Nitride-based transistors having laterally grown active region and methods of fabricating same

1. A high electron mobility transistor, comprising:

a first Group III-nitride layer having vertically grown regions, laterally grown regions between adjacent vertically grown regions and a coalescence region between adjacent laterally grown regions, wherein the first Group-III nitride layer is provided on a trench wherein the coalescence region is positioned substantially at the center of the trench;

a Group III-nitride channel layer on the first Group III-nitride layer;

a Group III-nitride barrier layer on the Group III-nitride channel layer;

a drain contact on the barrier layer;

a source contact on the barrier layer; and

a gate contact on the barrier layer,

wherein the gate contact is disposed on a portion of the barrier layer on a laterally grown region of the first Group III-nitride layer and at least a portion of one of the source contact and/or the drain contact is disposed on a portion of the barrier layer on a vertically grown region of the first Group III-nitride layer; and

wherein the trench is oriented with respect to a crystal structure to provide one of a vertical sidewall growth of the laterally grown regions of the first Group III-nitride layer and trapezoidal sidewall growth of the laterally grown regions of the first Group III-nitride layer.

2. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends toward the drain contact at least as far as a depletion region of a two-dimensional electron gas extends from the gate contact under expected operating conditions.

3. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends toward the drain contact at least as far as a point where a strength of an electric field is 50% of a strength of an electric field at a drain side edge of the gate contact under expected operating conditions.

4. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends toward the drain contact at least as far as a point where a strength of an electric field is an order of magnitude less than a strength of an electric field at a drain side edge of the gate contact under expected operating conditions.

5. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends toward the drain contact at least as far as a point where a strength of an electric field is two orders of magnitude less than a strength of an electric field at a drain side edge of the gate contact under expected operating conditions.

6. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends toward the drain contact but not to the drain contact such that the drain contact is disposed on a vertically grown region of the first Group III-nitride layer.

7. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends to but not beyond the drain contact.

8. The transistor of claim 1 , wherein the laterally grown region on which the gate contact is disposed extends beyond an edge of the drain contact such that at least a portion of the drain contact is disposed on the laterally grown region on which the gate contact is disposed.

9. A high electron mobility transistor, comprising:

a semiconductor layer defining a trench therein; and

a group III-nitride layer on the semiconductor layer, the group III-nitride layer having laterally grown portions that are cantilevered over the trench defining a cavity in the trench between the cantilevered laterally grown portions of the group III-nitride layer and a floor of the trench,

wherein the group III-nitride layer further includes vertically grown portions;

wherein the laterally grown portions extend from the vertically grown portions and begin above sidewalls of the trench; and

wherein the trench is oriented with respect to a crystal structure to provide one of a vertical sidewall growth of the laterally grown portions of the group III-nitride layer and trapezoidal sidewall growth of the laterally grown portions of the group III-nitride layer,

wherein the group III-nitride layer further includes a coalescence region between adjacent laterally grown regions, the transistor further comprising:

a group III-nitride channel layer on the group III-nitride layer;

a group III-nitride barrier layer on the group III-nitride channel layer;

a drain contact on the barrier layer;

a source contact on the barrier layer;

a gate contact on the barrier layer, wherein the gate contact is disposed on a portion of the barrier layer on a laterally grown region of the group III-nitride layer and at least a portion of one of the source contact and/or the drain contact is disposed on a portion of the barrier layer on a vertically grown region of the group III-nitride layer.

10. The transistor of claim 9 , wherein the laterally grown portions contact first and second sides of a coalescence region that is positioned substantially in a center portion of the trench.

11. The transistor of claim 9 , wherein a depth of the trench is dependent on a width of the trench such that the laterally grown portions coalesce before the trench is filled to provide the cavity between the cantilevered portions of the laterally grown portions of the group III-nitride layer and the floor of the trench.

12. The transistor of claim 11 , wherein the width of the trench is about 8.0 μm and the depth of the trench is about 4.0 μm.

13. The transistor of claim 9 , further comprising a mask on a floor of the trench to suppress growth within the trench to provide the cavity.

14. The transistor of claim 9 , wherein the semiconductor layer comprises a semiconductor substrate.

15. The transistor of claim 14 , wherein the semiconductor substrate comprises a silicon carbide substrate.

16. The transistor of claim 14 , wherein the trench extends perpendicular or parallel to a crystal plane of the semiconductor substrate.

17. The transistor of claim 16 , wherein the trench extends parallel or perpendicular to a plane in the {11 2 0} family of planes or the {10 1 0} family of planes of the semiconductor substrate.

18. The transistor of claim 14 , wherein the trench extends parallel or perpendicular to a major flat of a wafer that provides the semiconductor substrate.

19. The transistor of claim 9 , wherein the vertical the sidewall growth is substantially perpendicular to a surface of the semiconductor substrate.

20. The transistor of claim 9 , wherein the first group-III nitride layer comprises a gallium nitride based layer.

21. The transistor of claim 9 , wherein the semiconductor layer comprises a semiconductor substrate, the transistor further comprising a buffer layer on the semiconductor substrate, wherein the trench is defined by the buffer layer and the semiconductor substrate.