Bulk silicon germanium FinFET

1. A bulk SiGe FinFET comprising:

a plurality of SiGe fins and a bulk semiconductor substrate, the SiGe fins extending from the bulk semiconductor substrate;

the SiGe fins having a top SiGe portion and a bottom SiGe portion, a part of the bottom SiGe portion being doped to form a punchthrough stop;

the bulk semiconductor substrate having a top portion in contact with the bottom SiGe portions of the SiGe fins and comprising a gradient of germanium and silicon, and a bottom portion of silicon in contact with the top portion such that the gradient has a composition of SiGe at the top portion in contact at an interface with the SiGe fins that is the same composition of SiGe as in the SiGe fins, the proportion of germanium atoms in the gradient gradually decreasing from the interface and the proportion of silicon atoms in the gradient gradually increasing in the gradient from the interface until the bulk semiconductor substrate top portion contacts at a second interface the bulk semiconductor substrate bottom portion.

2. The bulk SiGe FinFET of claim 1 wherein the SiGe gradient in the top portion of the bulk semiconductor substrate comprises 0% germanium and 100% silicon where the top portion of the bulk semiconductor substrate contacts the bottom portion of the bulk semiconductor substrate.

3. The bulk SiGe FinFET of claim 2 wherein the bottom portion of the bulk semiconductor substrate is silicon.

4. The bulk SiGe FinFET of claim 1 further comprising shallow trench isolation between bottom portions of adjacent SiGe fins.

5. The bulk SiGe FinFET of claim 4 wherein the punchthrough stop is at a first height of the bottom portion of the SiGe measured from the bulk semiconductor top portion and the shallow trench isolation is at a second height of the bottom portion of the SiGe fins measured from the bulk semiconductor top portion such that the first height is greater than the second height.

6. The bulk SiGe FinFET of claim 5 wherein the first height is greater than the second height by 3 to 10 nm.

7. The bulk SiGe FinFET of claim 1 further comprising a gate dielectric wrapping around the top portion of the SiGe fins and a gate wrapping around the gate dielectric.

8. The bulk SiGe FinFET of claim 1 wherein the bottom SiGe portions of the SiGe fins are in direct contact with the top portion of the bulk semiconductor substrate.

9. The bulk SiGe FinFET of claim 1 wherein the top portion of the bulk semiconductor substrate is in direct contact with the bottom portion of the bulk semiconductor substrate.

10. The bulk SiGe FinFET of claim 1 wherein the SiGe gradient has a thickness of 5 to 30 nm.

11. The bulk SiGe FinFET of claim 1 further comprising dislocation defects at the second interface of the top portion of the bulk semiconductor substrate with the bottom portion of the bulk semiconductor substrate.

12. A method of forming a bulk SiGe FinFET comprising:

forming silicon fins from a bulk silicon substrate;

epitaxially growing a SiGe layer on the silicon fins and on the bulk silicon substrate;

heating the silicon fins and the bulk silicon substrate in an oxygen ambient to cause the Ge of the SiGe layer to react with the silicon in the silicon fins and a top portion of the bulk silicon substrate to form SiGe;

forming a punchthrough stop in a bottom portion of the SiGe fins;

forming shallow trench isolation in contact with the bottom portion of the SiGe fins;

forming a gate dielectric wrapping around a top portion of the SiGe fins;

forming a gate wrapping around the gate dielectric; and

forming a source and a drain.

13. The method of claim 12 wherein the SiGe in the top portion of the bulk silicon substrate forms a gradient of germanium and silicon wherein the SiGe gradient in the top portion of the bulk semiconductor substrate has a composition of SiGe at the top portion in contact with the SiGe fins that is the same composition of SiGe as in the SiGe fins, the proportion of germanium atoms in the gradient gradually decreasing and the proportion of silicon atoms in the gradient gradually increasing in the gradient until there is 0% germanium and 100% silicon where the top portion of the bulk semiconductor substrate contacts the bottom portion of the bulk semiconductor substrate.

14. The method of claim 13 wherein the bottom portion of the bulk semiconductor substrate is silicon.

15. The method of claim 13 further comprising shallow trench isolation between bottom portions of adjacent SiGe fins.

16. The method of claim 15 wherein the punchthrough stop is at a first height of the bottom portion of the SiGe fins measured from the bulk semiconductor top portion and the shallow trench isolation is at a second height of the bottom portion of the SiGe fins measured from the bulk semiconductor top portion such that the first height is greater than the second height.

17. The method of claim 16 wherein the first height is greater than the second height by 3 to 10 nm.

18. The method of claim 13 wherein the SiGe fins are in direct contact with the top portion of the bulk semiconductor substrate.

19. The method of claim 13 wherein the top portion of the bulk semiconductor substrate is in direct contact with bottom portion of the bulk semiconductor substrate.

20. The method of claim 13 wherein the SiGe gradient has a thickness of 5 to 30 nm.