III-Nitride insulating-gate transistors with passivation

1. A field-effect transistor (FET) comprising:

a plurality of semiconductor layers;

a source electrode contacting at least one of the semiconductor layers;

a drain electrode contacting at least one of the semiconductor layers;

a first dielectric layer covering a portion of semiconductor top surface between the source electrode and the drain electrode;

a first trench extending through the first dielectric layer and having a bottom located on a top surface of the semiconductor layers or within one of the semiconductor layers;

a second dielectric layer lining the first trench and covering a portion of the first dielectric layer;

a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer;

a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric layer on the first dielectric layer; and

a gate electrode filling the second trench;

wherein the second dielectric layer comprises a single-crystalline AlN layer at the bottom of the first trench; and

wherein the second dielectric layer comprises a polycrystalline AlN layer on the single crystalline AlN layer.

2. The FET of claim 1 further comprising:

a substrate comprising silicon (Si), silicon carbide (SiC), sapphire (Al 2 O 3 ), gallium nitride (GaN), or aluminum nitride (AlN);

wherein the plurality of semiconductor layers comprise:

a III-nitride channel layer; and

a barrier layer over the channel layer.

3. The FET of claim 2 wherein:

the channel layer comprises GaN and has a thickness ranging from 5 nanometers to a few micrometers; and

the barrier layer comprises AlGaN having a thickness ranging from 1-30 nanometers and having a 25% Al composition.

4. The FET of claim 2 wherein:

a distance between an interface of the channel layer and the barrier layer and the bottom of the first trench is equal to or greater than 0 nanometers and less than or equal to 10 nanometers.

5. The FET of claim 1 wherein:

the first dielectric layer is deposited by metal organic chemical vapor deposition; and

the third dielectric layer is deposited by plasma-enhanced chemical vapor deposition.

6. The FET of claim 1 wherein:

the first dielectric layer comprises SiN deposited at a temperature greater than 600 degrees centigrade by metal organic chemical vapor deposition; and

the third dielectric layer comprises SiN deposited at a temperature lower than 500 degrees centigrade by plasma-enhanced chemical vapor deposition.

7. The FET of claim 1 wherein:

the gate electrode extends over the third dielectric layer partially toward the source electrode and the drain electrode to form an integrated gate field-plate.

8. The FET of claim 1 wherein:

the first dielectric layer comprises SiN having a thickness of 1 nm to 100 nm; and

the third dielectric layer comprises SiN having a thickness of 10 nm to 500 nm.

9. The FET of claim 1 wherein the second dielectric layer comprises:

a single-crystalline AlN layer at the bottom of the first trench;

a polycrystalline AlN layer on the single crystalline AlN layer; and

an insulating layer comprising SiN on the polycrystalline AlN layer.

10. The FET of claim 9 wherein:

the single crystalline AlN is grown at a temperature greater than 600 C. and less than 1100 C.;

the poly crystalline AlN is grown at a temperature greater than 300 C. and less than 900 C.

11. The FET of claim 9 wherein:

the single crystalline AlN layer is up to 2 nm thick;

the polycrystalline AlN layer is 1 nm to 50 nm thick; and

the insulating layer is 1 nm to 50 nm thick.

12. The FET of claim 1 wherein:

the first dielectric layer is only in a gate area and the second dielectric layer covers the first dielectric layer in the gate area; or

the first dielectric layer extends from the source electrode to the drain electrode and the second dielectric layer covers the first dielectric layer only in the gate area; or

the first dielectric layer extends from the source electrode to the drain electrode and the second dielectric layer covers the first dielectric layer and extends from the source electrode to the drain electrode.

13. The FET of claim 1 wherein:

the single crystalline AlN layer is up to 2 nm thick.

14. The FET of claim 1 wherein:

the single crystalline AlN is grown at a temperature greater than 600 C. and less than 1100 C.

