Group III-nitride HEMT having a well region formed on the surface of substrate and contacted the buffer layer to increase breakdown voltage and the method for forming the same
The buffer breakdown of a group III-N HEMT on a p-type Si substrate is significantly increased by forming an n-well in the p-type Si substrate to lie directly below the metal drain region of the group III-N HEMT. The n-well forms a p-n junction which becomes reverse biased during breakdown, thereby increasing the buffer breakdown by the reverse-biased breakdown voltage of the p-n junction and allowing the substrate to be grounded. The buffer layer of a group III-N HEMT can also be implanted with n-type and p-type dopants which are aligned with the p-n junction to minimize any leakage currents at the junction between the substrate and the buffer layer.
1. A method of forming a transistor with increased buffer breakdown voltage comprising:
forming a substrate of a first conductivity type, the substrate having a top surface;
forming a well of a second conductivity type in the substrate, the well having a top surface;
forming a buffer layer to touch the top surface of the substrate and the top surface of the well, the buffer layer having a top surface;
forming a channel layer to touch the top surface of the buffer layer, the channel layer including a group III-nitride and having a top surface;
forming a barrier layer to touch the top surface of the channel layer, the barrier layer including a group III-nitride;
forming spaced-apart metal source and drain regions that make ohmic contact with the channel and barrier layers, the metal drain region lying directly over the well;
wherein the well is under the drain region and not under the source region, and also wherein a junction formed by the substrate and the well form a junction isolation barrier that acts in series with, and thereby increases the buffer breakdown voltage.
2. The method of claim 1 wherein the metal drain region is spaced apart from the well.
3. The method of claim 1 and further comprising removing a portion of the barrier layer and the channel layer to form an opening, a bottom of the opening exposing a portion of the buffer layer, a side wall of the opening exposing a portion of the channel layer and the barrier layer.
4. The method of claim 1 and further comprising forming an alignment mark opening in the substrate, the metal drain region having a predefined spatial relationship to the alignment mark opening.
5. The method of claim 1 and further comprising forming a metal gate to touch the barrier layer to form a Schottky contact, or be separated from the barrier layer by an insulating layer.
6. The method of claim 1 wherein forming the buffer layer includes:
forming a first buffer layer to touch the top surface of the substrate and the top surface of the well, the first buffer layer having a top surface;
forming a region of the second conductivity type in the first buffer layer; and
forming a second buffer layer to touch the top surface of the first buffer layer, the channel layer touching a top surface of the second buffer layer.
7. The method of claim 6 wherein the first buffer layer is thinner than the second buffer layer.
8. The method of claim 6 wherein the region of the second conductivity type in the first buffer layer lies directly over and touches the well.
9. The method of claim 6 and further comprising forming a region of the first conductivity type in the first buffer layer.
10. The method of claim 9 wherein the region of the first conductivity type in the first buffer layer touches the region of the second conductivity type in the first buffer layer.
11. A transistor with increased buffer breakdown voltage comprising:
a substrate of a first conductivity type, the substrate having a top surface;
a well of a second conductivity type located in the substrate, the well having a top surface;
a buffer layer that touches the top surface of the substrate and the top surface of the well, the buffer layer having a top surface;
a channel layer that touches the top surface of the buffer layer, the channel layer including a group III-nitride and having a top surface;
a barrier layer that touches the top surface of the channel layer, the barrier layer including a group III-nitride;
spaced-apart metal source and drain regions that make ohmic contact with the channel and barrier layer, the metal drain region lying directly over the well;
wherein the well is under the drain region and not under the source region, and also wherein a junction formed by the substrate and the well form a junction isolation barrier that acts in series with, and thereby increases the buffer breakdown voltage.
12. The transistor of claim 11 wherein the metal drain region is spaced apart from the well.
13. The transistor of claim 11 and further comprising an implant isolation region that touches the buffer layer, and touches and lies laterally between regions of the channel layer and the barrier layer.
14. The transistor of claim 11 and further comprising an alignment mark opening located in the substrate, the metal drain region having a predefined spatial relationship to the alignment mark opening.
15. The transistor of claim 11 and further comprising a metal gate that touches the barrier layer, or is separated from the barrier layer by an insulating layer.
16. The transistor of claim 11 wherein the buffer layer includes:
a first buffer layer that touches the top surface of the substrate and the top surface of the well;
a region of the second conductivity type located in the first buffer layer; and
a second buffer layer that touches the first buffer layer, the channel layer touching a top surface of the second buffer layer.
17. The transistor of claim 16 wherein the first buffer layer is thinner than the second buffer layer.
18. The transistor of claim 16 wherein the region of the second conductivity type located in the first buffer layer lies directly over and touches the well.
19. The transistor of claim 16 and further comprising a region of the first conductivity type located in the first buffer layer.
20. The transistor of claim 19 wherein the region of the first conductivity type located in the first buffer layer touches the region of the second conductivity type located in the first buffer layer.