MOSFET and power conversion circuit

A MOSFET comprising:

• a semiconductor base substrate having a super junction structure which is formed of an n-type column region and a p-type column region;
• and
• a gate electrode formed on a first main surface side of the semiconductor base substrate by way of a gate insulation film, wherein
• using a deepest depth position of a surface of a depletion layer on the first main surface side when the depletion layer extends most in the super junction structure by turning off the MOSFET as a reference, in a graph where a depth x at a predetermined depth position of the super junction structure is taken on an axis of abscissas, average positive charge density ρ(x) at the predetermined depth position of the super junction structure expressed by a following formula (1) when the super junction structure is depleted by turning off the MOSFET is taken on an axis of ordinates, and assuming a depth of a surface of the depletion layer on a second main surface side at a shallowest depth position when the depletion layer extends most in the super junction structure by turning off the MOSFET as a,
• in a curve which expresses the average positive charge density ρ(x), a value of the average positive charge density ρ(0) when x=0 becomes a negative value and a value of the average positive charge density ρ(a) when x=a becomes a positive value,
• an area of a region surrounded by a curve which expresses the average positive charge density ρ(x), a straight line when x=0, and an x axis is equal to an area of a region surrounded by the curve which expresses the average positive charge density ρ(x), a straight line when x=a, and the x axis,
• in a case that a state where a total amount of dopant in the n-type column region differs from a total amount of dopant in the p-type column region is brought about,
• using a surface of the super junction structure on the first main surface side as a reference, in the state where the total amount of dopant in the n-type column region differs from the total amount of dopant in the p-type column region, assuming a depth position where the average positive charge density ρ(x) becomes 0 as Xm′, and assuming a deepest depth position of the surface of the depletion layer on the first main surface side when the depletion layer extends most in the super junction structure by turning off the MOSFET as X0′, and
• using the surface of the super junction structure on the first main surface side as a reference, in a case where the average positive charge density when the super junction structure is depleted by turning off the reference MOSFET which is configured such that the total amount of dopant in the n-type column region is set equal to the total amount of dopant in the p-type column region is set as a reference average positive charge density ρ0(x), assuming a depth position where the reference average positive charge density ρ0(x) becomes 0 as Xm, and assuming a deepest depth position of the depletion layer on the first main surface side when the depletion layer extends most in the super junction structure by turning off the reference MOSFET as X0, a relationship of |X0−X0′|<|Xm−Xm′| is satisfied,
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• (In the formula (1), Wn(x) indicates a width of the n-type column region at the predetermined depth position, Nd(x) indicates average density of a positive charge at the predetermined depth position of the n-type column region when the super junction structure is depleted by turning off the MOSFET, Wp(x) indicates a width of the p-type column region at the predetermined depth position, Na(x) indicates average density of a negative charge at the predetermined depth position of the p-type column region when the super junction structure is depleted by turning off the MOSFET, q indicates an elementary charge, and w indicates a positive constant which satisfies Wn(x)+Wp(x)=2w).