To provide a semiconductor device with high design flexibility. A first transistor and a second transistor having different electrical characteristics from those of the first transistor are provided over the same layer without significantly increasing the number of manufacturing steps. A semiconductor layer where a channel of the first transistor is formed and a semiconductor layer where a channel of the second transistor is formed are formed using semiconductor materials having different electron affinities. When an oxide semiconductor is used for the semiconductor layer, an insulating layer containing excess oxygen is used as an insulating layer below the semiconductor layer. By increasing the thickness of the insulating layer, a large amount of oxygen can be supplied to the semiconductor layer.