1. Field of the Invention
The present invention relates to a transistor and a method for manufacturing the transistor. In particular, the present invention relates to a transistor in which a channel is formed in an oxide semiconductor layer and a method for manufacturing the transistor. Further, the present invention relates to a semiconductor device including the transistor and a method for manufacturing the semiconductor device.
Note that “semiconductor device” in this specification indicates all devices that can operate by utilizing semiconductor characteristics. For example, electro-optical devices, semiconductor circuits, and electronic devices are all included in the category of the semiconductor device.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films over a substrate having an insulating surface is known. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).