A semiconductor device including a substrate includes 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 lower TiN layer on and in contact with the first gate insulating layer, a first etch-stop layer on the first lower TiN layer and a first upper gate electrode on the first etch-stop layer. The second transistor includes a second gate insulating layer on the substrate, a second lower TiN layer on and in contact with the second gate insulating layer, a second etch-stop layer on the second lower TiN layer and a second upper gate electrode on the second etch-stop layer. A thickness of the first lower TiN layer is less than a thickness of the second lower TiN layer.