A semiconductor device having stable electrical characteristics is provided. Alternatively, a highly reliable semiconductor device suitable for miniaturization or high integration is provided. The semiconductor device includes a first barrier layer, a second barrier layer, a third barrier layer, a transistor including an oxide, an insulator, and a conductor. The insulator includes an oxygen-excess region. The insulator and the oxide are between the first barrier layer and the second barrier layer. The conductor is in an opening of the first barrier layer, an opening of the second barrier layer, and an opening of the insulator with the third barrier layer positioned therebetween.