A semiconductor device includes a diffusion barrier structure, a bottom electrode, a top electrode over the bottom electrode, a switching layer and a capping layer. The bottom electrode is over the diffusion barrier structure. The top electrode is over the bottom electrode. The switching layer is between the bottom electrode and the top electrode, and configured to store data. The capping layer is between the top electrode and the switching layer. A thermal conductivity of the diffusion barrier structure is greater than approximately 20 W/mK.