Semiconductor device and method for fabricating the same

1. A semiconductor device comprising:

a drift diffusion region of a first conductivity type formed in an upper portion of a semiconductor substrate;

a body diffusion region of a second conductivity type formed in an upper portion of the drift diffusion region;

a source diffusion region of the first conductivity type formed in an upper portion of the body diffusion region;

an insulating film buried in a trench formed in an upper portion of the drift diffusion region, and spaced apart from the body diffusion region;

a drain diffusion region of the first conductivity type formed in an upper portion of the drift diffusion region and adjacent to the insulating film on an opposite side of the insulating film from the source diffusion region; and

a gate electrode formed on a portion of the body diffusion region, the drift diffusion region, and a portion of the insulating film with a gate insulating film being interposed between the gate electrode, and the portion of the body diffusion region, the drift diffusion region, and the portion of the insulating film,

wherein

the drift diffusion region includes both a substrate inner region and a surface region, said surface region directly contacts the body diffusion region,

the surface region is formed between the substrate inner region and the gate electrode, and

the surface region contains an impurity of the first conductivity type at a higher concentration than that of the substrate inner region.

2. The semiconductor device of claim 1 , wherein a portion of the drift diffusion region located at a bottom portion of the trench contains the impurity of the first conductivity type at a higher concentration than that of the substrate inner region located at a same depth.

3. The semiconductor device of claim 1 , wherein an upper end portion and a bottom corner portion of the trench are rounded.

4. The semiconductor device of claim 1 , wherein the concentration of the impurity of the first conductivity type contained in the drift diffusion region is lower than the concentration of the impurity of the first conductivity type contained in the source and drain diffusion regions.

5. The semiconductor device of claim 1 ,

wherein the surface region of the drift diffusion region has substantially the same depth along an interface with the substrate inner region of the drift diffusion region.

6. The semiconductor device of claim 1 , wherein

the surface region of the drift diffusion region has a first length in a depth direction and a second length in a direction perpendicular to the depth direction, and

the second length is larger than the first length.

7. A semiconductor device comprising:

a drift diffusion region of a first conductivity type formed in an upper portion of a semiconductor substrate;

a body diffusion region of a second conductivity type formed in an upper portion of the semiconductor substrate;

a source diffusion region of the first conductivity type formed in an upper portion of the body diffusion region;

an insulating film buried in a trench formed in an upper portion of the drift diffusion region, and spaced apart from the body diffusion region;

a drain diffusion region of the first conductivity type formed in an upper portion of the drift diffusion region and adjacent to the insulating film on an opposite side of the insulating film from the source diffusion region; and

a gate electrode formed on a portion of the body diffusion region, the drift diffusion region, and a portion of the insulating film with a gate insulating film being interposed between the gate electrode, and the portion of the body diffusion region, the drift diffusion region, and the portion of the insulating film,

wherein the drift diffusion region includes both a substrate inner region, and a surface region formed on the substrate inner region and below the gate electrode and containing an impurity of the first conductivity type at a higher concentration than that of the substrate inner region, and

wherein the body diffusion region and the drift diffusion region are spaced apart from each other, and a portion of the semiconductor substrate which is located between the body diffusion region and the drift diffusion region is of the second conductivity type.

8. The semiconductor device of claim 7 , wherein a portion of the drift diffusion region located at a bottom portion of the trench contains the impurity of the first conductivity type at a higher concentration than that of the substrate inner region located at a same depth.

9. The semiconductor device of claim 7 , wherein an upper end portion and a bottom corner portion of the trench are rounded.

10. The semiconductor device of claim 7 , wherein the concentration of the impurity of the first conductivity type contained in the drift diffusion region is lower than the concentration of the impurity of the first conductivity type contained in the source and drain diffusion regions.

11. The semiconductor device of claim 7 ,

wherein the surface region of the drift diffusion region has substantially the same depth along an interface with the substrate inner region of the drift diffusion region.

12. The semiconductor device of claim 7 , wherein

the surface region of the drift diffusion region has a first length in a depth direction and a second length in a direction perpendicular to the depth direction, and

the second length is larger than the first length.