The present invention provides a semiconductor device, which is formed on a semiconductor substrate, comprising a gate stack, a channel region, and source/drain regions, wherein the gate stack is on the channel region, the channel region is in the semiconductor substrate, the source/drain regions are embedded in the semiconductor substrate, and each of the source/drain regions comprises a sidewall and a bottom, a second semiconductor layer being sandwiched between the channel region and a portion of the sidewall distant from the bottom, a first semiconductor layer being sandwiched between the semiconductor substrate and at least a portion of the bottom distant from the sidewall, and an insulating layer being sandwiched between the semiconductor substrate and the other portions of the bottom and/or the other portions of the sidewall. The present invention also provides a method for forming the semiconductor device. The present invention helps preventing the dopants in the source/drain regions from diffusing into the substrate.