Light emitting diode having modulation doped layer

1. A light emitting diode (LED), comprising:

an n-type contact layer;

a p-type contact layer; and

an active region of a multiple quantum well structure comprising an InGaN well layer and being interposed between the n-type contact layer and the p-type contact layer,

wherein the n-type contact layer comprises a first modulation doped layer comprising first InGaN layers and second InGaN layers alternately laminated, the first InGaN layers being doped with an n-type impurity, the second InGaN layers comprising a lower doping concentration than that of the first InGaN layers, and a second modulation doped layer comprising third InGaN layers and fourth InGaN layers alternately laminated, the third InGaN layers being doped with an n-type impurity, the fourth InGaN layers comprising a lower doping concentration than that of the third InGaN layers,

the first InGaN layers and the second InGaN layers in the first modulation doped layer have the same composition, and the third InGaN layers and the fourth InGaN layers in the second modulation doped layer have the same composition;

the second modulation doped layer is interposed between the first modulation doped layer and the active region; and

an n-electrode contacts the first modulation doped layer.

2. The LED of claim 1 , wherein an In composition ratio in the first modulation doped layer is different from that of the second modulation doped layer.

3. The LED of claim 2 , wherein the In composition ratio in the first modulation doped layer is less than that of the second modulation doped layer.

4. The LED of claim 1 , wherein one of the third InGaN layers doped with the n-type impurity in the second modulation doped layer is adjacent to the active region.

5. The LED of claim 1 , wherein an In composition ratio of the second modulation doped layer is less than an In composition ratio of the InGaN well layer.

6. The LED of claim 1 , wherein the second InGaN layers are undoped layers.

7. The LED of claim 1 , wherein the fourth InGaN layers are undoped layers.

8. The LED of claim 1 , further comprising a sapphire substrate and a GaN buffer layer interposed between the sapphire substrate and the n-type contact layer.

9. The LED of claim 1 , wherein the p-type contact layer comprises a first p-type GaN layer, a second p-type GaN layer, and a third modulation doped layer interposed between the first p-type GaN layer and the second p-type GaN layer; the third modulation doped layer comprises fifth InGaN layers and sixth InGaN layers alternately laminated, the fifth InGaN layers being doped with a p-type impurity, the sixth InGaN layers comprising a lower doping concentration than that of the fifth InGaN layers; and the fifth InGaN layers and the sixth InGaN layers have the same composition.

10. The LED of claim 9 , wherein the second p-type GaN layer comprises the same doping concentration as the first p-type GaN layer.

11. The LED of claim 9 , wherein a doping concentration of the fifth InGaN layers differs from a doping concentration of the first p-type GaN layer and the second p-type GaN layer.

12. The LED of claim 11 , wherein the doping concentration of the fifth InGaN layers is less than the doping concentration of the first p-type GaN layer and the doping concentration of the second p-type GaN layer.

13. The LED of claim 9 , wherein the sixth InGaN layers are undoped layers.

14. The LED of claim 9 , further comprising a p-type AlGaN clad layer interposed between the active region and the p-type contact layer.

15. The LED of claim 14 , further comprising a superlattice layer interposed between the active region and the p-type AlGaN clad layer, the superlattice layer comprising InGaN layers doped with a p-type impurity and AlGaN layers doped with a p-type impurity alternately laminated.

16. An LED, comprising:

an n-type contact layer;

a p-type contact layer formed over the n-type contact layer; and

an active region of a multiple quantum well structure comprising an InGaN well layer, the active region being interposed between the n-type contact layer and the p-type contact layer,

wherein the p-type contact layer comprises a first p-type GaN layer, a second p-type GaN layer and a modulation doped layer interposed between the first p-type GaN layer and the second p-type GaN layer,

the modulation doped layer comprises fifth InGaN layers and sixth InGaN layers alternately laminated, the fifth InGaN layers being doped with a p-type impurity, the sixth InGaN layers comprising a lower doping concentration than that of the fifth InGaN layers, and

the fifth InGaN layers and the sixth InGaN layers have the same composition.

17. The LED of claim 16 , wherein the second p-type GaN layer comprises the same doping concentration as the first p-type GaN layer.

18. The LED of claim 16 , wherein a doping concentration of the fifth InGaN layers differs from a doping concentration of the first p-type GaN layer and a doping concentration of the second p-type GaN layer.

19. The LED of claim 18 , wherein the doping concentration of the fifth InGaN layers is less than the doping concentration of the first p-type GaN layer and the doping concentration of the second p-type GaN layer.

20. The LED of claim 16 , wherein the sixth InGaN layers are undoped layers.

21. The LED of claim 16 , further comprising a p-type AlGaN clad layer interposed between the active region and the p-type contact layer.

22. The LED of claim 21 , further comprising a superlattice layer interposed between the active region and the p-type AlGaN clad layer, the superlattice layer comprising InGaN layers doped with a p-type impurity and AlGaN layers doped with a p-type impurity alternately laminated.