Semiconductor device, semiconductor memory device, and method for manufacturing semiconductor device

A semiconductor device comprising:

• a first electrode;
• a second electrode;
• an oxide semiconductor layer provided between the first electrode and the second electrode in a first direction, and including a first region, a second region provided between the first region and the first electrode in the first direction, and a third region provided between the first region and the second electrode in the first direction;
• a gate electrode facing the first region;
• and
• a gate insulating layer provided between the first region and the gate electrode, separated from the first electrode, and in contact with the second electrode, wherein
• in a first cross section parallel to the first direction, a first length, which is a maximum length of the first region in a second direction perpendicular to the first direction, is greater than a second length, which is a minimum length of the second region in the second direction, and greater than a third length, which is a minimum length of the third region in the second direction, and
• a second thickness, which is ½ of a difference between the first length and an average value of the second length and the third length, is equal to or greater than 0.75 times a first thickness, which is a thickness of the gate insulating layer in the second direction.