A semiconductor device includes a substrate having an active region and a device isolation layer defining the active region, a gate electrode on the active region, source/drain regions at the active region at both sides of the gate electrode, a buffer insulating layer on the device isolation layer, an etch stop layer formed on the buffer insulating layer and extending onto the gate electrode and the source/drain region, a first interlayer insulating layer on the etch stop layer, a first contact and a second contact penetrating the first interlayer insulating layer and the etch stop layer. The first contact and the second contact are spaced apart from each other and are in contact with the source/drain region and the buffer insulating layer, respectively.