A semiconductor device 101 includes: a substrate 1; an active layer 4 provided on the substrate 1 and including a channel region 4c, and a first region 4a and a second region 4b that are respectively located on opposite sides of the channel region 4c; first and second contact layers 6a and 6b respectively in contact with the first and second regions 4a and 4b of the active layer 4; a first electrode 7 electrically coupled to the first region 4a via the first contact layer 6a; a second electrode 8 electrically coupled to the second region 4b via the second contact layer 6b; and a gate electrode 2 provided such that a gate insulating layer 3 is interposed between the gate electrode 2 and the active layer 4, the gate electrode 2 being configured to control a conductivity of the channel region 4c. The active layer 4 contains silicon. The semiconductor device further includes an oxygen-containing silicon layer 5 between the active layer 4 and the first and second contact layers 6a, 6b. The layer 5 contains oxygen at a concentration higher than the active layer 4 and the first and second contact layers 6a, 6b.