Semiconductor device

1. A semiconductor device comprising:

a semiconductor layer made of a wide bandgap semiconductor having a gate trench provided with a sidewall and a bottom wall;

a gate insulating film formed on the sidewall and the bottom wall of the gate trench;

a gate electrode embedded in the gate trench to be opposed to the semiconductor layer through the gate insulating film, wherein

the semiconductor layer includes:

a first conductivity type source region formed to be exposed on a side of a front surface of the semiconductor layer for partially forming the sidewall of the gate trench;

a second conductivity type body region formed on a side of the first conductivity type source region closer to a rear surface of the semiconductor layer to be in contact with the first conductivity type source region for partially forming the sidewall of the gate trench;

a first conductivity type drift region formed on a side of the second conductivity type body region closer to the rear surface of the semiconductor layer to be in contact with the second conductivity type body region for forming the bottom wall of the gate trench;

a second conductivity type first breakdown voltage holding region selectively formed on an edge portion of the gate trench where the sidewall and the bottom wall of the gate trench intersect with each other in a partial region of the gate trench, wherein

the first conductivity type drift region includes:

a first region of a first impurity concentration forming the bottom wall of the gate trench; and

a second region of a second impurity concentration smaller than the first impurity concentration formed on a side of the first region closer to the rear surface of the semiconductor layer to be in contact with the first region, wherein

a thickness of the first region is not more than a thickness of the second conductivity type first breakdown voltage holding region, and

an interface between the first region and the second region is at a constant distance from the rear surface of the semiconductor layer.

2. The semiconductor device according to claim 1 , wherein

an impurity concentration in the second conductivity type first breakdown voltage holding region is higher than an impurity concentration in the first conductivity type drift region.

3. The semiconductor device according to claim 1 , wherein

the gate trench is formed in a latticed manner,

the semiconductor layer includes a plurality of unit cells partitioned by a latticed gate trench and provided in a form of polygonal prisms each having a plurality of corner portions, and each unit cell has the first conductivity type source region, the second conductivity type body region and the first conductivity type drift region, and

the second conductivity type first breakdown voltage holding region is selectively formed on a corner edge portion of the gate trench formed on the corner portion of the unit cell.

4. The semiconductor device according to claim 3 , wherein

the second conductivity type first breakdown voltage holding region is formed to reach a portion of the second conductivity type body region immediately above the corner edge portion.

5. The semiconductor device according to claim 3 , wherein

the second conductivity type first breakdown voltage holding region is selectively formed on an intersection portion of the latticed gate trench.

6. The semiconductor device according to claim 3 , wherein

the semiconductor layer further includes a second conductivity type second breakdown voltage holding region, formed on a bottom wall of a linear portion of the latticed gate trench, having a width narrower than a width of the linear portion.

7. The semiconductor device according to claim 6 , wherein

an impurity concentration in the second conductivity type second breakdown voltage holding region is higher than an impurity concentration in the second conductivity type first breakdown voltage holding region.

8. The semiconductor device according to claim 6 , wherein

a thickness of the second conductivity type second breakdown voltage holding region is smaller than the thickness of the second conductivity type first breakdown voltage holding region.

9. The semiconductor device according to claim 1 , wherein

the semiconductor layer further includes:

a source trench having a sidewall and a bottom wall, passing through the first conductivity type source region and the second conductivity type body region from the front surface and reaching the first conductivity type drift region; and

a second conductivity type second breakdown voltage holding region selectively formed on an edge portion of the source trench where the sidewall and the bottom wall of the source trench intersect with each other in a partial region of the source trench.

10. The semiconductor device according to claim 1 , wherein

a first portion of the gate insulating film on the bottom wall of the gate trench is thicker than a second portion of the gate insulating film on the sidewall of the gate trench, and a top portion of the first portion is below a deepest portion of the second conductivity type body region.

11. The semiconductor device according to claim 1 , wherein

the front surface of the semiconductor layer is a C plane.

12. The semiconductor device according to claim 1 , wherein

the second conductivity type body region is formed by ion implantation.

13. The semiconductor device according to claim 1 , wherein a bottom portion of the second conductivity type first breakdown voltage holding region projects beyond the interface between the first region and the second region toward a side of the second region, and is covered with the second region.

14. The semiconductor device according to claim 9 , wherein a bottom portion of the second conductivity type second breakdown voltage holding region projects beyond the interface between the first region and the second region toward a side of the second region, and is covered with the second region.