Semiconductor transistor structure and fabrication method thereof

1. A method of forming a semiconductor transistor structure, comprising:

providing a substrate with a first conductivity type;

growing a fin structure on the substrate, the fin structure comprising an epitaxial stack disposed at an uppermost portion of the fin, the epitaxial stack comprising a first epitaxial SiGe layer having a second conductivity type opposite to the first conductivity type, a second epitaxial SiGe layer on the first epitaxial SiGe layer and in direct contact with the first epitaxial layer, and a third epitaxial SiGe layer having the second conductivity type on the second epitaxial SiGe layer and in direct contact with the second epitaxial SiGe layer, wherein the first epitaxial SiGe layer and the third epitaxial SiGe layer having the second conductivity type comprise a SiGe layer doped with phosphorus or arsenic, wherein the second epitaxial SiGe layer is an intrinsic SiGe layer;

forming a gate on the fin structure;

forming a buffer layer at a bottom of the fin structure; and

forming a strain relaxed layer on the buffer layer.

2. The method according to claim 1 , wherein the first conductivity type is P type and the second conductivity type is N type.

3. The method according to claim 1 , wherein a concentration of the phosphorus or arsenic is between 1E18-1E19 atoms/cm 3 .

4. The method according to claim 1 , wherein a thickness of the SiGe layer is between 1-20 nm.

5. The method according to claim 1 , wherein the fin structure has a width of about 3-100 nm.

6. The method according to claim 1 further comprising:

forming a gate dielectric layer between the gate and the fin structure.