A device and a method of forming the same are provided. The device includes a substrate, a first dielectric layer over the substrate, a bottom electrode extending through the first dielectric layer, a first buffer layer over the bottom electrode, a phase-change layer over the first buffer layer, a top electrode over the phase-change layer, and a second dielectric layer over the first dielectric layer. The second dielectric layer surrounds the phase-change layer and the top electrode. A width of the top electrode is greater than a width of the bottom electrode.