III-N transistor structures with stepped cap layers
Described herein are III-N (e.g. GaN) devices having a stepped cap layer over the channel of the device, for which the III-N material is orientated in an N-polar orientation.
1. A III-N device, comprising:
a III-N channel layer over an N-face of a III-N barrier layer, the III-N channel layer having a smaller bandgap than the III-N barrier layer;
a first III-N cap layer over an N-face of the III-N channel layer, the first III-N cap layer having a larger bandgap than the III-N channel layer;
a second III-N cap layer over an N-face of the first III-N cap layer, wherein a portion of the second III-N cap layer adjacent to the first III-N cap layer has a smaller bandgap than the first III-N cap layer;
a gate contact between a source contact and a drain contact, and the gate contact is at least partially in a recess in the second III-N cap layer; wherein
the second III-N cap layer includes a first region and a second region, wherein the first region has a first end in contact with the gate contact and directly adjacent to a bottom surface of the recess and a second end between the first end and the drain contact, and the second region is directly adjacent to the first region and is between the first region and the drain contact;
a first thickness of the second III-N cap layer in the first region is everywhere less than a second thickness of the second III-N cap layer in the second region so that a charge density of a 2DEG channel below the first region is everywhere less than a charge density of the 2DEG channel below the second region;
the first end comprises a sidewall of the first thickness in contact with the gate contact; and
a lateral separation between the first end and the second end is greater than 25 nanometers.
2. The device of claim 1 , wherein the recess includes a sidewall proximal to the drain contact, and the gate contact is in contact with a first section of the sidewall but not to a second section of the sidewall.
3. The device of claim 2 , further comprising a gate dielectric layer between the gate contact and the sidewall of the recess.
4. The device of claim 1 , wherein the first thickness of the second III-N cap layer in the first region increases monotonically from the first end to the second end.
5. The device of claim 1 , wherein the recess includes a sidewall proximal to the drain contact, and the first region of the second III-N cap layer is directly below the sidewall of the recess.
6. The device of claim 1 , wherein the second III-N cap layer comprises alternating layers of GaN and AlGaN.
7. The device of claim 1 , wherein the second III-N cap layer comprises multiple layers of different III-N materials.
8. The device of claim 1 , wherein the III-N channel layer comprises GaN.
9. The device of claim 8 , wherein the first III-N cap layer comprises AlGaN, AlGaInN, AN or AlInN.
10. The device of claim 1 , wherein the second III-N cap layer comprises GaN.
11. The device of claim 1 , further comprising a cap layer including the first III-N cap layer and the second III-N cap layer, wherein the cap layer is etched to form a shape of the first region, the second region and the recess.
12. An electronic device, comprising:
a III-N material structure comprising a III-N channel layer over a III-N barrier layer and a III-N cap layer over the III-N channel layer, wherein a compositional difference between the III-N barrier layer and the III-N channel layer causes a 2DEG channel to be induced in the III-N channel layer; and
a gate contact between a source contact and a drain contact, wherein the gate contact is over an N-face of the III-N material structure and is at least partially in a recess in the III-N cap layer; wherein
the III-N cap layer in a drain-side access region of the device includes a first region and a second region, wherein the first region has a first end in contact with the gate contact and directly adjacent to a bottom surface of the recess and a second end between the first end and the drain contact, and the second region is directly adjacent to the first region and is between the first region and the drain contact;
a first thickness of the III-N cap layer in the first region is everywhere less than a second thickness of the III-N cap layer in the second region so that a charge density of the 2DEG channel below the first region is everywhere less than a charge density of the 2DEG channel below the second region; and
the first end comprises a sidewall of the first thickness in contact with the gate contact.
13. The electronic device of claim 12 , wherein the charge density of the 2DEG channel in the first region increases monotonically from a first charge density at the first end to a second charge density at the second end.
14. The electronic device of claim 13 , wherein the second charge density is at least 1.1 times the first charge density.
15. The electronic device of claim 12 , wherein the III-N cap layer is thicker in the second region than at the first end of the first region.
16. The electronic device of claim 12 , further comprising a gate dielectric layer between the gate contact and the first end of the first region.
17. The electronic device of claim 16 , wherein the III-N material structure further comprises an AlGaN layer between the III-N channel layer and the III-N cap layer, and the gate dielectric layer is between the gate contact and the AlGaN layer.
18. The electronic device of claim 12 , wherein the charge density of the 2DEG channel increases monotonically from the gate contact to the source contact.
19. The device of claim 1 , wherein the recess includes a sidewall proximal to the drain contact, and the sidewall includes a plurality of steps.
20. A method of making a III-N device, comprising:
providing a III-N channel layer over an N-face of a III-N barrier layer, the III-N channel layer having a smaller bandgap than the III-N barrier layer;
forming a first III-N cap layer over an N-face of the III-N channel layer, the first III-N cap layer having a larger bandgap than the III-N channel layer;
forming a second III-N cap layer over an N-face of the first III-N cap layer, wherein a portion of the second III-N cap layer adjacent to the first III-N cap layer has a smaller bandgap than the first III-N cap layer;
depositing a gate contact between a source contact and a drain contact, the gate contact at least partially in a recess in the second III-N cap layer; wherein:
the second III-N cap layer includes a first region and a second region, wherein the first region has a first end in contact with the gate contact and directly adjacent to a bottom surface of the recess and a second end between the first end and the drain contact, and the second region is directly adjacent to the first region and is between the first region and the drain contact;
a first thickness of the second III-N cap layer in the first region is everywhere less than a second thickness of the second III-N cap layer in the second region so that a charge density of a 2DEG channel below the first region is everywhere less than a charge density of the 2DEG channel below the second region;
the first end comprises a sidewall of the first thickness in contact with the gate contact; and
a lateral separation between the first end and the second end is greater than 25 nanometers.
21. The method of claim 20 , wherein:
the recess includes a sidewall proximal to the drain contact, and the gate contact is in contact with a first section of the sidewall but not to a second section of the sidewall, and
the method further comprises forming a gate dielectric layer between the gate contact and the sidewall of the recess.
22. The method of claim 20 , wherein the second III-N cap layer comprises alternating layers of GaN and AlGaN.
23. The method of claim 20 , wherein the III-N channel layer comprises GaN and the first III-N cap layer comprises AlGaN, AlGaInN, AN or AlInN.
24. The method of claim 20 , wherein the second III-N cap layer comprises GaN.
25. The method of claim 20 , wherein the recess includes a sidewall proximal to the drain contact, and the sidewall includes a plurality of steps.