Semiconductor device and method for manufacturing the same

A method for manufacturing a semiconductor device with a single-gate transistor, comprising the steps of:

• forming an oxide semiconductor layer containing indium and zinc over an insulating surface;
• forming a source electrode and a drain electrode in contact with part of the oxide semiconductor layer;
• forming a gate insulating film over the oxide semiconductor layer, the source electrode, and the drain electrode;
• and
• adding oxygen to the oxide semiconductor layer in a region that does not overlap with the source electrode and the drain electrode after the formation of the gate insulating film, wherein the oxygen is added by an ion implantation method, an ion doping method or a plasma immersion ion implantation method.