A method of fabricating a semiconductor device includes forming first gate structure and a second gate structure over a core device region of a substrate. The method further includes forming stressors at opposite sides of the first gate structure. The method further includes doping the stressors to form a first source region and a first drain region of a first device. The method further includes doping into the substrate and at opposite sides of the second gate structure to form a second source region and a second drain region of a second device, wherein the first source region, the first drain region, the second source region and the second drain region are of a same conductivity. The first source region includes a different material from the second source region.