Method for manufacturing semiconductor device

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

• forming a gate electrode layer over a substrate;
• forming a gate insulating layer over the gate electrode layer;
• forming a first oxide semiconductor layer over the gate insulating layer;
• forming a second oxide semiconductor layer over the first oxide semiconductor layer;
• performing dehydration or dehydrogenation on the first oxide semiconductor layer and the second oxide semiconductor layer;
• forming a source electrode layer and a drain electrode layer over the second oxide semiconductor layer;
• and
• forming an oxide insulating layer over the first oxide semiconductor layer so that a carrier concentration in a part of the first oxide semiconductor layer is reduced,
• wherein the steps of forming the first oxide semiconductor layer and forming the second oxide semiconductor layer are successively performed without exposure to air, and
• wherein the part of the first oxide semiconductor layer does not overlap with the source electrode layer and the drain electrode layer.

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

• forming a first oxide semiconductor layer over a substrate;
• forming a second oxide semiconductor layer over the first oxide semiconductor layer;
• performing a first treatment on the first oxide semiconductor layer and the second oxide semiconductor layer so that each of the first oxide semiconductor layer and the second oxide semiconductor layer are oxygen-deficiency type or n-type oxide semiconductor layers;
• forming a first conductive layer and a second conductive layer over and in direct contact the second oxide semiconductor layer;
• and
• performing a second treatment after forming the first conductive layer and the second conductive layer so that a part of the first oxide semiconductor layer is an oxygen-excess type or i-type oxide semiconductor layer,
• wherein the steps of forming the first oxide semiconductor layer and forming the second oxide semiconductor layer are successively performed without exposure to air, and
• wherein the part of the first oxide semiconductor layer is between the first conductive layer and the second conductive layer.