U.S. patent application number 16/497797 was filed with the patent office on 2020-12-03 for mosfet and power conversion circuit.
The applicant listed for this patent is SHINDENGEN ELECTRIC MANUFACTURING CO., LTD.. Invention is credited to Daisuke ARAI, Mizue KITADA.
Application Number | 20200381510 16/497797 |
Document ID | / |
Family ID | 1000005075989 |
Filed Date | 2020-12-03 |
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United States Patent
Application |
20200381510 |
Kind Code |
A1 |
ARAI; Daisuke ; et
al. |
December 3, 2020 |
MOSFET AND POWER CONVERSION CIRCUIT
Abstract
A MOSFET according to the present invention includes a
semiconductor base substrate having a super junction structure. A
gate electrode is on a first main surface side of the semiconductor
base substrate byway of a gate insulation film, wherein in a state
where a total amount of dopant in an n-type column region differs
from a total amount of dopant in a p-type column region, assuming a
depth position where an average positive charge density .rho.(x)
becomes 0 as X.sub.m', assuming a deepest depth position of the
surface of the depletion layer on the first main surface side as
X.sub.0', assuming a depth position where the reference average
positive charge density .rho..sub.0(x) becomes 0 as X.sub.m, and
assuming a deepest depth position of the depletion layer on the
first main surface side as X.sub.0, a relationship of
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied.
Inventors: |
ARAI; Daisuke; (Saitama,
JP) ; KITADA; Mizue; (Saitama, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SHINDENGEN ELECTRIC MANUFACTURING CO., LTD. |
Tokyo |
|
JP |
|
|
Family ID: |
1000005075989 |
Appl. No.: |
16/497797 |
Filed: |
May 26, 2017 |
PCT Filed: |
May 26, 2017 |
PCT NO: |
PCT/JP2017/019817 |
371 Date: |
September 26, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/1608 20130101;
H01L 29/7813 20130101; H02M 3/156 20130101; H01L 29/1095 20130101;
H01L 29/0688 20130101; H02M 7/537 20130101; H01L 29/0634
20130101 |
International
Class: |
H01L 29/06 20060101
H01L029/06; H01L 29/78 20060101 H01L029/78; H01L 29/10 20060101
H01L029/10; H02M 3/156 20060101 H02M003/156; H02M 7/537 20060101
H02M007/537 |
Claims
1. 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
.rho.(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 .rho.(x), a value of
the average positive charge density .rho.(0) when x=0 becomes a
negative value and a value of the average positive charge density
.rho.(a) when x=a becomes a positive value, an area of a region
surrounded by a curve which expresses the average positive charge
density .rho.(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 .rho.(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 .rho.(x) becomes 0 as X.sub.m', 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 X.sub.0', 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 .rho..sub.0(x), assuming a depth position where the
reference average positive charge density .rho..sub.0(x) becomes 0
as X.sub.m, 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 X.sub.0, a relationship of
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied. [ Formula 1
] .rho. ( x ) = q 2 w [ w n ( x ) N d ( x ) - w p ( x ) N a ( x ) ]
( 1 ) ##EQU00023## (In the formula (1), W.sub.n(x) indicates a
width of the n-type column region at the predetermined depth
position, N.sub.d(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, W.sub.p(x) indicates a width of the p-type column region at
the predetermined depth position, N.sub.a(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
W.sub.n(x)+W.sub.p(x)=2w.)
2. The MOSFET according to claim 1, wherein the average positive
charge density .rho.(x) is expressed by an upward convex curve
projecting in a right upward direction.
3. The MOSFET according to claim 1, wherein in a graph where a
depth x of the predetermined depth position of the super junction
structure is taken on an axis of abscissas and a width W.sub.p(x)
at the predetermined depth position of the p-type column region or
a width W.sub.n(x) at the predetermined depth position of the
n-type column region is taken on an axis of ordinates, the width
W.sub.p(x) at the predetermined depth position of the p-type column
region is expressed by a following formula (2), and [ Formula 2 ]
Wp ( x ) = - [ - A exp ( - x .lamda. ) + B ] + W ( 2 ) ##EQU00024##
(symbols A, B, .lamda. being constants) the width W.sub.n(x) at the
predetermined depth position of the n-type column region is
expressed by a following formula (3). [ Formula 3 ] Wn ( x ) = [ -
A exp ( - x .lamda. ) + B ] + W ( 3 ) ##EQU00025## (symbols A, B,
.lamda. being constants)
4. The MOSFET according to claim 1, wherein in a graph where a
depth x at a predetermined depth position of the super junction
structure is taken on an axis of abscissas and average density
N.sub.a(x) of a negative charge at the predetermined depth position
of the p-type column region or average density N.sub.d(x) 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 is taken on an axis of ordinates, the
average density N.sub.a(x) 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 is
expressed by a following formula (4), and [ Formula 4 ] Na ( x ) =
- [ - A exp ( - x .lamda. ) + B ] + N ( 4 ) ##EQU00026## (symbols
A, B, .lamda. being constants) the average density N.sub.d(x) of
the 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 is expressed by a following formula (5).
