Field effect transistor

1. A field effect transistor, comprising:

a substrate;

a C-doped semiconductor layer disposed on the substrate;

a channel layer disposed on the C-doped semiconductor layer;

an electron supply layer disposed on the channel layer, wherein a two-dimensional electron gas generates in an interface between the channel layer and the electron supply layer; and

a diffusion barrier layer comprising an Al content and disposed between the C-doped semiconductor layer and the channel layer, wherein

the diffusion barrier layer contacts the channel layer and the C-doped semiconductor layer directly, and

a thickness of the diffusion barrier layer is between 5 nm and 30 nm,

wherein the Al content in the diffusion barrier layer decreases or increases from the middle of the diffusion barrier layer toward the channel layer and the C-doped semiconductor layer, respectively.

2. The field effect transistor according to claim 1 , wherein the Al content in the diffusion barrier layer is between 2% and 20%.

3. The field effect transistor according to claim 1 , wherein the diffusion barrier layer comprises AlGaN, AlInGaN or AlN.

4. The field effect transistor according to claim 1 , wherein the thickness of the channel layer is 50 nm or less.

5. The field effect transistor according to claim 1 , further comprising an AlN layer disposed between the channel layer and the electron supply layer.

6. The field effect transistor according to claim 1 , wherein the C-doped semiconductor layer, the channel layer and the electron supply layer are III-V compound semiconductor materials.

7. A field effect transistor, comprising:

a substrate;

a first Al-containing semiconductor layer disposed over the substrate and being a C-doped layer;

a second Al-containing semiconductor layer disposed on the first Al-containing semiconductor layer;

a third Al-containing semiconductor layer disposed on the second Al-containing semiconductor layer;

a diffusion barrier layer comprising an Al content and disposed between the first Al-containing semiconductor layer and the second Al-containing semiconductor layer, wherein the Al content in the diffusion barrier layer is greater than an Al content in the first Al-containing semiconductor layer and an Al content in the second Al-containing semiconductor layer,

wherein the second Al-containing semiconductor layer and the third Al-containing semiconductor layer are arranged for generating a two-dimensional electron gas in an interface therebetween.

8. The field effect transistor according to claim 7 , wherein the Al content in the diffusion barrier layer is between 7% and 25%.

9. The field effect transistor according to claim 7 , wherein the Al content in the diffusion barrier layer decreases or increases continuously along a direction from the first Al-containing semiconductor layer to the second Al-containing semiconductor layer.

10. The field effect transistor according to claim 7 , wherein the Al content in the diffusion barrier layer decreases or increases from the middle of the diffusion barrier layer toward the second Al-containing semiconductor layer and the first Al-containing semiconductor layer, respectively.

11. The field effect transistor according to claim 7 , wherein a thickness of the diffusion barrier layer is between 5 nm and 30 nm.

12. The field effect transistor according to claim 7 , wherein the diffusion barrier layer comprises AlGaN, AlInGaN or AlN.

13. The field effect transistor according to claim 7 , wherein the thickness of the second Al-containing semiconductor layer is 50 nm or less.

14. The field effect transistor according to claim 7 , further comprising an AlN layer disposed between the second Al-containing semiconductor layer and the third Al-containing semiconductor layer.

15. The field effect transistor according to claim 7 , wherein the first Al-containing semiconductor layer, the second Al-containing semiconductor layer, and the third Al-containing semiconductor layer are III-V compound semiconductor materials.

16. The field effect transistor according to claim 1 , wherein the diffusion barrier layer is a single layer.

17. The field effect transistor according to claim 7 , wherein the diffusion barrier layer is a single layer.

18. The field effect transistor according to claim 5 , wherein a thickness of the AlN layer is 1 nm.

19. The field effect transistor according to claim 1 , wherein the substrate is a single crystal substrate or a composite substrate.

20. The field effect transistor according to claim 1 , wherein the material of the substrate comprises silicon, silicon carbide, gallium nitride, aluminum gallium nitride, or aluminum nitride.