Process for fabricating silicon-on-nothing MOSFETs

1. A semiconductor device comprising:

a gate stack;

an air-gap under the gate stack wherein semiconductor materials adjoin two ends of the air-gap;

a source/drain region, at least a portion of which is at a lower plane than the air-gap;

a semiconductor layer vertically between the gate stack and the air-gap; and

a first dielectric layer underlying and adjoining the semiconductor layer, wherein the first dielectric layer is exposed to the air-gap.

2. The semiconductor device of claim 1 further comprising a second dielectric layer underlying and exposed to the air-gap.

3. The semiconductor device of claim 2 further comprising:

a semiconductor substrate underlying and adjoining the second dielectric layer; and

the source/drain region on a side of the gate stack, wherein the source/drain region adjoins the semiconductor substrate and the semiconductor layer.

4. The semiconductor device of claim 1 further comprising semiconductor materials on opposite sides of the air-gap and exposed to the air-gap.

5. The semiconductor device of claim 4 , wherein the semiconductor materials are different from materials of the semiconductor substrate and the semiconductor layer.

6. The semiconductor device of claim 1 , wherein the semiconductor layer is formed of silicon.

7. The semiconductor device of claim 1 further comprising:

a pocket region in the semiconductor layer; and

a lightly doped source/drain region having at least a portion in the semiconductor layer.

8. The semiconductor device of claim 1 , wherein the first dielectric layer comprises an oxide of the semiconductor layer.

9. The semiconductor device of claim 1 , wherein an edge of the semiconductor layer is substantially vertically aligned to an edge of the first dielectric layer.

10. A semiconductor device comprising:

a semiconductor substrate;

an air-gap over the semiconductor substrate wherein semiconductor materials adjoin two ends of the air-gap;

a source/drain region, at least a portion of which is at a lower plane than the air-gap;

a semiconductor layer over the air-gap;

a gate dielectric over the semiconductor layer;

a gate electrode over the gate dielectric;

a first oxide layer underlying and adjoining the semiconductor layer, wherein the first oxide layer is exposed to the air-gap; and

a second oxide layer overlying and adjoining the semiconductor substrate, wherein the second oxide layer is exposed to the air-gap.

11. The semiconductor device of claim 10 further comprising a recess extending from a top surface of the semiconductor layer down into the semiconductor substrate, wherein the recess is adjacent to the gate dielectric, and is filled with a semiconductor material.

12. The semiconductor device of claim 11 , wherein the semiconductor material is different from a material of the semiconductor layer.

13. The semiconductor device of claim 12 , wherein the semiconductor material comprises a material selected from the group consisting essentially of silicon germanium and silicon carbon, and wherein the semiconductor layer is formed substantially of silicon.

14. The semiconductor device of claim 11 further comprising:

a pocket region in the semiconductor layer; and

a lightly doped source/drain region having at least a portion in the semiconductor layer;

the source/drain region in the semiconductor material and comprising at least a portion of the semiconductor material filled in the recess; and

a source/drain silicide region on the source/drain region.

15. The semiconductor device of claim 10 , wherein the semiconductor layer has a thickness of less than about 300 Å.

16. The semiconductor device of claim 10 , wherein the first and the second oxide layers have thicknesses of greater than about 5 Å.

17. The semiconductor device of claim 10 , wherein the first and the second oxide layers comprise an oxide of the semiconductor layer and an oxide of the semiconductor substrate, respectively.

18. A semiconductor device comprising:

a silicon substrate;

an air-gap over the silicon substrate wherein semiconductor materials adjoin two ends of the air-gap;

a source/drain region, at least a portion of which is at a lower plane than the air-gap;

a silicon layer over the air-gap;

a gate dielectric over the semiconductor layer;

a gate electrode over the gate dielectric;

a first silicon oxide layer between the silicon layer and the air-gap;

a second silicon oxide layer between the silicon substrate and the air-gap; and

a silicon germanium (SiGe) region extending into the silicon substrate, wherein the SiGe region adjoins the silicon layer, and is exposed to the air-gap.

19. The semiconductor device of claim 18 , wherein the first and the second silicon oxide layers have thickness greater than about 5 Å.

20. The semiconductor device of claim 18 , wherein an edge of the silicon layer is substantially vertically aligned to an edge of the first silicon oxide layer and an edge of the second silicon oxide layer.