A semiconductor comprises two transistors of the first conductivity type separated from two transistors of a second conductivity type by a first element isolation layer. Further, the two transistors of the first conductivity type are separated from each other by a second element isolation layer and the two transistors of the second conductivity type are separated from each other by a third element isolation layer. In example embodiments, the second and third element isolation layers are shallower than the first element isolation layer.