A semiconductor device has at least two main carbon-rich regions and two additional carbon-rich regions. The main carbon-rich regions are separately located in a substrate so that a channel region is located between them. The additional carbon-rich regions are respectively located underneath the main carbon-rich regions. The carbon concentrations is higher in the main carbon-rich regions and lower in the additional carbon-rich regions, and optionally, the absolute value of a gradient of the carbon concentration of the bottom portion of the main carbon-rich regions is higher than the absolute value of a gradient of the carbon concentration of the additional carbon-rich regions. Therefore, the leakage current induced by a lattice mismatch effect at the carbon-rich and the carbon-free interface can be minimized.