Semiconductor device and method for manufacturing the same

A semiconductor device comprising:

• a first protective insulating layer containing aluminum oxide over a semiconductor substrate;
• a first oxide layer over the first protective insulating layer;
• an oxide semiconductor layer comprising a channel formation region over the first oxide layer;
• a second oxide layer over the oxide semiconductor layer and overlapping with the channel formation region;
• an insulating layer over the second oxide layer and overlapping with the channel formation region;
• a gate electrode layer over the insulating layer;
• a low-resistance region on the oxide semiconductor layer;
• and
• a second protective insulating layer containing aluminum oxide over the gate electrode layer and the low-resistance region,
• wherein the low-resistance region has a lower resistance than the channel foimation region,
• wherein a side surface of the first oxide layer and a side surface of the oxide semiconductor layer are substantially aligned with each other,
• wherein the second oxide layer is in contact with a top surface and the side surface of the oxide semiconductor layer,
• wherein the gate electrode layer surrounds the top surface and the side surface of the oxide semiconductor layer,
• wherein the second protective insulating layer is in contact with a side surface of the low-resistance region and side surfaces of the second oxide layer,
• wherein the second protective insulating layer comprises a region in contact with the first protective insulating layer,
• wherein the first oxide layer, the oxide semiconductor layer, and the second oxide layer each comprises an oxide comprising In, an element M, and Zn,
• wherein the element M is any one of Al, Ti, Ga, Y, Zr, Sn, La, Ce, Nd, and Hf, and
• wherein an atomic ratio of the element M to In in each of the first and second oxide layers is higher than an atomic ratio of the element M to In in the oxide semiconductor layer.