A semiconductor device includes a substrate having a first region and a second region and a gate structure on the first region and the second region of the substrate. The gate structure includes a first bottom barrier metal (BBM) layer on the first region and the second region, a first work function metal (WFM) layer on the first region; and a diffusion barrier layer on a top surface and a sidewall of the first WFM layer on the first region and the first BBM layer on the second region. Preferably, a thickness of the first BBM layer on the second region is less than a thickness of the first BBM layer on the first region.