The object is to suppress deterioration in electrical characteristics in a semiconductor device comprising a transistor including an oxide semiconductor layer. In a transistor in which a channel layer is formed using an oxide semiconductor, a p-type silicon layer is provided in contact with a surface of the oxide semiconductor layer. Further, the p-type silicon layer is provided in contact with at least a region of the oxide semiconductor layer, in which a channel is formed, and a source electrode layer and a drain electrode layer are provided in contact with regions of the oxide semiconductor layer, over which the p-type silicon layer is not provided.