[ Formula 5 ] Nd ( x ) = [ - A exp ( - x .lamda. ) + B ] + N ( 5 )
##EQU00027## (symbols A, B, .lamda. being constants)
5. The MOSFET according to claim 1, wherein in the case that 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
is brought about, using the surface of the super junction structure
on the first main surface side as the 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
the average positive charge density at the deepest depth position
X.sub.0' 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 .rho.(X.sub.0'),
and assuming the reference average positive charge density at the
deepest depth position X.sub.0 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
.rho..sub.0(X.sub.0), a relationship of
.rho..sub.0(X.sub.0)>.rho.(X.sub.0') is satisfied.
6. The MOSFET according to claim 1, wherein in the case that 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
is brought about, using the surface of the super junction structure
on the first main surface side as the 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
the depth position where the average positive charge density
.rho.(x) becomes 0 as X.sub.m', and assuming the depth of the
surface of the depletion layer on the 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 X.sub.1',
assuming the depth position where the reference average positive
charge density .rho..sub.0(x) becomes 0 as X.sub.m, and assuming
the shallowest depth position of the surface of the depletion layer
on the second main surface side when the depletion layer extends
most in the super junction structure by turning off the reference
MOSFET as X.sub.1, a relationship of
|X.sub.m-X.sub.m'|<|X.sub.1-X.sub.1'| is satisfied.
7. The MOSFET according to claim 1, wherein in the case that 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
is brought about, using the surface of the super junction structure
on the first main surface side as the 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
the average positive charge density at a depth X.sub.1' of the
surface of the depletion layer on the second main surface side at
the shallowest depth position when the depletion layer extends most
in the super junction structure by turning off the MOSFET as
.rho.(X.sub.1'), and assuming the reference average positive charge
density at the shallowest depth position X.sub.1 of the surface of
the depletion layer on the second main surface side when the
depletion layer extends most in the super junction structure by
turning off the reference MOSFET as .rho..sub.0(X.sub.1), a
relationship of .rho..sub.0(X.sub.1)<.rho.(X.sub.1') is
satisfied.
8. The MOSFET according to claim 1, wherein in a graph where a
difference between the total amount of dopant in the n-type column
region and the total amount of dopant in the p-type column region
is taken on an axis of abscissas, and a depth X.sub.1 of a surface
of the depletion layer on the 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 using the
surface of the super junction structure on the first main surface
side as a reference is taken on an axis of ordinates, in a case
where the total amount of dopant in the p-type column region is
larger than the total amount of dopant in the n-type column region,
a curve which expresses the depth X.sub.1 is converged to a
predetermined value when a difference between the total amount of
dopant in the n-type column region and the total amount of dopant
in the p-type column region is increased.
9. The MOSFET according to claim 1, wherein 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, 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,
assuming a depth of a surface of the depletion layer on the 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', and assuming a depth of a surface of the
depletion layer on the second main surface side at a shallowest
depth position when the depletion layer extends most in the super
junction structure by turning off the reference MOSFET as a, an
area of a region surrounded by a curve which expresses the average
positive charge density .rho.(x), a straight line when x=a', and an
x axis is set equal to an area of a region surrounded by a curve
which expresses the reference average positive charge density
.rho..sub.0(x), a straight line when x=a, and an x axis.
10. The MOSFET according to claim 1, 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,
assuming a shallowest depth position of a surface of the depletion
layer on the second main surface side when the depletion layer
extends most in the super junction structure by turning off the
MOSFET as a, and assuming a depth of a depth position where the
average positive charge density .rho.(x) becomes 0 in the super
junction structure as d, a relationship of 0<d<a/2 is
satisfied.
11. The MOSFET according to claim 1, wherein the semiconductor base
substrate further includes a p-type base region formed on the
n-type column region and the p-type base region, and an n-type
source region formed on a surface of the base region, the MOSFET
further comprises, in a region where the n-type column region is
positioned as viewed in a plan view, a trench formed so as to
extend from a surface of the first main surface of the
semiconductor base substrate to a depth position deeper than a
deepest position of the base region and a portion of the source
region is exposed on an inner peripheral surface of the trench, the
gate insulation film is formed on the inner peripheral surface of
the trench, and the gate electrode is embedded in the trench by way
of the gate insulation film.
