Semiconductor devices (102) and fabrication methods (10) are provided, in which a nitride film (130) is formed over NMOS transistors to impart a tensile stress in ail or a portion of the NMOS transistor to improve carrier mobility. The nitride layer (130) is initially deposited over the transistors at low temperature with high hydrogen content to provide a moderate tensile stress in the semiconductor body prior to back-end processing. Subsequent back-end thermal processing reduces the film hydrogen content and causes an increase in the applied tensile stress.