Semiconductor device

1. A semiconductor device comprising:

a silicon substrate;

a first conductive type channel region formed in the silicon substrate;

a gate insulating film formed over the channel region;

a gate electrode formed over the gate insulating film;

a first diffusion region that includes a first impurity with a second conductive type and that is formed over the silicon substrate at both sides of the channel region; and

a Si mixed crystal layer with the second conductive type that is in contact with the first diffusion region and buried in a trench portion that is formed in the silicon substrate;

wherein, the Si mixed crystal layer includes:

a first Si mixed crystal layer that includes a second impurity with a first concentration;

a second Si mixed crystal layer formed over the first Si mixed crystal layer and that includes the second impurity with a second concentration higher than the first concentration; and

a third Si mixed crystal layer formed over the second Si mixed crystal layer and that includes the second impurity with a third concentration lower than the second concentration.

2. The semiconductor device according to claim 1 ; further comprising:

a second diffusion region with the second conductive type formed between the silicon substrate and the first Si mixed crystal layer.

3. The semiconductor device according to claim 1 ; further comprising:

a silicide layer formed over the third Si mixed crystal layer.

4. The semiconductor device according to claim 1 , wherein the first conductive type is an n-type, the second conductive type is a p-type, and the Si mixed crystal layer is a p-type SiGe layer.

5. The semiconductor device according to claim 1 , wherein the first conductive type is a p-type, the second conductive type is an n-type, and the Si mixed crystal layer is one of an n-type SiC layer and an n-type SiGeC layer.