Method for manufacturing semiconductor device

A method for manufacturing a semiconductor device, comprising:

• forming a first conductive film that serves as a first gate electrode of a transistor over a substrate;
• forming a first insulating film over the first conductive film;
• forming an oxide semiconductor layer that comprises a channel formation region over the first insulating film;
• forming a source electrode and a drain electrode that are electrically connected to the oxide semiconductor layer;
• forming a second insulating film over the oxide semiconductor layer, the source electrode, and the drain electrode;
• and
• forming a transparent film that comprises an oxide including indium and zinc over the second insulating film by a sputtering method,
• wherein the transparent film serves as a second gate electrode of the transistor,
• wherein the first conductive film and the transparent film overlap each other with the oxide semiconductor layer therebetween, and
• wherein the transparent film has a larger width than the oxide semiconductor layer both in a channel length direction and a channel width direction of the transistor.

A method for manufacturing a semiconductor device, comprising:

• forming a first conductive film that serves as a first gate electrode of a transistor over a substrate;
• forming a first insulating film over the first conductive film;
• forming an oxide semiconductor layer that comprises a channel formation region over the first insulating film;
• forming a source electrode and a drain electrode by patterning a second conductive film formed over the oxide semiconductor layer, whereby a top surface of a first region of the oxide semiconductor layer that is located between the source electrode and the drain electrode is etched to have a smaller thickness than a second region of the oxide semiconductor layer that is below the source electrode or the drain electrode;
• forming a second insulating film over the oxide semiconductor layer, the source electrode, and the drain electrode;
• and
• forming a transparent film that comprises an oxide including indium and zinc over the second insulating film by a sputtering method,
• wherein the transparent film serves as a second gate electrode of the transistor,
• wherein the first conductive film and the transparent film overlap each other with the oxide semiconductor layer therebetween.

A method for manufacturing a semiconductor device, comprising:

• forming a first conductive film that serves as a first gate electrode of a transistor over a substrate;
• forming a first insulating film over the first conductive film;
• forming an oxide semiconductor layer that comprises a channel formation region over the first insulating film;
• forming a second insulating film over the oxide semiconductor layer;
• and
• forming a transparent film that comprises an oxide including indium and zinc over the second insulating film by a sputtering method,
• wherein the transparent film serves as a second gate electrode of the transistor,
• wherein the first conductive film and the transparent film overlap each other with the oxide semiconductor layer therebetween,
• wherein the transparent film has a larger width than the oxide semiconductor layer in a channel width direction of the transistor, and
• wherein the second insulating film is an oxide insulating film.