A semiconductor device having favorable electrical characteristics is provided. The semiconductor device is manufactured by a first step of forming a semiconductor layer containing a metal oxide, a second step of forming a first insulating layer, a third step of forming a first conductive film over the first insulating layer, a fourth step of etching part of the first conductive film to form a first conductive layer, thereby forming a first region over the semiconductor layer that overlaps with the first conductive layer and a second region over the semiconductor layer that does not overlap with the first conductive layer, and a fifth step of performing first treatment on the conductive layer. The first treatment is plasma treatment in an atmosphere including a mixed gas of a first gas containing an oxygen element but not containing a hydrogen element, and a second gas containing a hydrogen element but not containing an oxygen element.

In a transistor including an oxide semiconductor layer, an oxide insulating layer is formed so as to be in contact with the oxide semiconductor layer. Then, oxygen is introduced (added) to the oxide semiconductor layer through the oxide insulating layer, and heat treatment is performed. Through these steps of oxygen introduction and heat treatment, impurities such as hydrogen, moisture, a hydroxyl group, or hydride are intentionally removed from the oxide semiconductor layer, so that the oxide semiconductor layer is highly purified.

Provided is a novel structure of a field effect transistor using a metal-semiconductor junction. The field effect transistor includes a wiring which is provided over a substrate and also functions as a gate electrode; an insulating film which is provided over the wiring, has substantially the same shape as the wiring, and also functions as a gate insulating film; a semiconductor layer which is provided over the insulating film and includes an oxide semiconductor and the like; an oxide insulating layer which is provided over the semiconductor layer and whose thickness is 5 times or more as large as the sum of the thickness of the insulating film and the thickness of the semiconductor layer or 100 nm or more; and wirings which are connected to the semiconductor layer through openings provided in the oxide insulating layer.

Provided is a novel structure of a field effect transistor using a metal-semiconductor junction. The field effect transistor includes a wiring which is provided over a substrate and also functions as a gate electrode; an insulating film which is provided over the wiring, has substantially the same shape as the wiring, and also functions as a gate insulating film; a semiconductor layer which is provided over the insulating film and includes an oxide semiconductor and the like; an oxide insulating layer which is provided over the semiconductor layer and whose thickness is 5 times or more as large as the sum of the thickness of the insulating film and the thickness of the semiconductor layer or 100 nm or more; and wirings which are connected to the semiconductor layer through openings provided in the oxide insulating layer.

A semiconductor device having favorable electrical characteristics is provided. The semiconductor device is manufactured by a first step of forming a semiconductor layer containing a metal oxide, a second step of forming a first insulating layer, a third step of forming a first conductive film over the first insulating layer, a fourth step of etching part of the first conductive film to form a first conductive layer, thereby forming a first region over the semiconductor layer that overlaps with the first conductive layer and a second region over the semiconductor layer that does not overlap with the first conductive layer, and a fifth step of performing first treatment on the conductive layer. The first treatment is plasma treatment in an atmosphere including a mixed gas of a first gas containing an oxygen element but not containing a hydrogen element, and a second gas containing a hydrogen element but not containing an oxygen element.

A method for manufacturing a semiconductor device is provided. The method includes forming an organosilicon compound layer on a surface of an oxide semiconductor substrate, heating the oxide semiconductor substrate provided with the organosilicon compound layer at a first temperature to form a silicon diffusion layer inside the oxide semiconductor substrate, and removing the organosilicon compound layer from the surface of the oxide semiconductor substrate after heating the oxide semiconductor substrate at the first temperature.

A method for manufacturing a semiconductor device is provided. The method includes forming an organosilicon compound layer on a surface of an oxide semiconductor substrate, heating the oxide semiconductor substrate provided with the organosilicon compound layer at a first temperature to form a silicon diffusion layer inside the oxide semiconductor substrate, and removing the organosilicon compound layer from the surface of the oxide semiconductor substrate after heating the oxide semiconductor substrate at the first temperature.

According to one embodiment, a method for manufacturing a semiconductor device, includes forming a first insulating film which covers a gate electrode, forming an island-shaped oxide semiconductor in contact with the first insulating film, forming a second insulating film which covers the oxide semiconductor, forming a source electrode in contact with the oxide semiconductor, forming a drain electrode in contact with the oxide semiconductor, forming a third insulating film which covers the source electrode and the drain electrode and forming a channel region between the source electrode and the drain electrode by supplying oxygen from the third insulating film to the oxide semiconductor via the second insulating film.

According to one embodiment, a method for manufacturing a semiconductor device, includes forming a first insulating film which covers a gate electrode, forming an island-shaped oxide semiconductor in contact with the first insulating film, forming a second insulating film which covers the oxide semiconductor, forming a source electrode in contact with the oxide semiconductor, forming a drain electrode in contact with the oxide semiconductor, forming a third insulating film which covers the source electrode and the drain electrode and forming a channel region between the source electrode and the drain electrode by supplying oxygen from the third insulating film to the oxide semiconductor via the second insulating film.

A thin film transistor includes a gate electrode, an active layer formed of oxide semiconductor material on a substrate, and a gate insulation layer therebetween. The active layer includes a channel region corresponding to the gate electrode, a source region at one side of the channel region, and a drain region at the other side of the channel region. The source region includes a first upper portion and the drain region includes a second upper portion that includes the oxide semiconductor material and Si.