Silicon carbide semiconductor device

1. A silicon carbide semiconductor device comprising:

a silicon carbide substrate having a first main surface and a second main surface opposite to the first main surface; and

a source electrode provided on the first main surface,

the silicon carbide substrate including a first silicon carbide layer, and a second silicon carbide layer provided on the first silicon carbide layer and constituting the first main surface,

the first silicon carbide layer having

a first drift region having a first conductivity type,

a first impurity region having a second conductivity type different from the first conductivity type, and

a second impurity region having the second conductivity type and surrounding the first impurity region as seen in a direction perpendicular to the first main surface,

the second silicon carbide layer having

a second drift region having the first conductivity type,

a third impurity region having the second conductivity type and facing the first impurity region with the second drift region interposed therebetween, and

a fourth impurity region having the second conductivity type, facing the second impurity region with the second drift region interposed therebetween, and surrounding the third impurity region as seen in the direction perpendicular to the first main surface,

a first conductivity type impurity in the second drift region having a concentration higher than a concentration of a first conductivity type impurity in the first drift region,

the third impurity region being electrically connected to the source electrode,

the silicon carbide substrate having at least one of a first structure and a second structure,

the first structure being such that the first impurity region is in contact with the second impurity region, the third impurity region is separated from the fourth impurity region by the second drift region, and the second impurity region has a width greater than a width of the fourth impurity region in a direction parallel to the first main surface, and

the second structure being such that the first impurity region is separated from the second impurity region by the first drift region, the third impurity region is in contact with the fourth impurity region, and the fourth impurity region has a width greater than a width of the second impurity region in the direction parallel to the first main surface.

2. The silicon carbide semiconductor device according to claim 1 , wherein

the silicon carbide substrate has the first structure.

3. The silicon carbide semiconductor device according to claim 2 , wherein

the fourth impurity region includes one or more guard rings.

4. The silicon carbide semiconductor device according to claim 2 , wherein

the width of the second impurity region in the direction parallel to the first main surface is two or more times greater than a sum of a thickness of the first silicon carbide layer and a thickness of the second silicon carbide layer.

5. The silicon carbide semiconductor device according to claim 2 , wherein

the fourth impurity region is floating.

6. The silicon carbide semiconductor device according to claim 2 , wherein

the first impurity region is floating.

7. The silicon carbide semiconductor device according to claim 2 , wherein

the first impurity region is electrically connected to the source electrode.

8. The silicon carbide semiconductor device according to claim 2 , wherein

each of the first impurity region, the second impurity region and the fourth impurity region is electrically connected to the source electrode.

9. The silicon carbide semiconductor device according to claim 2 , wherein

a dose amount of a second conductivity type impurity in the second impurity region is not less than 0.4×10 13 cm −2 and not more than 2.0×10 13 cm −2 .

10. The silicon carbide semiconductor device according to claim 2 , wherein

a dose amount of a second conductivity type impurity in the fourth impurity region is not less than 0.4×10 13 cm −2 and not more than 2.0×10 13 cm −2 .

11. The silicon carbide semiconductor device according to claim 1 , wherein

the silicon carbide substrate has the second structure.

12. The silicon carbide semiconductor device according to claim 11 , wherein

the second impurity region includes one or more guard rings.

13. The silicon carbide semiconductor device according to claim 11 , wherein

the width of the fourth impurity region in the direction parallel to the first main surface is two or more times greater than a sum of a thickness of the first silicon carbide layer and a thickness of the second silicon carbide layer.

14. The silicon carbide semiconductor device according to claim 11 , wherein

the second impurity region is floating.

15. The silicon carbide semiconductor device according to claim 11 , wherein

the first impurity region is floating.

16. The silicon carbide semiconductor device according to claim 11 , wherein

the first impurity region is electrically connected to the source electrode.

17. The silicon carbide semiconductor device according to claim 11 , wherein

each of the first impurity region and the second impurity region is electrically connected to the source electrode.

18. The silicon carbide semiconductor device according to claim 11 , wherein

a dose amount of a second conductivity type impurity in the fourth impurity region is not less than 0.4×10 13 cm −2 and not more than 2.0×10 13 cm −2 .

19. The silicon carbide semiconductor device according to claim 11 , wherein

a dose amount of a second conductivity type impurity in the second impurity region is not less than 0.4×10 13 cm −2 and not more than 2.0×10 13 cm −2 .

20. The silicon carbide semiconductor device according to claim 1 , wherein

the first conductivity type impurity in the first drift region has a concentration of not less than 5×10 13 cm −3 and not more than 1×10 16 cm −3 .

21. The silicon carbide semiconductor device according to claim 1 , wherein

the first conductivity type impurity in the second drift region has a concentration of not less than 5×10 15 cm −3 and not more than 1×10 18 cm −3 .