A semiconductor device and a method for fabricating the same are provided. The semiconductor device includes first and second gate stack structures formed in first and second regions, respectively, wherein the first gate stack structure is formed adjacent a first channel region and comprises a first gate insulating film having a first thickness formed on the first channel region, a first function film having a second thickness formed on the first gate insulating film and a first filling film having a third thickness formed on the first function film, wherein the second gate stack structure is formed adjacent a second channel region and comprises a second gate insulating film having the first thickness formed on the second channel region, a second function film having the second thickness formed on the second gate insulating film and a second filling film having the third thickness formed on the second function film, wherein the first and second function films, respectively, comprise TiN and Si concentrations that are different from each other.