A device includes a substrate, a channel layer, a gate structure, a first source/drain epitaxial structure, a second source/drain epitaxial structure, and a bottom dielectric structure. The channel layer is over the substrate. The gate structure is across the channel layer. The first source/drain epitaxial structure and the second source/drain epitaxial structure are on opposite sides of the channel layer and are connected to the channel layer. The bottom dielectric structure is between the first source/drain epitaxial structure and the substrate. A maximum width of the first source/drain epitaxial structure is greater than or equal to a maximum width of the bottom dielectric structure in a cross-sectional view.