Abstract: A semiconductor device includes a fin structure, a metal gate stack, a barrier structure and an epitaxial source/drain region. The fin structure is over a substrate. The metal gate stack is across the fin structure. The barrier structure is on opposite sides of the metal gate stack. The barrier structure comprises one or more passivation layers and one or more barrier layers, and the one or more passivation layers have a material different from a material of the one or more barrier layers. The epitaxial source/drain region is over the barrier structure.