Semiconductor device and method for manufacturing the same

A semiconductor device comprising a transistor, the transistor comprising:na first oxide semiconductor layer;a second oxide semiconductor layer over the first oxide semiconductor layer;a source electrode over the second oxide semiconductor layer; anda drain electrode over the second oxide semiconductor layer,wherein the first oxide semiconductor layer includes a channel formation region,wherein the second oxide semiconductor layer includes a first region overlapping with the channel formation region,wherein the second oxide semiconductor layer includes a second region in contact with the source electrode,wherein the second oxide semiconductor layer includes a third region in contact with the drain electrode,wherein the first region is located between the source electrode and the drain electrode,wherein a thickness of each of the second region and the third region is thicker than a thickness of the first region,wherein the first region has a first electrical conductivity,wherein the second region has a second electrical conductivity,wherein the third region has a third electrical conductivity, andwherein each of the second electrical conductivity and the third electrical conductivity is higher than the first electrical conductivity.

A semiconductor device comprising a transistor, the transistor comprising:na first oxide semiconductor layer;a second oxide semiconductor layer over the first oxide semiconductor layer;a source electrode over the second oxide semiconductor layer; anda drain electrode over the second oxide semiconductor layer,wherein the first oxide semiconductor layer includes a channel formation region,wherein the second oxide semiconductor layer includes a first region overlapping with the channel formation region,wherein the second oxide semiconductor layer includes a second region in contact with the source electrode,wherein the second oxide semiconductor layer includes a third region in contact with the drain electrode,wherein the first region is located between the source electrode and the drain electrode,wherein a thickness of each of the second region and the third region is thicker than a thickness of the first region,wherein the first region has a first electrical conductivity,wherein the second region has a second electrical conductivity,wherein the third region has a third electrical conductivity,wherein each of the second electrical conductivity and the third electrical conductivity is higher than the first electrical conductivity, andwherein the first region contains oxygen at a higher concentration than the second region and the third region.