Patent ID: 8216878

Claim:
A method for manufacturing a semiconductor device comprising: forming a gate electrode layer over a substrate having an insulating surface; 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 to be dehydrated or dehydrogenated; then slowly cooling the first oxide semiconductor film and the second oxide semiconductor film in an oxygen atmosphere; selectively etching the first oxide semiconductor film and the second oxide semiconductor film after the slowly cooling 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; selectively etching the first oxide semiconductor layer, the second oxide semiconductor layer, and the conductive film to form a semiconductor layer, a source region, a drain region, a source electrode layer, and a drain electrode layer; and forming an oxide insulating film which is contact with part of the semiconductor layer over the gate insulating layer, the semiconductor layer, the source region, the drain region, the source electrode layer, and the drain electrode layer so that carrier density is reduced.