Patent ID: 8609478

Claim:
A method for manufacturing a semiconductor device, comprising the steps of: forming a gate electrode layer; forming a gate insulating layer over the gate electrode layer; forming a first oxide semiconductor film over the gate insulating layer; forming a second oxide semiconductor film over the first oxide semiconductor film; heating the first oxide semiconductor film and the second oxide semiconductor film under an inert gas atmosphere so that a carrier concentration in the second oxide semiconductor film is increased; selectively etching the first oxide semiconductor film and the second oxide semiconductor film so as to form a first oxide semiconductor layer and a second oxide semiconductor layer; forming a conductive film over the first oxide semiconductor layer and the second oxide semiconductor layer; forming a semiconductor layer, a source region, a drain region, a source electrode layer, and a drain electrode layer by selectively etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film, wherein a part of the semiconductor layer has a depression; and forming an oxide insulating film over the semiconductor layer, the source electrode layer, and the drain electrode layer, wherein the oxide insulating film is in contact with the part of the semiconductor layer so that a carrier concentration in the part of the semiconductor layer is reduced.