A device includes a substrate, a semiconductor layer, a gate structure, source/drain regions, a bottom isolation layer, and a bottom spacer. The semiconductor layer is above the substrate. The gate structure is above the substrate and surrounds the semiconductor layer. The source/drain regions are on opposite sides of the semiconductor layer. The bottom isolation layer is between the substrate and the semiconductor layer. The bottom spacer is on a sidewall of the bottom isolation layer. The bottom isolation layer has a seam therein, and the seam exposes a sidewall of the bottom spacer.