15. A method of fabricating a field-effect transistor (FET) comprising:

forming a plurality of semiconductor layers;

forming a source electrode contacting at least one of the semiconductor layers;

forming a drain electrode contacting at least one of the semiconductor layers;

forming a first dielectric layer covering a portion of semiconductor top surface between the source electrode and the drain electrode;

forming a first trench extending through the first dielectric layer and having a bottom located on a top surface of the semiconductor layers or within one of the semiconductor layers;

forming a second dielectric layer lining the first trench and covering a portion of the first dielectric layer;

forming a third dielectric layer over the semiconductor layers, the first dielectric layer, and the second dielectric layer;

forming a second trench extending through the third dielectric layer and having a bottom located in the first trench on the surface of or within the second dielectric layer, and extending over a portion of the second dielectric layer on the first dielectric layer; and

forming a gate electrode filling the second trench;

wherein the second dielectric layer comprises a single-crystalline AlN layer at the bottom of the first trench; and

wherein the second dielectric layer comprises a polycrystalline AlN layer on the single crystalline AlN layer.

16. The method of claim 15 further comprising:

providing a substrate comprising silicon (Si), silicon carbide (SiC), sapphire (Al 2 O 3 ), gallium nitride (GaN), or aluminum nitride (AlN);

wherein the plurality of semiconductor layers comprise:

a III-nitride channel layer; and

a barrier layer over the channel layer.

17. The method of claim 16 wherein:

the channel layer comprises GaN and has a thickness ranging from 5 nanometers to a few micrometers; and

the barrier layer comprises AlGaN having a thickness ranging from 1-30 nanometers and having a 25% Al composition.

18. The method of claim 16 wherein:

a distance between an interface of the channel layer and the barrier layer and the bottom of the first trench is equal to or greater than 0 nanometers and less than or equal to 10 nanometers.

19. The method of claim 15 wherein:

forming the first dielectric layer comprises depositing the first dielectric layer by metal organic chemical vapor deposition; and

forming the third dielectric layer comprises depositing the third dielectric layer by plasma-enhanced chemical vapor deposition.

20. The method of claim 15 wherein:

the first dielectric layer comprises SiN deposited at a temperature greater than 600 degrees centigrade by metal organic chemical vapor deposition; and

the third dielectric layer comprises SiN deposited at a temperature lower than 500 degrees centigrade by plasma-enhanced chemical vapor deposition.

21. The method of claim 15 further comprising:

forming the gate electrode to extend over the third dielectric layer partially toward the source electrode and the drain electrode to form an integrated gate field-plate.

22. The method of claim 15 wherein:

the first dielectric layer comprises SiN having a thickness of 1 nm to 100 nm; and

the third dielectric layer comprises SiN having a thickness of 10 nm to 500 nm.

23. The method of claim 15 wherein the forming the second dielectric layer comprises:

forming a single-crystalline AlN layer at the bottom of the first trench;

forming a polycrystalline AlN layer on the single crystalline AlN layer; and

forming an insulating layer comprising SiN on the polycrystalline AlN layer.

24. The method of claim 23 wherein:

the single crystalline AlN is grown at a temperature greater than 600 C. and less than 1100 C.;

the poly crystalline AlN is grown at a temperature greater than 300 C. and less than 900 C.

25. The method of claim 23 wherein:

the single crystalline AlN layer is up to 2 nm thick;

the polycrystalline AlN layer is 1 nm to 50 nm thick; and

the insulating layer is 1 nm to 50 nm thick.

26. The method of claim 15 wherein:

the first dielectric layer is only formed in a gate area and the second dielectric layer covers the first dielectric layer in the gate area; or

the first dielectric layer is formed to extend from the source electrode to the drain electrode and the second dielectric layer covers the first dielectric layer only in the gate area; or

the first dielectric layer is formed to extend from the source electrode to the drain electrode and the second dielectric layer covers the first dielectric layer and extends from the source electrode to the drain electrode.

27. The method of claim 15 wherein:

the first dielectric layer provides an etch stop for patterning of the second dielectric layer.

28. The method of claim 15 wherein:

forming the first trench comprises etching; and

forming the second trench comprises etching and the second dielectric layer provides an etch stop.

29. The method of claim 15 wherein:

the single crystalline AlN layer is formed up to 2 nm thick.

30. The method of claim 15 wherein:

the single crystalline AlN is grown at a temperature greater than 600 C. and less than 1100 C.