GaN HEMTs and GaN diodes
Consistent with an example embodiment, a GaN heterojunction structure has a three-layer dielectric structure. The lowermost and middle portions of the gate electrode together define the gate foot, and this is associated with two dielectric layers. A thinner first dielectric layer is adjacent the gate edge at the bottom of the gate electrode. The second dielectriclayer corresponds to the layer in the conventional structure, and it is level with the main portion of the gate foot.
1. A GaN heterojunction semiconductor device comprising:
a substrate;
a layer structure having a first layer having a first bandgap and a second layer having a second bandgap, wherein the first layer is located between the substrate and the second layer; and
a Schottky gate electrode and a first further electrode each conductively coupled to a different area of the the GaN heterojunction semiconductor device, said Schottky gate electrode comprising a central region and an edge region,
wherein the Schottky gate electrode comprises:
a lowermost portion which is co-planar with a first dielectric layer portion over the second layer;
a middle portion which is co-planar with a second dielectric layer, the middle portion overlapping the lowermost portion; and
an upper portion which is co-planar with a third dielectric layer, the upper portion overlapping the middle portion.
2. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein the first dielectric layer is over the second dielectric layer, and the second dielectric layer has a well in the region of the gate and in which the first dielectric layeris formed such that the first dielectric layer portion is the only one of the first, second and third dielectric layers beneath the overlap of the middle portion over the lowermost portion.
3. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein the first dielectric layer comprises an LPCVD layer, and the second and third dielectric layers comprise PECVD layers.
4. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein all three dielectric layers comprise SiN.
5. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein the length of the overlap of the middle portion over the lowermost portion is at least twice the thickness of the first dielectric layer.
6. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein the thickness of the first dielectric layer is in the range 5 to 50 nm.
7. The GaN heterojunction semiconductor device as claimed in claim 1 , wherein the first layer comprises GaN and the second layer comprises AlGaN.
8. A method of manufacturing a semiconductor device comprising:
providing a substrate carrying a layer structure having a first layer having a first bandgap and a second layer having a second bandgap, wherein the first layer is located between the substrate and the second layer, the interface between the first layer and second layer defining a heterojunction;
forming a dielectric layer structure over the second layer;
forming a Schottky gate electrode and a first further electrode each conductively coupled to a different area of the heterojunction, said Schottky gate electrode comprising a central region and an edge region, wherein the Schottky gate electrode comprises:
a lowermost portion which is co-planar with a first dielectric layer portion over the second layer;
a middle portion which is co-planar with a second dielectric layer, the middle portion overlapping the lowermost portion; and
an upper portion which is co-planar with a third dielectric layer, the upper portion overlapping the middle portion.
9. The method as claimed in claim 8 , wherein forming the dielectric layer structure comprises:
forming the second dielectric layer;
forming a well in the second dielectric layer in the region of the gate;
forming the first dielectric layer over the second dielectric layer and into the well;
removing a region of the first dielectric layer in the well;
forming the third dielectric layer over the first dielectric layer,
wherein the first dielectric layer portion is the only one of the first, second and third dielectric layers beneath the overlap of the middle portion over the lowermost portion.
10. The method as claimed in claim 8 , wherein the first dielectric layer is formed by LPCVD.
11. The method as claimed in claim 10 , wherein the second and third dielectric layers are formed by PECVD.
12. The method as claimed in claim 8 , wherein all three dielectric layers comprise SiN.
13. The method as claimed in claim 8 , wherein the length of the overlap of the middle portion over the lowermost portion is at least twice the thickness of the first dielectric layer.
14. The method as claimed in claim 8 , wherein the first dielectric layer is formed to a thickness in the range 5 to 50 nm.
15. The method as claimed in claim 8 , wherein the first layer comprises GaN and the second layer comprises AlGaN.