High field-effect mobility is provided for a semiconductor device including an oxide semiconductor. Further, a highly reliable semiconductor device including the transistor is provided. In a transistor in which a stack of oxide semiconductor layers is provided over a gate electrode layer with a gate insulating layer provided therebetween, an oxide semiconductor layer functioning as a current path (channel) of the transistor and containing an n-type impurity is sandwiched between oxide semiconductor layers having lower conductivity than the oxide semiconductor layer. In the oxide semiconductor layer functioning as the channel, a region on the gate insulating layer side contains the n-type impurity at a higher concentration than a region on the back channel side. With such a structure, the channel can be separated from the interface between the oxide semiconductor stack and the insulating layer in contact with the oxide semiconductor stack, so that a buried channel can be formed.