Semiconductor device and manufacturing method thereof

To offer a semiconductor device including a thin film transistor having excellent characteristics and high reliability and a method for manufacturing the semiconductor device without variation. The summary is to include an inverted-staggered (bottom-gate structure) thin film transistor in which an oxide semiconductor film containing In, Ga, and Zn is used for a semiconductor layer and a buffer layer is provided between the semiconductor layer and source and drain electrode layers. An ohmic contact is formed by intentionally providing a buffer layer containing In, Ga, and Zn and having a higher carrier concentration than the semiconductor layer between the semiconductor layer and the source and drain electrode layers.