A semiconductor device includes a gate structure on a substrate, a single diffusion break (SDB) structure adjacent to the gate structure, a first spacer adjacent to the gate structure, a second spacer adjacent to the SDB structure, a source/drain region between the first spacer and the second spacer, an interlayer dielectric (ILD) layer around the gate structure and the SDB structure, and a contact plug in the ILD layer and on the source/drain region. Preferably, a top surface of the second spacer is lower than a top surface of the first spacer.