A semiconductor device includes first and second transistors on a substrate. The first transistor includes a first N-type active region, a first gate electrode having a first work function layer, and a first gate dielectric layer having high-k dielectrics containing La. The first work function layer includes a first layer having TiON, a second layer having TiN or TiON, a third layer having TiON, a fourth layer having TiN, and a fifth layer having TiAlC. The second transistor includes a first P-type active region, a second gate electrode having a second work function layer, and a second gate dielectric layer having high-k dielectrics. The second work function layer includes the fifth layer directly contacting the second gate dielectric layer.