semiconductor switching element;

a protection diode for protecting the semiconductor switching element; and

a semiconductor substrate having a first surface and a second surface, wherein

the semiconductor switching element and the protection diode are integrated into one chip so that the semiconductor switching element and the protection diode are disposed in the semiconductor substrate,

the semiconductor substrate has a first conductive type,

the protection diode includes:

a first trench extending from the first surface of the semiconductor substrate in a depth direction perpendicular to the first surface of the semiconductor substrate; and

an epitaxial semiconductor layer having a second conductive type, wherein the epitaxial semiconductor layer is disposed in the first trench,

the first trench includes an inner wall as an interface between the semiconductor substrate and the epitaxial semiconductor layer so that the interface provides a PN junction,

the first trench has an aspect ratio between a depth and a width, the aspect ratio being equal to or larger than 1,

the semiconductor substrate includes a high concentration layer having the first conductive type, a low concentration layer having the first conductive type, and a second trench,

the high concentration layer has an impurity concentration higher than an impurity concentration of the low concentration layer of the protection diode,

the first trench of the protection diode is disposed inside of the second trench so that the low concentration layer is embedded between the second trench and the first trench,

the PN junction is provided by an interface between the epitaxial semiconductor layer and the low concentration layer,

the second trench of the protection diode extends from the first surface of the semiconductor substrate in the depth direction,

the semiconductor switching element is a three dimensional power MOSFET,

the three dimensional power MOSFET includes:

a third trench extending from the first surface of the semiconductor substrate in the depth direction;

a drift semiconductor layer disposed in the third trench, wherein the drift semiconductor layer has the first conductive type and a low impurity concentration lower than the high concentration layer of the semiconductor substrate;

a base semiconductor layer embedded in the drift semiconductor layer, wherein the base semiconductor layer has the second conductive type;

a source semiconductor layer embedded in the base semiconductor layer, wherein the source semiconductor layer has the first conductive type;

a fourth trench extending from the first surface of the semiconductor substrate in the depth direction, wherein the fourth trench is disposed inside of the third trench, and wherein the fourth trench extends between the source semiconductor layer and the drift semiconductor layer so that the fourth trench penetrates the base semiconductor layer;

a gate electrode disposed in the fourth trench through a gate insulation film, wherein the gate insulation film is disposed on an inner wall of the fourth trench; and

a drain region provided by the high concentration layer of the semiconductor substrate,

the first trench of the protection diode has a depth substantially equal to a depth of the third trench of the three dimensional power MOSFET,

the low concentration layer of the protection diode and the drift semiconductor layer of the MOSFET are made of a same epitaxial film, and

the epitaxial semiconductor layer of the protection diode and the base semiconductor layer of the MOSFET are made of a same epitaxial film; and the method comprising:

forming the third trench of the three dimensional power MOSFET together with the second trench of the semiconductor substrate;

forming the drift semiconductor layer in the third trench of the MOSFET together with forming the low concentration layer in the second trench of the semiconductor substrate, wherein the drift semiconductor layer and the low concentration layer are made of an epitaxial film; and

embedding the base semiconductor layer in the drift semiconductor layer together with embedding the epitaxial semiconductor layer in the low concentration layer, wherein the base semiconductor layer and the epitaxial semiconductor layer are made of an epitaxial film.