Field effect transistors and methods of forming same

1. A semiconductor device comprising:

a substrate;

a first gate stack over the substrate, wherein the first gate stack comprises a first ferroelectric layer;

a source/channel/drain stack over the first gate stack, wherein the source/channel/drain stack comprises one or more solid 2D material layers; and

a second gate stack over the source/channel/drain stack, wherein the second gate stack comprises a second ferroelectric layer, and wherein at least a portion of the first gate stack laterally extends beyond an edge of the second gate stack.

2. The semiconductor device of claim 1 , wherein the substrate is a silicon-on-insulator substrate, the first gate stack being formed on a buried oxide layer of the silicon-on-insulator substrate.

3. The semiconductor device of claim 1 , wherein the first gate stack further comprises a first dielectric layer and a first conductive layer, the first conductive layer being interposed between the first dielectric layer and the first ferroelectric layer.

4. The semiconductor device of claim 1 , wherein the second gate stack further comprises a second dielectric layer and a second conductive layer, the second conductive layer being interposed between the second dielectric layer and the second ferroelectric layer.

5. The semiconductor device of claim 1 , wherein each of the one or more solid 2D material layers comprises a transition metal dichalcogenide, graphene, or boron nitride.

6. The semiconductor device of claim 1 , wherein the first ferroelectric layer and the second ferroelectric layer are formed of HfO 2 , HfSiO x , HfZrO x , Al 2 O 3 , TiO 2 , LaO x , BaSrTiO x , or PbZr x Ti y O z .

7. The semiconductor device of claim 1 , wherein each of the one or more solid 2D material layers comprises a first source/drain region, a second source/drain region, and a channel region interposed between the first source/drain region and the second source/drain region.

8. The semiconductor device of claim 1 , further comprising a first spacer along a sidewall of the first gate stack and a second spacer along a sidewall of the second gate stack, wherein source/drain regions of the source/channel/drain stack are interposed between the first spacer and the second spacer.

9. A semiconductor device comprising:

a first ferroelectric layer over a substrate;

a first metal layer over the first ferroelectric layer;

a first dielectric layer over the first metal layer;

2D material layers over the first dielectric layer, wherein neighboring 2D material layers are separated by a corresponding dielectric layer;

a second dielectric layer over the 2D material layers;

a second metal layer over the second dielectric layer;

a second ferroelectric layer over the second metal layer; and

a spacer adjacent to the first ferroelectric layer, wherein the 2D material layers extend over the spacer, and wherein at least one of the 2D material layers physically contacts the spacer.

10. The semiconductor device of claim 9 , further comprising a third metal layer, wherein the first ferroelectric layer is interposed between the first metal layer and the third metal layer.

11. The semiconductor device of claim 10 , wherein the third metal layer is wider than the first metal layer.

12. The semiconductor device of claim 10 , further comprising a conductive plug in electrical contact with the third metal layer.

13. The semiconductor device of claim 9 , further comprising a fourth metal layer, wherein the second ferroelectric layer is interposed between the second metal layer and the fourth metal layer.

14. The semiconductor device of claim 9 , further comprising a first source/drain electrode and a second source/drain electrode adjacent the 2D material layers, wherein the 2D material layers are interposed between the first source/drain electrode and the second source/drain electrode.

15. A method of forming a semiconductor device, the method comprising:

forming a first gate stack over a substrate, wherein the first gate stack comprises a first ferroelectric layer;

forming a first spacer along a sidewall of the first gate stack;

forming a source/channel/drain stack over the first gate stack, wherein the source/channel/drain stack comprises one or more 2D material layers, and wherein a portion of the one or more 2D material layers extends directly over the first spacer; and

forming a second gate stack over source/channel/drain stack, wherein the second gate stack comprises a second ferroelectric layer.

16. The method of claim 15 , wherein the forming the first gate stack further comprises forming a first conductive layer and a second conductive layer, the first ferroelectric layer being interposed between the first conductive layer and the second conductive layer.

17. The method of claim 15 , wherein the forming the second gate stack further comprises forming a third conductive layer and a fourth conductive layer, the second ferroelectric layer being interposed between the third conductive layer and the fourth conductive layer.

18. The method of claim 15 , further comprising forming a first source/drain electrode and a second source/drain electrode, wherein the source/channel/drain stack over is interposed between the first source/drain electrode and the second source/drain electrode.

19. The method of claim 15 , further comprising forming a first conductive plug in electrical contact with a first conductive layer below the first ferroelectric layer and a second conductive plug in electrical contact with a second conductive layer above the second ferroelectric layer.

20. The method of claim 15 , further comprising forming a second spacer along a sidewall of the second gate stack, the portion of the one or more 2D material layers being interposed between the first spacer and the second spacer.