Strained MOS device and methods for forming the same

A metal-oxide-semiconductor (MOS) device comprising:

• a semiconductor substrate;
• a gate stack on the semiconductor substrate, wherein the gate stack and the semiconductor substrate have an interface;
• a gate spacer on a sidewall of the gate stack;
• a silicon germanium (SiGe) stressor in the semiconductor substrate, wherein the SiGe stressor has a first top surface substantially lower than the interface, and wherein the first top surface has an inner end substantially aligned with an outer sidewall of the gate spacer;
• and
• a contact etch stop layer (CESL) over the SiGe stressor, the gate spacer and the gate stack, wherein the CESL has an inherent compressive stress.