The present disclosure provides a semiconductor device. The semiconductor device includes a silicon substrate. The semiconductor device includes first and second regions that are disposed in the substrate. The first and second regions have a silicon compound material. The semiconductor device includes first and second source/drain structures that are partially disposed in the first and second regions, respectively. The semiconductor device includes a first gate that is disposed over the substrate. The first gate has a first proximity to the first region. The semiconductor device includes a second gate that is disposed over the substrate. The second gate has a second proximity to the second region. The second proximity is different from the first proximity. The first source/drain structure and the first gate are portions of a first transistor, and the second source/drain structure and the second gate are portions of a second transistor.