A semiconductor device including a substrate; a first and second active region on the substrate; a first recess intersecting with the first active region; a second recess intersecting with the second active region; a gate spacer extending along sidewalls of the first and second recess; a first lower high-k dielectric film in the first recess and including a first high-k dielectric material in a first concentration and a second high-k dielectric material; a second lower high-k dielectric film in the second recess and including the first high-k dielectric material in a second concentration that is greater than the first concentration, and the second high-k dielectric material; a first metal-containing film on the first lower high-k dielectric film and including silicon in a third concentration; and a second metal-containing film on the second lower high-k dielectric film and including silicon in a fourth concentration that is smaller than the third concentration.