Semiconductor device comprising an oxide semiconductor

A semiconductor device comprising:

• a base insulating layer;
• a first oxide layer over the base insulating layer;
• an oxide semiconductor layer over the first oxide layer;
• a second oxide layer over the oxide semiconductor layer;
• a first source electrode layer and a first drain electrode layer each of which is in contact with a top surface of the second oxide layer;
• a second source electrode layer and a second drain electrode layer over the first source electrode layer and the first drain electrode layer, respectively, and in contact with the top surface of the second oxide layer;
• a gate insulating layer over the second source electrode layer and the second drain electrode layer, and in contact with the top surface of the second oxide layer between the second source electrode layer and the second drain electrode layer;
• and
• a gate electrode layer overlapping with the oxide semiconductor layer with the gate insulating layer provided therebetween,
• wherein the base insulating layer and the gate insulating layer are in contact with each other.