Fully isolated LIGBT and methods for forming the same

1. A device comprising:

a dielectric layer;

a heavily doped semiconductor layer over the dielectric layer, wherein the heavily doped semiconductor layer is of a first conductivity type;

a semiconductor region over the heavily doped semiconductor layer, wherein the semiconductor region is of a second conductivity type opposite the first conductivity type; and

a Lateral Insulated Gate Bipolar Transistor (LIGBT) at a surface of the semiconductor region.

2. The device of claim 1 , further comprising:

a well region forming a ring encircling the semiconductor region, wherein the heavily doped semiconductor layer is underlying, and contacts, a bottom surface of the well region, and wherein the heavily doped semiconductor layer forms a basin with the well region.

3. The device of claim 1 , wherein the LIGBT comprises:

a body region of the first conductivity type in the semiconductor region;

a gate dielectric overlapping a portion of the body region;

a gate electrode overlying the gate dielectric;

a cathode/source in the body region and of the first conductivity type; and

an anode/drain in the semiconductor region and of the first conductivity type, wherein the cathode/source and the anode/drain are on opposite sides of the gate electrode.

4. The device of claim 1 , wherein the semiconductor region is in physical contact with the heavily doped semiconductor layer.

5. The device of claim 1 , wherein the heavily doped semiconductor layer is in physical contact with the dielectric layer.

6. The device of claim 1 , further comprising an additional dielectric layer underlying and in contact with the dielectric layer, wherein the dielectric layer and the additional dielectric layer comprise different dielectric materials.

7. The device of claim 1 , wherein the heavily doped semiconductor layer has an impurity concentration higher than about 1×10 19 /cm 3 .

8. A device comprising:

a first dielectric layer;

a heavily doped semiconductor layer over the first dielectric layer, wherein the heavily doped semiconductor layer is of a first conductivity type;

a semiconductor region over the heavily doped semiconductor layer, wherein the semiconductor region is of a second conductivity type opposite the first conductivity type;

a well region forming a first ring encircling and level with the semiconductor region, wherein the well region is of the first conductivity type; and

a Lateral Insulated Gate Bipolar Transistor (LIGBT) at a surface of the semiconductor region, wherein the LIGBT comprises:

a body region of the first conductivity type in the semiconductor region;

a gate stack overlapping a portion of the body region;

a cathode/source in the body region and of the first conductivity type; and

an anode/drain in the semiconductor region and of the first conductivity type, wherein the cathode/source and the anode/drain are on opposite sides of the gate stack.

9. The device of claim 8 , wherein the first conductivity type is p-type, and the second conductivity type is n-type.

10. The device of claim 8 , further comprising an additional well region forming a second ring encircling the first ring, wherein the additional well region is of the second conductivity type, and wherein the first dielectric layer extends underlying the second ring.

11. The device of claim 8 , wherein the semiconductor region is in physical contact with the heavily doped semiconductor layer.

12. The device of claim 8 , wherein the heavily doped semiconductor layer is in physical contact with the first dielectric layer.

13. The device of claim 8 further comprising a second dielectric layer underlying and in contact with the first dielectric layer, wherein the first dielectric layer and the second dielectric layer comprise different dielectric materials.

14. The device of claim 8 , wherein the heavily doped semiconductor layer has an impurity concentration higher than about 1×10 19 /cm 3 .

15. A device comprising:

a dielectric layer;

a heavily doped semiconductor layer over the dielectric layer, wherein the heavily doped semiconductor layer is of a first conductivity type, and wherein the heavily doped semiconductor layer has an impurity concentration higher than about 1×10 19 /cm 3 ;

a semiconductor region over the heavily doped semiconductor layer, wherein the semiconductor region is of a second conductivity type opposite the first conductivity type, and wherein the heavily doped semiconductor layer fully separates the semiconductor region from the dielectric layer; and

a Lateral Insulated Gate Bipolar Transistor (LIGBT) at a surface of the semiconductor region.

16. The device of claim 15 further comprising:

a well region forming a ring encircling the semiconductor region, wherein the well region is of the first conductivity type, wherein the heavily doped semiconductor layer is underlying, and contacts, a bottom surface of the well region, and wherein the heavily doped semiconductor layer forms a basin with the well region.

17. The device of claim 16 further comprising:

a deep well region forming a ring encircling the well region, wherein the deep well region is of the second conductivity type, and wherein a bottom surface of the deep well region is in contact with a top surface of the dielectric layer.

18. The device of claim 15 , wherein the LIGBT comprises:

a body region of the first conductivity type in the semiconductor region;

a gate dielectric overlapping a portion of the body region;

a gate electrode overlying the gate dielectric;

a cathode/source in the body region and of the first conductivity type; and

an anode/drain in the semiconductor region and of the first conductivity type, wherein the cathode/source and the anode/drain are on opposite sides of the gate electrode.

19. The device of claim 15 , wherein the semiconductor region is in physical contact with the heavily doped semiconductor layer.

20. The device of claim 15 , wherein the heavily doped semiconductor layer is in physical contact with the dielectric layer.