Conventionally, a transistor using silicon as a semiconductor layer has been used in a display device such as a liquid crystal display device or an organic EL display device. In recent years, demands for increasing the area, increasing resolution and increasing the frame rate and the like in display devices have been increasing, and efforts to satisfy these demands have been actively carried out.
Thus, recently, development of a transistor using an oxide semiconductor instead of silicon has been proceeding. It is expected that a transistor including an oxide semiconductor can realize high mobility. In particular, it is possible to form an oxide semiconductor layer formed from IGZO at a relatively low temperature with a large area. As a result, oxide semiconductors are attracting attention as a material which satisfies the demands described above (for example, Japanese Laid Open Patent Application No. 2015-135962).