A semiconductor device includes a semiconductor substrate, first gate structure, a first metal layer, a first protective layer, and a first contact plug. The first gate structure includes a plurality of first U-shaped layers stacked one another between the first gate spacers in a cross-sectional view and first gate spacers on opposite sides of the first U-shaped layers. The first metal layer is over the first U-shaped layers and has a different shape than the first U-shaped layers in the cross-sectional view. The first protective layer is over the first metal layer and between the first gate spacers. The first contact plug extends through the first protective layer and the first metal layer, and is in contact with the first gate structure.