Stacked FET SRAM design

1. A semiconductor structure comprising:

a bottom device level including a first bottom field effect transistor (FET) of a first conductivity type, a second bottom FET of the first conductivity type, a third bottom FET of the first conductivity type, and a fourth bottom FET of the first conductivity type, wherein the first bottom FET is electrically connected to the second bottom FET and the third bottom FET is electrically connected to the fourth bottom FET, and the first bottom FET is located diagonally to the fourth bottom FET and the second bottom FET is located diagonally to the third bottom FET, and the first bottom FET and the fourth bottom FET are pass gates; and

a top device level stacked above the bottom device level and including a first top FET of a second conductivity type different from the first conductivity type located directly above the first bottom FET, a second top FET of the second conductivity type located directly above the second bottom FET, a third top FET of the second conductivity type located directly above the third bottom FET, and a fourth top FET of the second conductivity type located directly above the fourth bottom FET, wherein the first top FET is electrically connected to the second top FET and the third top FET is electrically connected to the fourth top FET, and the first top FET is a first dummy transistor which is wired to cross-couple with the third top FET and the fourth top FET is a second dummy transistor which is wired to cross-couple with the second top FET, and the second bottom FET and the second top FET are wired to provide a first inverter and the third bottom FET and the third top FET are wired to provide a second inverter.

2. The semiconductor structure of claim 1 , wherein the first top FET comprises a gate electrode that forms an extension of an input of the first inverter, and the fourth top FET comprises another gate electrode that forms an extension of an input of the second inverter.

3. The semiconductor structure of claim 1 , wherein the first bottom FET comprises a gate electrode that is isolated from a gate electrode of the first top FET and the fourth bottom FET comprises a gate electrode that is isolated from a gate electrode of the fourth top FET.

4. The semiconductor structure of claim 1 , wherein the first conductivity type is n-type and the second conductivity type is p-type.

5. The semiconductor structure of claim 1 , wherein the first conductivity type is p-type and the second conductivity type is n-type.

6. The semiconductor structure of claim 1 , wherein the first top FET has a source/drain region located on one side that is shared with the second top FET, the third top FET has a source/drain region located on one side that is shared with the fourth top FET, the first bottom FET has a source/drain region located on one side that is shared with the second bottom FET, the third bottom FET has a source/drain region located on one side that is shared with the fourth bottom FET.

7. The semiconductor structure of claim 6 , wherein the shared source/drain region of the first top FET and the second top FET is electrically connected to the shared source/drain region of the first bottom FET and the second bottom FET, and the shared source/drain region of the third top FET and the fourth top FET is electrically connected to the shared source/drain region of the third bottom FET and the fourth bottom FET.

8. The semiconductor structure of claim 1 , further comprising a word line electrically connected to a gate electrode of both the first bottom FET and the fourth bottom FET.

9. The semiconductor structure of claim 8 , further comprising a bit line electrically connected to a source/drain region of both the first bottom FET and the fourth bottom FET.

10. The semiconductor structure of claim 1 , wherein the first bottom FET includes a gate electrode that spaced apart from a gate electrode of the second bottom FET by a dielectric structure, and the third bottom FET includes a gate electrode that spaced apart from a gate electrode of the fourth bottom FET by another dielectric structure.

11. The semiconductor structure of claim 1 , wherein the first top FET and the second top FET comprise a shared gate electrode, and the third top FET and fourth top FET comprise another shared gate electrode.

12. The semiconductor structure of claim 1 , wherein the second top FET and the third top FET are both wired to a power source, and the second bottom FET and the third bottom FET are both grounded.

13. The semiconductor structure of claim 1 , wherein each of the first bottom FET, the second bottom FET, the third bottom FET, the fourth bottom FET, the first top FET, the second top FET, the third top FET and the fourth top FET are located on a semiconductor channel material structure.

14. The semiconductor structure of claim 13 , wherein the semiconductor channel material structure is a semiconductor fin.

15. The semiconductor structure of claim 13 , wherein the semiconductor channel material structure is a semiconductor nanowire.

16. The semiconductor structure of claim 1 , wherein the cross-couple of the first dummy transistor with the third top FET comprises a first contact metal that contacts both a gate electrode and a source/drain region a first dummy gate region, and the cross-couple of the second dummy transistor with the second top FET comprises a second contact metal that contacts both a gate electrode and a source/drain region of a second dummy gate region.

17. The semiconductor structure of claim 16 , wherein both the first dummy gate region and the second dummy gate region are non-functional gate structure with a removed or partially removed source/drain region.

18. The semiconductor structure of claim 1 , wherein the first top FET, the third top FET and the third bottom FET electrically share a common gate electrode, and the fourth top FET, the second top FET and the second bottom FET electrically share a common gate electrode.