A semiconductor device includes a semiconductor layer formed on part of an insulating layer. The semiconductor layer includes a diffusion region and a channel region. The insulating layer is etched so that the semiconductor layer is separated from the insulating layer below at least part of the diffusion region. The space left below this part of the semiconductor layer is filled by an etch stop film that also covers the side surfaces of the insulating layer. The etch stop film prevents contact holes targeted at the diffusion region from penetrating the insulating layer due to alignment error or defects in the semiconductor layer. Since the etch stop film is not present below the channel region, the electrical characteristics of the semiconductor device are not altered.