Enhancement mode III-N HEMTs

1. A method of forming a III-N semiconductor device, comprising:

forming a nitride channel layer, the composition of the nitride channel layer being selected from the group consisting of nitrides of gallium, indium, and aluminum, and combinations thereof;

forming an AlXN layer adjacent the channel layer, wherein X is selected from the group consisting of gallium, indium, and their combination;

forming a gate, wherein the nitride channel layer includes a first channel region beneath the gate, and two channel access regions on opposite sides of the first channel region; and

forming an n-doped GaN layer adjacent the AlXN layer in the areas adjacent to the channel access regions, but not in the area adjacent to the first channel region; wherein

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

2. The method of claim 1 , the III-N semiconductor device including a substrate, wherein the method further comprises forming 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 channel layer comprises GaN.

3. The method of claim 1 , wherein the n-doped GaN layer is doped with silicon.

4. A method of making a III-N semiconductor device, comprising:

forming a nitride channel layer, the nitride channel layer including a first channel region and two 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, aluminum, and combinations thereof;

forming a first AlXN layer atop the channel layer, wherein X is selected from the group consisting of gallium, indium, and their combination;

forming a second AlXN layer adjacent the first AlXN layer, the second AlXN layer having a substantially lower concentration of Al than the first AlXN layer;

etching an opening through the second AlXN layer down to the first AlXN layer using a selective etchant that etches faster through the second AlXN layer that has a lower concentration of Al, than it etches through the first AlXN layer having a higher concentration of Al; and

depositing a gate in the opening; wherein

the concentration of Al in each of the first and second AlXN layers, respectively, and their respective thicknesses are selected to induce a 2DEG charge in the channel access regions without inducing any substantial 2DEG charge in the first channel region in the absence of a switching voltage applied to the gate, so that a channel of the device is not conductive in the absence of the switching voltage applied to the gate, but becomes conductive when a switching voltage above a threshold voltage is applied to the gate.

5. The method of claim 4 , wherein the second AlXN layer is n-doped.

6. The method of claim 4 , wherein before forming the first AlXN layer, an additional nitride layer is deposited on the substrate, 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.

7. The method of claim 6 , wherein the additional nitride layer comprises AlXN.

8. The method of claim 4 , wherein the opening through the second AlXN layer is etched in a manner so that it has tapered sides.

9. The method of claim 4 , further comprising depositing a gate insulator in the opening prior to depositing the gate, wherein the insulator covers at least part of the aperture.

10. A method of forming a III-N semiconductor device, comprising:

forming a nitride channel layer, the nitride channel layer including a first channel region and two 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, aluminum, and combinations thereof;

forming a first AlXN layer adjacent the channel layer, wherein X is selected from the group consisting of gallium, indium, and their combination;

forming a second AlXN layer adjacent the first AlXN layer, the second AlXN layer being over the channel access regions but not over the first channel region, wherein the first AlXN layer has a substantially higher concentration of Al than the second AlXN layer; and

forming a conductive gate contact over the first channel region; wherein

the concentration of Al in each of the first and second AlXN layers, respectively, and their respective thicknesses are selected to induce a 2DEG charge in the channel access regions without inducing any substantial 2DEG charge in the first channel region in the absence of a switching voltage applied to the gate, so that a channel of the device is not conductive in the absence of the switching voltage applied to the gate, but is conductive when a switching voltage greater than a threshold voltage is applied to the gate.

11. The method of claim 10 , further comprising forming an insulator beneath the conductive gate contact.

12. The method of claim 10 , further comprising forming source and drain contacts at opposite ends of the channel.

13. The method of claim 12 , wherein a passivating layer is applied to the top surface of the device, including at least part of the gate, source, and drain contacts.

14. A method of forming a GaN semiconductor device, comprising:

forming a GaN channel layer, the GaN channel layer including a first channel region and two channel access regions on opposite sides of the first channel region;

forming a layer of Al x GaN on the channel layer, where x is between about 0.05 and 0.3; and

forming an n-doped GaN layer adjacent the Al x GaN layer in the areas adjacent to the channel access regions, but not in an area adjacent to the first channel region; wherein

the concentration of Al in the Al x GaN layer, the Al x GaN layer thickness, and the n-doping concentration in the n-doped GaN layer are selected to induce a 2DEG charge in the channel access regions without inducing any substantial 2DEG charge in the first channel region in the absence of a switching voltage applied to a gate of the device, so that a channel of the device is not conductive in the absence of the switching voltage applied to the gate, but becomes conductive when a switching voltage greater than a threshold voltage is applied to the gate.

15. The method of claim 14 , further comprising forming an additional SiN layer on top of the n-doped GaN layer.

16. The method of claim 14 , wherein the n-doped GaN layer is doped with silicon.

17. The method of claim 14 , wherein the n-doped GaN layer is delta-doped.

18. The method of claim 14 , further comprising including an Al z GaN buffer layer between the GaN channel layer and the substrate, where z is between 1 and a finite value greater than 0.

19. The method of claim 14 , further comprising including an additional AlXN layer on top of the n-doped GaN layer, wherein X is selected from the group consisting of gallium, indium, and their combination.

20. The method of claim 19 , further comprising including an additional SiN layer on top of the additional AlXN layer.

21. The method of claim 14 , wherein current through the channel when the channel is conductive is at least 10,000 times current when the channel is not conductive.

22. A method of forming a semiconductor device, comprising:

forming a first layer of a first material, the first layer including two 2DEG-containing channel access regions, one coupled to a source and the other coupled to a drain;

forming a second layer of a second material having a channel region, the channel region being coupled between the two channel access regions and being beneath a gate of the device; wherein

the channel region is depleted of conducting charge and is not conductive in the absence of a switching voltage being applied to the gate, but which comprises a 2DEG channel and becomes conductive when a switching voltage greater than a threshold voltage is applied to the gate, thereby completing a conductive path between the source and the drain.

23. The method of claim 22 , wherein when the switching voltage is applied to the gate, the two channel access regions are connected to the channel region via substantially vertical conducting regions induced by the applied switching voltage.

24. The method of claim 22 , wherein the first and second materials each comprise III-N materials.

25. A method of forming a III-N semiconductor device, comprising:

forming a nitride channel layer including a first channel region, the composition of the nitride channel layer being selected from the group consisting of nitrides of gallium, indium, aluminum, and combinations thereof;

forming a gate over the first channel region;

forming a III-N layer over the nitride channel layer, the III-N layer including channel access regions on opposite sides of the first channel region, wherein the channel access regions each include a conductive channel respectively connected to a source and a drain; and

forming an AlYN layer over the III-N layer, wherein Y is selected from the group consisting of gallium, indium, and their combination; wherein

the first channel region is non-conductive in the absence of a switching voltage applied to the gate, but becomes conductive in the presence of a switching voltage greater than a threshold voltage applied to the gate, creating a 2DEG region in the first channel region which is connected to the conductive channels in the channel access regions, thereby completing a conduction path from the source to the drain through the channel access regions and the first channel region.

26. The method of claim 25 , further comprising including an additional AlN layer between about 4 and 30 Å thick between the III-N layer and the AlYN layer, wherein the concentration of Al in the AlYN layer and its thickness are selected to induce the conductive channels in the channel access regions.

27. The method of claim 26 , wherein the conductive channels in the channel access regions each comprise 2DEGs.

28. The method of claim 25 , further comprising forming an AlXN layer between the nitride channel layer and the III-N layer, wherein X is selected from the group consisting of gallium, indium, and their combination.

29. The method of claim 25 , further comprising forming an insulator beneath the gate.