A semiconductor device includes a first transistor comprising a first dielectric film on a substrate and a first work function metal film of a first conductivity type on the first dielectric film, a second transistor comprising a second dielectric film on the substrate and a second work function metal film of the first conductivity type on the second dielectric film, and a third transistor comprising a third dielectric film on the substrate and a third work function metal film of the first conductivity type on the third dielectric film. The first dielectric film comprises a work function tuning material and the second dielectric film does not comprise the work function tuning material. The first work function metal film has different thickness than the third work function metal film. Related methods are also described.