Semiconductor device and manufacturing method thereof

A manufacturing method of a semiconductor device comprising:

• providing a single crystalline semiconductor substrate of a first conductivity type;
• forming a first epitaxial semiconductor layer of a second conductivity type on the single crystalline semiconductor substrate;
• introducing a first impurity of the first conductivity type into a predetermined region in the first epitaxial semiconductor layer;
• forming a second epitaxial semiconductor layer of the second conductivity type on the first epitaxial semiconductor layer;
• forming a buried layer of the first conductivity type by diffusing the first impurity of the first conductivity type;
• introducing a second impurity of the first conductivity type into a region in the second epitaxial semiconductor layer above the buried layer of the first conductivity type;
• forming a well region of the first conductivity type abutting on the buried layer of the first conductivity type by diffusing the second impurity of the first conductivity type;
• and
• forming a high voltage MOS transistor in the well region of the first conductivity type.