A device includes a channel layer, a gate structure, a source/drain epitaxial structure, and a gate via. The gate structure wraps around the channel layer. The gate structure includes a gate dielectric layer and a gate electrode over the gate dielectric layer. The source/drain epitaxial structure is adjacent the gate structure and is electrically connected to the channel layer. The gate via is under the gate structure and is in contact with a bottom surface of the gate electrode.