress the removal of oxygen from the oxide semiconductor layer surrounded by the insulating layer including an aluminum oxide film, and the entry of impurities such as hydrogen into the oxide semiconductor layer; as a result, the oxide semiconductor layer can be made highly intrinsic. In addition, gate electrode layers over and under the oxide semiconductor layer control the threshold voltage effectively.

A thin film transistor includes: a gate electrode; a gate insulating layer above the gate electrode; an oxide semiconductor layer disposed above the gate insulating layer; and a source electrode and a drain electrode disposed above the oxide semiconductor layer and electrically connected to the oxide semiconductor layer, wherein metallic elements included in the oxide semiconductor layer include at least indium (In), fluorine is included in a region which is an internal region in the oxide semiconductor layer and is close to the gate insulating layer, and a fluorine concentration of the region close to the gate insulating layer in the oxide semiconductor layer is higher than a fluorine of a contact region for the source electrode or the drain electrode in the oxide semiconductor layer.

A thin-film transistor includes a substrate, a gate electrode formed over the substrate, a gate insulating layer formed over the gate electrode and the substrate, an oxide semiconductor layer formed over the gate insulating layer and comprising a source section and a drain section, a first electrode formed over the substrate and electrically connected to the source section, and a second electrode formed over the substrate and electrically connected to the drain section. The thin-film transistor further includes a first barrier layer disposed between the oxide semiconductor layer and the first electrode, a second barrier layer disposed between the first barrier layer and the first electrode, and the first electrode being electrically connected to the oxide semiconductor layer via the first barrier layer and the second barrier layer.

An object is to reduce to reduce variation in threshold voltage to stabilize electric characteristics of thin film transistors each using an oxide semiconductor layer. An object is to reduce an off current. The thin film transistor using an oxide semiconductor layer is formed by stacking an oxide semiconductor layer containing insulating oxide over the oxide semiconductor layer so that the oxide semiconductor layer and source and drain electrode layers are in contact with each other with the oxide semiconductor layer containing insulating oxide interposed therebetween; whereby, variation in threshold voltage of the thin film transistors can be reduced and thus the electric characteristics can be stabilized. Further, an off current can be reduced.

A semiconductor device including a transistor is provided. The transistor includes a gate electrode, a first insulating film over the gate electrode, a second insulating film over the first insulating film, an oxide semiconductor film over the second insulating film, a source electrode and a drain electrode electrically connected to the oxide semiconductor film, a third insulating film over the source electrode, and a fourth insulating film over the drain electrode. A fifth insulating film including oxygen is provided over the transistor. The third insulating film includes a first portion, the fourth insulating film includes a second portion, and the fifth insulating film includes a third portion. The amount of oxygen molecules released from each of the first portion and the second portion is smaller than the amount of oxygen molecules released from the third portion when the amounts are measured by thermal desorption spectroscopy.

This thin film transistor comprises, on a substrate, at least a gate electrode, a gate insulating film, an oxide semiconductor layer, a source-drain electrode, and two or more protective films. The oxide semiconductor layer comprises Sn, O and one or more elements selected from the group consisting of In, Ga and Zn. In addition, the two or more protective films are composed of at least a first protective film that is in contact with the oxide semiconductor film, and one or more second protective films other than the first protective film. The first protective film is a SiO.sub.x film having a hydrogen concentration of 3.5 atomic % or lower.

A thin film transistor includes a gate electrode on a substrate, a gate insulating layer on the gate electrode, a semiconductor layer on the gate insulating layer, the semiconductor layer overlapping at least a portion of the gate electrode, a plurality of etch stoppers on the semiconductor layer, and a source electrode and a drain electrode spaced apart from each other and disposed on the etch stoppers and the semiconductor layer, wherein a plurality of channel regions are defined in the semiconductor layer by the etch stoppers on the semiconductor layer.

A thin film transistor substrate includes a gate electrode disposed on a base substrate, an active pattern overlapping the gate electrode, a source metal pattern including both a source electrode disposed on the active pattern and a drain electrode spaced apart from the source electrode, a buffer layer disposed on the source metal pattern and contacting the active pattern, a first passivation layer disposed on the buffer layer and a second passivation layer disposed on the first passivation layer. The density of hydrogen in the buffer layer is greater than the density of hydrogen in the first passivation layer and less than the density of hydrogen in the second passivation layer.

A change in electrical characteristics is inhibited and reliability is improved in a semiconductor device using a transistor including an oxide semiconductor. One embodiment of a semiconductor device including a transistor includes a gate electrode, first and second insulating films over the gate electrode, an oxide semiconductor film over the second insulating film, and source and drain electrodes electrically connected to the oxide semiconductor film. A third insulating film is provided over the transistor and a fourth insulating film is provided over the third insulating film. The third insulating film includes oxygen. The fourth insulating film includes nitrogen. The amount of oxygen released from the third insulating film is 110.sup.19/cm.sup.3 or more by thermal desorption spectroscopy, which is estimated as oxygen molecules. The amount of oxygen molecules released from the fourth insulating film is less than 110.sup.19/cm.sup.3.