The present invention provides a semiconductor device which includes an oxide semiconductor to be used in a power device for high power. Provided is a transistor which has a large on current and performs the secured on and off operations by using an oxide semiconductor layer including an n-type region as a main path of a current (channel forming region) and providing the oxide semiconductor layer including an i-type region between a source electrode layer and the oxide semiconductor layer including the n-type region. Also, a dual well structure is formed in an energy band diagram in the thickness direction of a laminate structure by laminating an oxide layer including one kind of the component of the oxide semiconductor layer on the lower and upper sides of the oxide semiconductor layer including the n-type region.
