A semiconductor device includes a substrate having a first region and a second region, a first gate structure disposed on the substrate within the first region, a first S/D region, a first S/D contact, a second gate structure on the substrate within the second region, a second S/D region and a second S/D contact. The first S/D region is disposed in the substrate within the first region and beside the first gate structure. The first S/D contact is connected to the first S/D region. The second S/D region is disposed in the substrate within the second region and beside the second gate structure. The second S/D contact is connected to the second S/D region. The contact area between the second S/D region and the second S/D contact is larger than a contact area between the first S/D region and the first S/D contact.