Semiconductor device and method of manufacturing the same

A method of manufacturing a semiconductor device, comprising:

• forming an oxide semiconductor layer;
• forming a first insulating layer over the oxide semiconductor layer;
• forming a crystalline oxide semiconductor layer by performing heat treatment on the oxide semiconductor layer;
• forming a gate electrode over the crystalline oxide semiconductor layer with the first insulating layer positioned therebetween;
• forming a gate insulating layer by etching the first insulating layer with the use of the gate electrode as a mask;
• forming a crystalline region containing nitrogen in the crystalline oxide semiconductor layer by performing nitrogen plasma treatment on part of the crystalline oxide semiconductor layer, the part is not covered with the gate insulating layer;
• forming a second insulating layer covering the crystalline oxide semiconductor layer and the gate electrode;
• forming, in the second insulating layer, openings in regions overlapping with a source region and a drain region;
• and
• forming, over the second insulating layer, a source electrode and a drain electrode respectively in contact with the source region and the drain region through the openings.

A semiconductor device comprising:

• a gate electrode;
• a gate insulating layer over the gate electrode;
• a crystalline oxide semiconductor layer over the gate electrode with the gate insulating layer positioned therebetween, the crystalline oxide semiconductor layer comprising an n-type source region, an n-type drain region, and a channel formation region;
• and
• a channel protective layer overlapping with the channel formation region.