A semiconductor device includes a substrate, a carbon-containing diffusion barrier, a phosphorus-containing source/drain feature, a gate structure, and a gate spacer. The substrate has a channel region. The carbon-containing diffusion barrier is present in the substrate. The phosphorus-containing source/drain feature is present in the substrate, and the carbon-containing diffusion barrier is between the channel region and the phosphorus-containing source/drain feature. The gate is present over the channel region of the substrate. The gate spacer abuts the gate structure and is present over a portion of the phosphorus-containing source/drain feature.