Methods of manufacturing a semiconductor device include forming a gate insulation layer including a high-k dielectric material on a substrate that is divided into a first region and a second region; forming a diffusion barrier layer including a first metal on a second portion of the gate insulation layer in the second region; forming a diffusion layer on the gate insulation layer and the diffusion barrier layer; and diffusing an element of the diffusion layer into a first portion of the gate insulation layer in the first region.