Semiconductor device

A method for forming a semiconductor device comprising the steps of:

• forming an oxide semiconductor film comprising a channel formation region;
• forming a metal oxide film over and in contact with the oxide semiconductor film;
• performing a heat treatment after forming the metal oxide film;
• forming a source electrode and a drain electrode over the metal oxide film, the source electrode and the drain electrode being in contact with the oxide semiconductor film;
• and
• forming an electrode over the metal oxide film,
• wherein the electrode overlaps with the channel formation region with the metal oxide film interposed therebetween,
• wherein the oxide semiconductor film contains one or more metal elements,
• wherein the metal oxide film contains at least one of the metal elements contained in the oxide semiconductor film, and
• wherein a side surface of the oxide semiconductor film is in contact with the metal oxide film.

A method for forming a semiconductor device comprising the steps of:

• forming a gate electrode;
• forming a gate insulating film over the gate electrode;
• forming a first metal oxide film over the gate insulating film;
• forming an oxide semiconductor film comprising a channel formation region over the first metal oxide film;
• forming a second metal oxide film over and in contact with the oxide semiconductor film;
• performing a heat treatment after forming the second metal oxide film;
• and
• forming a source electrode and a drain electrode over the second metal oxide film, the source electrode and the drain electrode being in contact with the oxide semiconductor film,
• wherein the oxide semiconductor film contains one or more metal elements,
• wherein each of the first metal oxide film and the second metal oxide film contains at least one of the metal elements contained in the oxide semiconductor film, and
• wherein a side surface of the oxide semiconductor film is in contact with the second metal oxide film.