A semiconductor device including a substrate with a first region and a second region and first and second transistors in the first and second regions, respectively. The first transistor includes a first gate insulating layer on the substrate, a first etch-stop layer, and a first work function layer on the first etch-stop layer. The second transistor includes a second gate insulating layer on the substrate, a second etch-stop layer, and a second work function layer on the second etch-stop layer. At least one of the first and second work function layers is chamfered.