III-Nitride devices with recessed gates
III-nitride devices are described with recessed gates. In some embodiments, the material around the gates is formed by epitaxially depositing different III-nitride layers on a substrate and etching through at least the top two layers in the gate region. Because adjacent layers in the top three layers of the structure have different compositions, some of the layers act as etch stops to allow for precision etching. In some embodiments, a regrowth mask is used to prevent growth of material in the gate region. A gate electrode is deposited in the recess.
1. A method of forming a III-nitride device, comprising:
forming a layer of GaN on a substrate;
forming a first portion of a layer of AlGaN on the layer of GaN;
depositing a regrowth mask over the gate region of the first portion of the layer of AlGaN;
forming a second portion of the layer of AlGaN in an access region of the device;
forming a gate electrode in a gate region of the device; and
forming a source and a drain outside of the gate region and defining in part the access region; wherein
the device is an enhancement mode device with a threshold voltage greater than 0V.
2. The method of claim 1 , further comprising removing the regrowth mask.
3. The method of claim 2 , wherein the regrowth mask is one of aluminum nitride, silicon nitride or silicon oxide.
4. The method of claim 1 , further comprising doping the second portion of the layer of AlGaN.
5. The method of claim 1 , wherein the first portion of the layer of AlGaN is Al x Ga 1-x N and the second portion of the layer of AlGaN is Al y Ga 1-y N, where x≠y.
6. The method of claim 5 , wherein x<y.
7. The method of claim 1 , wherein a top surface of the gate region has a flatness within 2 nanometers.
8. The method of claim 1 , wherein the layer of AlGaN has a uniform composition throughout a thickness of the layer.
9. The method of claim 1 , wherein forming the layer of AlGaN controls a thickness of the layer of AlGaN to within 2 nanometers.
10. The method of claim 1 , wherein the layer of AlGaN is doped.
11. The method of claim 10 , wherein the layer of AlGaN is doped with iron.
12. The method of claim 1 , further comprising forming an insulating region between the gate electrode and the layer of AlGaN.
13. The method of claim 1 , wherein forming a first portion of a layer of AlGaN includes epitaxially growing AlGaN.
14. The method of claim 1 , wherein a two-dimensional electron gas is present in the access region but not in the gate region when 0 gate voltage is applied to the III-nitride device.
15. The method of claim 1 , wherein the second portion of the layer of AlGaN includes a surface, further comprising forming a passivation layer over the surface in the access region.
16. The method of claim 15 , wherein the passivation layer is a dielectric.
17. The method of claim 15 , wherein the passivation layer is SiN.
18. The method of claim 1 , wherein the first portion of the layer of AlGaN is made thin enough to ensure that the threshold voltage of the III-nitride device is greater than 0V.