Gallium nitride compound semiconductor element

1. A gallium nitride semiconductor device comprising an active layer made of an n-type gallium nitride semiconductor that includes In and is doped with n-type impurity and a p-type cladding layer made of a p-type gallium nitride semiconductor that includes Al and is doped with p-type impurity,

wherein a first cap layer, made of a gallium nitride semiconductor that includes n-type impurity of lower concentration than that of said active layer and p-type impurity of lower concentration than that of said p-type cladding layer, and a second cap layer made of a p-type gallium nitride semiconductor that includes Al and is doped with p-type impurity are stacked one on another between said active layer and said p-type cladding layer.

2. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer is formed in contact with said active layer and said second cap layer is formed in contact with said first cap layer.

3. The gallium nitride semiconductor device according to claim 1 , wherein concentration of the n-type impurity and the p-type impurity in said first cap layer is 1.0×10 17 cm −3 or lower, and concentration of the p-type impurity in said second cap layer is from 8.0×10 18 or to 2.0×10 19 cm −3 .

4. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer is grown without being doped with the n-type impurity and the p-type impurity, and includes the n-type impurity due to thermal diffusion from said active layer and the p-type impurity due to thermal diffusion from said second cap layer.

5. The gallium nitride semiconductor device according to claim 1 , wherein thickness of said first cap layer is equal to or less than the sum of thermal diffusion distance traveled in said first cap layer by the n-type impurity that is injected into said active layer and thermal diffusion distance traveled in said first cap layer by the p-type impurity that is injected into said second cap layer.

6. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer comprises one selected from a group consisting of GaN layer, In x Ga 1-x N layer (0<x<1) and Al y Ga 1-y N layer (0<y<1), and a stack of some of these layers.

7. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer comprises GaN layer and has a thickness in a range from 15 to 100 Å.

8. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer comprises In x Ga 1-x N layer (0<x<1) and has a thickness in a range from 15 to 150 Å.

9. The gallium nitride semiconductor device according to claim 1 , wherein said first cap layer comprises Al y Ga 1-y N layer (0<y<1) and has a thickness in a range from 15 to 50 Å.

10. The gallium nitride semiconductor device according to claim 1 , wherein the thickness of said second cap layer is in a range from 15 to 500 Å.

11. The gallium nitride semiconductor device according to claim 1 , wherein said active layer is formed by stacking well layers made of a gallium nitride semiconductor that includes In and barrier layers made of a gallium nitride semiconductor doped with n-type impurity alternately one on another, while said first cap layer includes n-type impurity in a concentration lower than that in said barrier layer.

12. The gallium nitride semiconductor device according to claim 1 ,

wherein said active layer is formed by stacking well layers made of gallium nitride semiconductor and barrier layers made of gallium nitride semiconductor, and

wherein the layer that is doped with an n-type impurity and is nearest to the first cap layer among said well layers and barrier layers has the concentration of n-type impurity in a range from 5.0×10 17 to 1.0×10 19 cm −3 .