Normally-off cubic phase GaN (c-GaN) HEMT having a gate electrode dielectrically insulated from a c-AlGaN capping layer

1. A normally-off, heterojunction field effect transistor, comprising:

an intrinsic cubic-phase gallium nitride (c-GaN) substrate;

an aluminum gallium nitride (AlGaN) capping layer disposed on the c-GaN substrate, wherein the AlGaN capping layer comprises:

a first sublayer of intrinsic c-phase Al x Ga 1-x N disposed on the c-GaN substrate, wherein the first sublayer is of a first thickness; and

a second sublayer of doped c-phase Al x Ga 1-x N disposed on the first sublayer, wherein the second sublayer is of a second thickness and is doped with a dopant;

an insulating layer disposed on an entirety of the AlGaN capping layer, wherein the insulating layer is of a fourth thickness;

a source electrode positioned adjacent a first side of the insulating layer;

a drain electrode positioned adjacent a second side of the insulating layer opposite from the first side; and

a gate electrode disposed on the insulating layer and positioned between the source electrode and the drain electrode, wherein the source electrode, the drain electrode, and the gate electrode are connectable to function as a logic device.

2. The transistor of claim 1 , wherein the dopant is a p-type impurity within a delta-doped layer, the dopant comprising one of magnesium, carbon, or a combination thereof.

3. The transistor of claim 1 , wherein the dopant is an n-type impurity within a delta-doped layer, the dopant comprising one of silicon, germanium, or a combination thereof.

4. The transistor of claim 1 , wherein the insulating layer is of a material comprising one of AlN, SiO 2 , SiN, HfO 2 , ZrO 2 , Al 2 O, or a combination thereof.

5. The transistor of claim 1 , wherein the first thickness is between approximately 2 nm and 10 nm.

6. The transistor of claim 1 , wherein the second thickness is between approximately 1 nm and 5 nm.

7. The transistor of claim 1 , wherein the AlGaN capping layer further comprises a third sublayer of intrinsic c-phase Al x Ga 1-x N disposed on the second sublayer, wherein the third sublayer is of a third thickness.

8. The transistor of claim 7 , wherein the third thickness is between approximately 10 nm and 25 nm.

9. The transistor of claim 1 , wherein the fourth thickness is between approximately 1 nm and 10 nm.

10. The transistor of claim 1 , wherein aluminum content, x, of the AlGaN capping layer is between approximately 10% and 50% of total content.

11. The transistor of claim 1 , wherein the dopant is a p-type impurity, and the aluminum content, x, of the AlGaN capping layer is between approximately 38% and 42% of total content.

12. The transistor of claim 1 , wherein the dopant is an n-type impurity, and wherein the aluminum content, x, of the AlGaN capping layer is between approximately 23% and 27% of total content.

13. The transistor of claim 1 , wherein the gate electrode is made of a metal selected from one of Pt, Ni, Cu, W, Ti, TiN, or a combination thereof, which comprises a work function greater than 4V.

14. The transistor of claim 1 , further comprising:

a first GaN contact region disposed between the c-phase GaN substrate and the source electrode to create a first ohmic contact between the source electrode and the first GaN contact region; and

a second GaN contact region disposed between the c-phase GaN substrate and the drain electrode to create a second ohmic contact between the drain electrode and the second GaN contact region, and wherein the AlGaN capping layer is disposed between the first GaN contact region and the second GaN contact region.

15. The transistor of claim 1 , wherein the dopant is a p-type impurity, and wherein the source electrode and the drain electrode are made of a metal comprising one of Ni, Pt, Au, Pb, or a combination thereof.

16. The transistor of claim 1 , wherein the dopant is an n-type impurity, and wherein the source electrode and the drain electrode are of a metal comprising one of Ti, Al, Pd, Ag, Nd, or a combination thereof.

17. A complementary logic circuit comprising a first of the transistor of claim 1 , wherein the second sublayer is n-doped and a second of the transistor of claim 1 , wherein the second sublayer is p-doped.

18. A normally-off, heterojunction field effect transistor, comprising:

an intrinsic cubic-phase gallium nitride (c-GaN) substrate;

an aluminum gallium nitride (AlGaN) capping layer disposed on the c-GaN substrate, wherein the AlGaN capping layer comprises:

a first sublayer of intrinsic c-phase Al x Ga 1-x N disposed on the c-GaN substrate, wherein the first sublayer is of a first thickness;

a second sublayer of doped c-phase Al x Ga 1-x N disposed on the first sublayer, wherein the second sublayer is of a second thickness and is doped with a dopant; and

a third sublayer of intrinsic c-phase Al x Ga 1-x N disposed on the second sublayer, wherein the third sublayer is of a third thickness;

an insulating layer disposed on an entirety of the third sublayer, wherein the insulating layer is of a fourth thickness;

a source electrode positioned adjacent to a first side of the insulating layer;

a drain electrode positioned adjacent to a second side of the insulating layer opposite from the first side; and

a gate electrode disposed on the insulating layer and positioned between the source electrode and the drain electrode, wherein the source electrode, the drain electrode, and the gate electrode are connectable to function as a logic device.

19. A complementary logic circuit comprising a first of the transistor of claim 18 , wherein the second sublayer is n-doped and a second of the transistor of claim 18 , wherein the second sublayer is p-doped.

20. The transistor of claim 18 , wherein the fourth thickness is between approximately 1 nm and 10 nm.

21. The transistor of claim 18 , wherein the dopant is an n-type impurity within a delta-doped layer, the dopant comprising one of silicon, germanium, or a combination thereof.