A semiconductor substrate includes a first transistor area having a first gate electrode and first source/drain areas, a second transistor area having a second gate electrode and second source/drain areas, and an interface area provided at an interface of the first transistor area and the second transistor area and having a third gate electrode. A first stress film is on the first gate electrode and the first source/drain areas of the first transistor area and at least a portion of the third gate electrode of the interface area. A second stress film is on the second gate electrode and the second source/drain areas of the second transistor area and not overlapping the first stress film on the third gate electrode of the interface area or overlapping at least a portion of the first stress film. The second stress film overlapping at least the portion of the first stress film is thinner than the second stress film in the second transistor area. Related methods are also described.