Patent ID: 8207523

Claim:
A metal oxide semiconductor field effect transistor, comprising: a substrate; a gate structure, disposed on the substrate; a spacer, disposed on a side wall of the gate structure; a source/drain extension layer, disposed in the substrate below the spacer; a source/drain layer, disposed in the substrate outside of the spacer, wherein the depth of the source /drain layer is larger than the depth of the source/drain extension layer, the source/drain extension layer and an entire of the source /drain layer are both strained layers, the strained layer of the source/drain extension layer comprises silicon germanium, and the germanium composition ratio of the portion of the source/drain extension layer adjacent to the substrate is larger than the germanium composition ratio of the portion of the source/drain extension layer disposed at a farther distance to the substrate; and a dopant diffusion barrier layer, only disposed directly under the same source/drain extension layer.