Bulk semiconductor substrate configured to exhibit semiconductor-on-insulator behavior

1. An integrated circuit device that includes a bulk substrate that exhibits semiconductor-on-insulator behavior, the integrated circuit device comprising:

a channel region defined in the bulk substrate, wherein the channel region has a first portion disposed over a second portion, and further wherein:

the first portion is disposed between epitaxial source/drain features disposed in the bulk substrate,

the second portion is defined by a first isolation feature disposed in the bulk substrate, wherein the epitaxial source/drain features are disposed on the first isolation feature, and

a thickness of the channel region is about equal to a sum of a thickness of the first isolation feature and a thickness of the epitaxial source/drain features;

a second isolation feature disposed in the bulk substrate, wherein the second isolation feature defines an active region that includes the first portion of the channel region disposed between the epitaxial source/drain features; and

wherein a resistance of a portion of the bulk substrate underlying the first isolation feature and the channel region is greater than a resistance of the bulk substrate.

2. The integrated circuit device of claim 1 , wherein the channel region has a tapered width that decreases as the thickness of the channel region increases.

3. The integrated circuit device of claim 2 , wherein a width of the first portion is substantially the same as the thickness of the channel region increases and a width of the second portion decreases as the thickness of the channel region increases.

4. The integrated circuit device of claim 1 , wherein a thickness of the second isolation feature is greater than or equal to the thickness of the epitaxial source/drain features.

5. The integrated circuit device of claim 4 , wherein the thickness of the second isolation feature is less than the thickness of the channel region.

6. The integrated circuit device of claim 4 , wherein the thickness of the second isolation feature is greater than the thickness of the channel region.

7. The integrated circuit device of claim 4 , wherein the thickness of the second isolation feature is greater than a sum of the thickness of the channel region and a thickness of the portion of the bulk substrate underlying the first isolation feature and the channel region.

8. The integrated circuit device of claim 1 , wherein the bulk substrate includes a material having a crystalline structure and the portion of the bulk substrate underlying the first isolation feature and the channel region includes the material having a non-crystalline structure.

9. The integrated circuit device of claim 1 , further comprising an air gap disposed in the second isolation feature, wherein the second isolation feature has a tapered width that increases with depth.

10. The integrated circuit device of claim 1 , further comprising a gate structure disposed over the channel region.

11. A semiconductor-on-insulator like structure comprising:

a bulk substrate;

a dual-isolation structure that includes:

a first isolation trench defined in the bulk substrate, wherein the first isolation trench is configured to define a channel region of the bulk substrate, the first isolation trench includes a first isolation trench portion and a second isolation trench portion disposed over the first isolation trench portion, and:

a first isolation material fills the first isolation trench portion of the first isolation trench; and

an epitaxial material fills the second isolation trench portion of the first isolation trench, wherein the epitaxial material is disposed on the first isolation material, and

a second isolation trench defined in the bulk substrate, wherein the second isolation trench defines an active region that includes the channel region, and

further wherein a second isolation material fills the second isolation trench; and

an amorphous layer disposed in the bulk substrate underlying the first isolation trench and the channel region, wherein a resistance of the amorphous layer is greater than a resistance of the bulk substrate.

12. The semiconductor-on-insulator like structure of claim 11 , wherein the first isolation trench has negatively sloped sidewalls and the second isolation trench has positively sloped sidewalls.

13. The semiconductor-on-insulator like structure of claim 11 , wherein the first isolation trench has negatively sloped sidewalls and the second isolation trench has negatively sloped sidewalls.

14. The semiconductor-on-insulator like structure of claim 11 , wherein an air gap is disposed in the second isolation material.

15. The semiconductor-on-insulator like structure of claim 11 , wherein the first isolation trench portion has substantially vertical sidewalls and the second isolation trench portion has negatively sloped sidewalls.

16. The semiconductor-on-insulator like structure of claim 11 , wherein the second isolation trench extends through at least a portion of the first isolation trench.

17. The semiconductor-on-insulator like structure of claim 16 , wherein the second isolation trench further extends through at least a portion of the amorphous layer.

18. A transistor comprising:

a bulk substrate;

a first epitaxial source/drain feature and a second epitaxial source/drain feature disposed in the bulk substrate, wherein a first portion of the bulk substrate is defined between the first epitaxial source/drain feature and the second epitaxial source/drain feature;

a first dielectric feature disposed in the bulk substrate, wherein the first dielectric feature is disposed directly adjacent to and under the first epitaxial source/drain feature and the second epitaxial source/drain feature, and further wherein the first dielectric feature surrounds a second portion of the bulk substrate, wherein the second portion of the bulk substrate is disposed under the first portion of the bulk substrate;

a third portion of the bulk substrate disposed directly adjacent to and under the first dielectric feature and the second portion of the bulk substrate, wherein the third portion of the bulk substrate has a non-crystalline structure, the first portion of the bulk substrate has a crystalline structure, and the second portion of the bulk substrate has a crystalline structure;

a second dielectric feature disposed in the bulk substrate, wherein the second dielectric feature surrounds the first dielectric feature and the third portion of the bulk substrate; and

a gate structure disposed over the first portion of the bulk substrate.

19. The transistor of claim 18 , wherein the second dielectric feature is disposed directly adjacent to the first epitaxial source/drain feature and the second epitaxial source/drain feature.

20. The transistor of claim 18 , wherein the second dielectric feature is disposed directly adjacent to the first portion of the bulk substrate.