Semiconductor device and manufacturing method thereof

A semiconductor device comprising:

• a gate electrode layer over an insulating surface;
• a gate insulating layer over the gate electrode layer;
• an oxide semiconductor layer over the gate insulating layer;
• an oxide insulating layer over the oxide semiconductor layer;
• a source electrode layer and a drain electrode layer over the oxide semiconductor layer;
• and
• an insulating layer over the source electrode layer and the drain electrode layer,
• wherein the oxide semiconductor layer comprises a first region in contact with the oxide insulating layer, a second region in contact with the source electrode layer or the drain electrode layer, and a third region in contact with the insulating layer,
• wherein, in the first region, a channel formation region overlaps with the gate electrode layer with the gate insulating layer interposed therebetween, and
• wherein the third region is located between the channel formation region and the second region.

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

• forming a gate insulating layer over a gate electrode layer;
• forming an oxide semiconductor layer over the gate insulating layer;
• dehydrating or dehydrogenating the oxide semiconductor layer;
• forming an oxide insulating layer being in contact with part of the oxide semiconductor layer and covering a peripheral portion and a side surface of the oxide semiconductor layer;
• forming a source electrode layer and a drain electrode layer over the oxide insulating layer;
• and
• forming an insulating layer in contact with the oxide insulating layer, the source electrode layer, the drain electrode layer, and the oxide semiconductor layer,
• wherein the oxide semiconductor layer is prevented from being exposed to air between the step of dehydrating or dehydrogenating the oxide semiconductor layer and the step of forming the oxide insulating layer, so as to prevent water or hydrogen contamination.