Semiconductor device

A semiconductor device comprising:

• an oxide semiconductor film over a substrate;
• and
• an insulating film over the oxide semiconductor film, wherein the insulating film comprises an oxide containing silicon,
• wherein the oxide semiconductor film comprises a region in which a concentration of silicon is lower than or equal to 1.0 at. %, and
• wherein the region is located at an interface between the oxide semiconductor film and the insulating film and in contact with the insulating film.

A semiconductor device comprising:

• a first transistor comprising:
• a channel formation region comprising silicon;
• a gate insulating film over the channel formation region;
• and
• a gate electrode;
• and
• a second transistor over the first transistor, the second transistor comprising:
• an oxide semiconductor film;
• and
• an insulating film over the oxide semiconductor film, wherein the insulating film comprises an oxide containing silicon,
• wherein the oxide semiconductor film comprises a region in which a concentration of silicon is lower than or equal to 1.0 at. %, and
• wherein the region is located at an interface between the oxide semiconductor film and the insulating film and in contact with the insulating film.