To provide a transistor with stable electric characteristics, provide a transistor having a small current in a non-conductive state, provide a transistor having a large current in a conductive state, provide a semiconductor device including the transistor, or provide a durable semiconductor device, a semiconductor device includes a first insulator containing excess oxygen, a semiconductor over the first insulator, a second insulator over the semiconductor, and a conductor having a region overlapping with the semiconductor with the second insulator provided therebetween. A region containing boron or phosphorus is located between the first insulator and the semiconductor.