Enhancement mode III-N HEMTs

1. A III-N semiconductor device comprising:

a nitride channel layer including a first channel region beneath a gate, and channel access regions on opposite sides of the first channel region, the composition of the nitride channel layer being selected from the group consisting of nitrides of gallium, indium and aluminum, and combinations thereof; and

an AlXN layer adjacent the channel layer in the areas adjacent to the channel access regions but not in the area adjacent to the first channel region, wherein X is selected from the group consisting of gallium, indium or their combination; wherein

the concentration of Al and thickness of the AlXN layer is selected to induce a 2DEG charge in the channel access regions adjacent the AlXN layer without inducing any substantial 2DEG charge in the first channel region, so that a channel comprising the 2DEG charge is not conductive in the absence of a switching voltage applied to the gate, but is conductive when a switching voltage greater than a device threshold voltage is applied to the gate.

2. The III-N semiconductor device of claim 1 , wherein the AlXN layer is n-doped.

3. The III-N semiconductor device of claim 2 , wherein the n-dopant is Si.

4. The III-N semiconductor device of claim 1 , further comprising a substrate and an additional nitride layer between the substrate and the nitride channel layer, the additional nitride layer being selected from the group consisting of nitrides of gallium, indium, aluminum, and combinations thereof, wherein the nitride channel layer comprises GaN.

5. The III-N semiconductor device of claim 4 , wherein the additional nitride layer comprises AlGaN.

6. The III-N semiconductor device of claim 1 , further comprising a gate insulator layer between the gate and the nitride channel layer.

7. The III-N semiconductor device of claim 6 , wherein the gate insulator layer includes SiN.

8. The semiconductor device of claim 6 , further comprising a source and a drain.

9. The semiconductor device of claim 8 , wherein the gate insulator layer extends from the source to the drain.

10. The III-N semiconductor device of claim 1 , further comprising a second AlXN layer between the AlXN layer and the channel layer, the second AlXN layer having a substantially higher concentration of Al than the AlXN layer.

11. A semiconductor device comprising:

a nitride channel layer including a first channel region beneath a conductive gate contact, and channel access regions on opposite sides of the first channel region, the composition of the nitride channel layer being selected from the group consisting of nitrides of gallium, indium and aluminum, and combinations thereof; and

an AlXN layer over the channel layer, wherein X is selected from the group consisting of gallium, indium or their combination; wherein

an aperture is formed in the AlXN layer in a gate region of the device, and an insulator covers at least part of the aperture;

the conductive gate contact is atop the insulator and insulated from the AlXN layer; and

the Al concentration and thickness of the AlXN layer is selected such that a 2DEG charge is induced in the channel access regions adjacent the AlXN layer without inducing any substantial 2DEG charge in the first channel region, so that a channel comprising the 2DEG charge is not conductive in the absence of a switching voltage applied to the conductive gate contact, but is conductive when a switching voltage greater than a threshold voltage is applied to the conductive gate contact.

12. The semiconductor device of claim 11 , wherein the aperture has slanted sides.

13. The III-N semiconductor device of claim 11 , wherein conductive source and drain contacts are formed on opposite sides of the conductive gate contact.

14. The III-N semiconductor device of claim 11 , wherein the AlXN layer is n-doped.

15. The III-N semiconductor device of claim 14 , wherein the n-dopant is Si.

16. The III-N semiconductor device of claim 11 , further comprising a substrate and an additional nitride layer between the substrate and the nitride channel layer, the additional nitride layer being selected from the group consisting of nitrides of gallium, indium, aluminum, and combinations thereof, wherein the nitride channel layer comprises GaN.

17. The III-N semiconductor device of claim 16 , wherein the additional nitride layer comprises AlGaN.

18. A III-N semiconductor device comprising:

a nitride channel layer including a first channel region beneath a recessed gate region and channel access regions on opposite sides of the first channel region, the channel access regions respectively connected to a source and a drain, the composition of the nitride channel layer being selected from the group consisting of nitrides of gallium, indium and aluminum, and combinations thereof;

a III-N layer adjacent the nitride channel layer and surrounding the recessed gate region;

an Al m YN layer adjacent to the III-N layer and surrounding the recessed gate region, wherein Y is selected from the group consisting of gallium, indium or their combination; and

a gate in the recessed gate region; wherein

the first channel region is non-conductive in the absence of a switching voltage applied to the gate, but is conductive in the presence of a switching voltage greater than a threshold voltage applied to the gate.

19. The III-N semiconductor device of claim 18 , further comprising a substrate and an additional nitride layer between the substrate and the nitride channel layer, the additional nitride layer being selected from the group consisting of nitrides of gallium, indium, aluminum, and combinations thereof, wherein the nitride channel layer comprises GaN.

20. The III-N semiconductor device of claim 18 , further comprising a gate insulator layer between the gate and the nitride channel layer.

21. The III-N semiconductor device of claim 20 , wherein the gate insulator layer includes SiN.

22. The III-N semiconductor device of claim 20 , wherein the gate insulator layer extends from the source to the drain.