Abstract: A semiconductor device that has a dual gate having different work functions is simply formed by using a selective nitridation. A gate insulating layer is formed on a semiconductor substrate including a first region and a second region, on which devices having different threshold voltages are to be formed. A diffusion inhibiting material is selectively injected into the gate insulating layer in one of the first region and the second region. A diffusion layer is formed on the gate insulating layer. A work function controlling material is directly diffused from the diffusion layer to the gate insulating layer using a heat treatment, wherein the gate insulting layer is self-aligned capped with the selectively injected diffusion inhibiting material so that the work function controlling material is diffused into the other of the first region and the second region. The gate insulating layer is entirely exposed by removing the diffusion layer. A gate electrode layer is formed on the exposed gate insulating layer.