Semiconductor device

1. A semiconductor device, comprising:

a channel layer formed on a substrate;

a top barrier layer formed on the channel layer, wherein a first heterojunction is formed between the channel layer and the top barrier layer so that a first two-dimensional electron gas is generated in the channel layer;

a buffer structure formed between the substrate and the channel layer;

a back barrier layer formed between the buffer structure and the channel layer, wherein a second heterojunction is formed between the buffer structure and the back barrier layer so that a second two-dimensional electron gas is generated in the buffer structure; and

a source electrode, a drain electrode, and a gate electrode formed on the top barrier layer, respectively;

wherein a sheet carrier density of the second two-dimensional electron gas is less than 8E+10 cm −2 .

2. The semiconductor device of claim 1 , wherein the back barrier layer comprises undoped Al y Ga 1−y N and wherein y>0.3.

3. The semiconductor device of claim 2 , wherein a thickness of the back barrier layer is less than 20 nm.

4. The semiconductor device of claim 3 , comprising a breakdown voltage BV DS , wherein BV DS ≥800 V.

5. The semiconductor device of claim 1 , wherein the channel layer comprises GaN and a thickness thereof is greater than or equal to 150 nm.

6. The semiconductor device of claim 1 , wherein the buffer structure is carbon-doped and comprises a carbon doping concentration greater than 1E+18 cm −3 .

7. The semiconductor device of claim 1 , wherein the buffer structure comprises a plurality of layers.

8. The semiconductor device of claim 7 , wherein the back barrier layer has an energy band gap larger than that of a top most layer of the plurality of layers.

9. A semiconductor device, comprising:

a first heterojunction on a substrate;

a second heterojunction between the first heterojunction and the substrate;

a source electrode, a drain electrode, and a gate electrode formed on the first heterojunction, respectively;

a first two-dimensional electron gas formed between the first heterojunction and the second heterojunction; and

a second two-dimensional electron gas formed between the second heterojunction and the substrate;

wherein a ratio of a sheet carrier density of the second two-dimensional electron gas to a sheet carrier density of the first two-dimensional electron gas is less than 3%.

10. The semiconductor device of claim 9 , wherein the sheet carrier density of the second two-dimensional electron gas is less than 8E+10 cm −3 .

11. The semiconductor device of claim 9 , further comprising a buffer structure on the substrate; and a back barrier layer on the buffer structure;

wherein an interface between the buffer structure and the back barrier layer forms the second heterojunction; and

wherein the back barrier layer comprises undoped Al y Ga 1−y N and y>0.3, and/or a thickness of the back barrier layer is less than 20 nm.

12. The semiconductor device of claim 11 , wherein the buffer structure is carbon-doped and comprises a carbon doping concentration greater than 1E+18 cm −3 .

13. The semiconductor device of claim 11 , wherein the buffer structure comprises a plurality of layers.

14. The semiconductor device of claim 13 , wherein the back barrier layer has an Al composition larger than that of a top most layer of the plurality of layers.

15. The semiconductor device of claim 11 , further comprising a channel layer on the substrate; and a top barrier layer on the channel layer;

wherein an interface between the channel layer and the top barrier layer forms the first heterojunction.

16. The semiconductor device of claim 15 , wherein the channel layer comprises GaN and/or a thickness thereof is greater than or equal to 150 nm.

17. The semiconductor device of claim 11 , wherein the buffer structure comprises a thickness of 0.5 μm to 5.5 μm.