Light-emitting device

1. A light-emitting device, comprising:

a first conductivity type semiconductor layer;

a second conductivity type semiconductor layer; and

an active region comprising a material having a composition of Al x In y Ga (1-x-y) N (0≦x≦1, 0≦y≦1, 0≦x+y≦1) between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, wherein the active region comprising:

a plurality of barriers and one well disposed between any two of adjacent barriers, wherein the barriers comprises a composite barrier and a single barrier while the composite barrier is composed of a gradient layer having an element with a gradient concentration therein and a first non-gradient layer having a non-gradient composition, and the single barrier is composed of a second non-gradient layer adjacent to the first conductivity type semiconductor layer or the second conductivity type semiconductor layer,

wherein the first non-gradient layer directly contacts with the well.

2. The light-emitting device as claimed in claim 1 , wherein the single barrier is adjacent to the first conductivity type semiconductor layer.

3. The light-emitting device as claimed in claim 2 , wherein the first non-gradient layer of each composite barrier is disposed closer to the first conductivity type semiconductor layer than the gradient layer is.

4. The light-emitting device as claimed in claim 3 , wherein the element with the gradient concentration is In, and the concentration of In gradually decreases toward the second conductivity type semiconductor layer.

5. The light-emitting device as claimed in claim 3 , wherein the element with the gradient concentration is Al, and the concentration of Al gradually increases toward the second conductivity type semiconductor layer.

6. The light-emitting device as claimed in claim 4 , wherein the well comprises In element, and the In concentration in the gradient layer closest to the first non-gradient layer is substantially the same as the concentration of In element in the well.

7. The light-emitting device as claimed in claim 4 , wherein the first non-gradient layer comprises In, and the smallest concentration of In in the gradient layer is substantially the same as the concentration of In element in the first non-gradient layer.

8. The light-emitting device as claimed in claim 1 , wherein the single barrier is adjacent to the second conductivity type semiconductor layer.

9. The light-emitting device as claimed in claim 8 , wherein the gradient layer of each composite barrier is disposed closer to the first conductivity type semiconductor layer than the first non-gradient layer is.

10. The light-emitting device as claimed in claim 9 , wherein the element with the gradient concentration is In, and the concentration of In gradually increases toward the second conductivity type semiconductor layer.

11. The light-emitting device as claimed in claim 9 , wherein the element with the gradient concentration is Al, and the concentration of Al gradually decreases toward the second conductivity type semiconductor layer.

12. The light-emitting device as claimed in claim 10 , wherein the well comprises In element, and the In concentration in the gradient layer closest to the first non-gradient layer is substantially the same as the concentration of In element in the well.

13. The light-emitting device as claimed in claim 10 , wherein the first non-gradient layer comprises In, and the smallest concentration of In in the gradient layer is substantially the same as the concentration of In element in the first non-gradient layer.

14. The light-emitting device as claimed in claim 1 , wherein a thickness of the single barrier is in a range from about 5 nm to about 15 nm.

15. The light-emitting device as claimed in claim 1 , wherein the single barrier comprises GaN.

16. The light-emitting device as claimed in claim 1 , wherein a thickness of the first non-gradient layer is less than or equal to about 5 nm.

17. The light-emitting device as claimed in claim 1 , wherein the first non-gradient layer comprises GaN.

18. The light-emitting device as claimed in claim 1 , wherein a thickness of the gradient layer is in a range from about 3 nm to about 15 nm.

19. The light-emitting device as claimed in claim 1 , wherein the gradient layer has a gradient concentration of In and has a thickness of about 10 nm.

20. A light-emitting device, comprising:

a first conductivity type semiconductor layer;

a second conductivity type semiconductor layer; and

an active region between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, the active region comprising:

a plurality of barriers and one well disposed between any two of adjacent barriers, wherein the barriers comprises a composite barrier and a single barrier while the composite barrier is composed of a gradient layer having a gradient energy band gap and a first non-gradient layer having a first constant energy band gap, and the single barrier is composed of a second non-gradient layer having a second constant energy band gap adjacent to the first conductivity type semiconductor layer or the second conductivity type semiconductor layer,

wherein the first non-gradient layer directly contacts with the well.