III-nitride semiconductor device with stepped gate trench and process for its manufacture

1. A III-nitride semiconductor device, comprising:

a III-nitride heterojunction that includes a first III-nitride body having one band gap, and a second III-nitride body having another band gap disposed over said first III-nitride body to form a two dimensional electron gas;

a passivation body formed over said second III-nitride body,

said passivation body including a gate well having a first mouth at said second III-nitride body and a second mouth wider than said first mouth at the top of said passivation body,

said second mouth defined by sidewalls within said passivation body,

said passivation body comprising a first ledge and a second ledge to define said first mouth, wherein said first ledge is wider than said second ledge; and

a gate arrangement disposed at least partially within said gate well, said gate arrangement including a gate insulation disposed at said first mouth of said well and a gate electrode over said gate insulation, said gate arrangement filling said second mouth of said gate well.

2. The device of claim 1 , further comprising a drain electrode and a source electrode on respective sides of said gate arrangement, wherein said first ledge is at a sidewall of said gate well closer to said drain electrode than said source electrode.

3. The device of claim 1 , wherein said ledges are comprised of silicon nitride.

4. The device of claim 1 , wherein said passivation body includes a first insulation body over said second III-nitride body and a second insulation body over said first insulation body.

5. The device of claim 4 , wherein said first insulation body is comprised of silicon nitride, and said second insulation body is comprised of a nitride dielectric.

6. The device of claim 5 , wherein said second insulation is comprised of AlN.

7. The device of claim 5 , wherein said second insulation is comprised of TiN.

8. The device of claim 1 , wherein said first III-nitride body is comprised of GaN and said second III-nitride body is comprised of AlGaN.

9. A method of fabricating a III-nitride device, comprising:

forming a first insulation body over a III-nitride heterojunction that includes a first III-nitride body having one band gap, and a second III-nitride body having another band gap disposed over said first III-nitride body to form a two dimensional electron gas;

forming a second insulation body over said first insulation body;

forming a trench through said first and second insulation bodies, said trench including a first mouth at said second III-nitride body and a second mouth at the top of said second insulation body; and

etching sidewalls of said trench to remove portions of said second insulation body thereby opening said second mouth wider than said first mouth and forming a first and a second ledge from said first insulation body to define said first mouth, said first ledge wider than said second ledge.

10. The method of claim 9 , wherein said first insulation body is comprised of silicon nitride, and said second insulation body is comprised of a nitride dielectric.

11. The method of claim 10 , wherein said second insulation is comprised of AlN.

12. The method of claim 10 , wherein said second insulation is comprised of TiN.

13. The method of claim 9 , wherein said first III-nitride body is comprised of GaN and said second III-nitride body is comprised of AlGaN.

14. The method of claim 9 , further comprising forming a gate arrangement in said trench.

15. The method of claim 14 , wherein said gate arrangement includes a gate dielectric body over said second III-nitride body and a gate electrode over said gate dielectric.

16. The method of claim 9 , further comprising forming a drain electrode and a source electrode on respective sides of said gate arrangement, wherein said first ledge is at a side of said gate arrangement closer to said drain electrode than said source electrode.