Patent ID: 8759167

Claim:
A method of manufacturing a semiconductor device comprising the steps of: forming a gate insulating film over a gate electrode; forming a first non-single crystalline oxide semiconductor film comprising indium by sputtering over the gate electrode with the gate insulating film interposed therebetween; forming a second non-single crystalline oxide semiconductor film comprising indium by sputtering over the first non-single crystalline oxide semiconductor film, the second non-single crystalline oxide semiconductor film having a lower electrical conductivity than the first non-single crystalline oxide semiconductor film; etching the first non-single crystalline oxide semiconductor film and the second non-single crystalline oxide semiconductor film to form a first oxide semiconductor layer and a second oxide semiconductor layer on the first oxide semiconductor layer; forming a conductive layer over the second oxide semiconductor layer; etching the conductive layer to form a source electrode and a drain electrode, wherein part of the second oxide semiconductor layer between the source electrode and the drain electrode is etched so that a thickness of the second oxide semiconductor layer between the source electrode and the drain electrode is smaller than thicknesses of the second oxide semiconductor layer below the source electrode and the drain electrode; and forming an insulating film comprising silicon over the second oxide semiconductor layer, the source electrode and the drain electrode, wherein an upper surface of the second oxide semiconductor layer between the source electrode and the drain electrode contacts the insulating film.