A semiconductor device includes a bottom electrode, a top electrode over the bottom electrode, a switching layer between the bottom electrode and the top electrode, wherein the switching layer is configured to store data, a capping layer in contact with the switching layer, wherein the capping layer is configured to extract active metal ions from the switching layer, an ion reservoir region formed in the capping layer, a diffusion barrier layer between the bottom electrode and the switching layer, wherein the diffusion barrier layer includes palladium (Pd), cobalt (Co), or a combination thereof and is configured to obstruct diffusion of the active metal ions between the switching layer and the bottom electrode, and the diffusion layer has a concaved top surface, and a passivation layer covering a portion of the top electrode, and wherein the passivation layer directly contacts a top surface of the switching layer.