A semiconductor device includes a semiconductor substrate, a capacitor structure, a first contact plug, and a spacer. The capacitor structure is over the semiconductor substrate. The capacitor structure includes a bottom electrode, a capacitor dielectric, and a top electrode. The bottom electrode is over the semiconductor substrate. The capacitor dielectric is over a first portion of the bottom electrode. The top electrode is over the capacitor dielectric. The first contact plug is on and connected to a second portion of the bottom electrode. The spacer is on at least a sidewall of the second portion of the bottom electrode.