12. The MOSFET according to claim 1, wherein the semiconductor base
substrate further includes: a base region formed on a surface of
the first main surface of the semiconductor base substrate, a whole
surface of the p-type column region, and a portion of a surface of
the n-type column region; an n-type surface high concentration
region formed on a surface of the first main surface of the
semiconductor base substrate and a surface of the n-type column
region such that the n-type surface high concentration region is
disposed adjacently to the base region; and an n-type source region
formed on a surface of the base region, and the gate electrode is
formed on a surface of the base region sandwiched between the
source region and the n-type surface high concentration region by
way of the gate insulation film.
13. The MOSFET according to claim 11, 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,
assuming an axis along a depth direction as an x axis, an x
coordinate at a depth position of a lowermost surface of the base
region as -t, an x coordinate at a depth position of a lowermost
portion of the p-type column region as b, and an x coordinate at a
depth position where the average positive charge density .rho.(x)
becomes 0 as d, a relationship of 0<t+d<(t+b)/2 is
satisfied.
14. A power conversion circuit comprising at least: a reactor; a
power source which supplies an electric current to the reactor; the
MOSFET according to claim 1 for controlling an electric current
supplied from the power source to the reactor; and a rectifier
element which performs a rectifying operation of the electric
current supplied from the power source to the reactor or an
electric current from the reactor.
15. The power conversion circuit according to claim 14, wherein the
rectifier element is a fast recovery diode.
16. The power conversion circuit according to claim 14, wherein the
rectifier element is a built-in diode of the MOSFET.
17. The power conversion circuit according to claim 14, wherein the
rectifier element is a silicon-carbide Schottky barrier diode.
Description
RELATED APPLICATIONS
[0001] The present application is a National Phase of International
Application Number PCT/JP2017/019817, filed May 26, 2017.
TECHNICAL FIELD
[0002] The present invention relates to a Metal-Oxide Semiconductor
Field-Effect Transistor (MOSFET) and a power conversion
circuit.
BACKGROUND ART
[0003] Conventionally, there has been known a MOSFET which includes
a semiconductor base substrate having a super junction structure
which is formed of an n-type column region and a p-type column
region (see patent document 1, for example).
[0004] In this specification, "super junction structure" means a
structure where an n-type column region and a p-type column region
are alternately and repeatedly arranged as viewed in a
predetermined cross section.
[0005] As shown in FIG. 25, a conventional MOSFET 900 includes: a
semiconductor base substrate 910 having a super junction structure
917 formed of n-type column regions 914 and p-type column regions
916, base regions 918 formed on a surface of a first main surface,
all surfaces of the p-type column regions 916 and portions of
surfaces of the n-type column regions 914, n-type surface high
concentration regions 919 formed on the surface of the first main
surface and surfaces of the n-type column regions 914 such that the
n-type surface high concentration region 919 is disposed adjacently
to the base region 918, and n-type source regions 920 formed on
surfaces of the base regions 918; and a gate electrode 936 formed
on surfaces of the base regions 918 each of which is sandwiched
between the source region 920 and the n-type surface high
concentration region 919 by way of a gate insulation film 934.
[0006] In the conventional MOSFET 900, the n-type column region 914
and the p-type column region 916 are formed such that a total
amount of dopant in the n-type column region 914 is equal to a
total amount of dopant in the p-type column region 916. That is,
the n-type column region 914 and the p-type column region 916 are
well-balanced with each other in terms of a charge. Further, both
the dopant concentration in the n-type column region 914 and the
dopant concentration in the p-type column region 916 are
respectively set to a fixed value regardless of a depth. Still
further, side walls of the n-type column region 914 are formed into
a tapered shape which is narrowed toward a first main surface side,
and side walls of the p-type column region 916 are formed into a
tapered shape which is narrowed toward a bottom thereof.
[0007] In the specification, "total amount of dopant" means a total
amount of dopant in a constitutional element (the n-type column
region or the p-type column region) in the MOSFET.
[0008] The conventional MOSFET 900 includes the semiconductor base
substrate 910 having the super junction structure 917 formed of the
n-type column regions 914 and the p-type column regions 916 thus
providing a switching element which has a low ON resistance and a
high withstand voltage.
PRIOR ART DOCUMENT
Patent Document
[0009] Patent document 1: JP-A-2004-119611
SUMMARY OF INVENTION
Problem to be Solved by the Invention
[0010] The conventional MOSFET 900 has a drawback that in a case
where an irregularity exists in a charge balance around the gate,
an irregularity is liable to occur in switching characteristics
when the MOSFET 900 is turned off.
[0011] Accordingly, the present invention has been made so as to
overcome the above-mentioned drawback, and it is an object of the
present invention to provide a MOSFET where an irregularity
minimally occurs in switching characteristics when the MOSFET is
turned off even in a case where an irregularity occurs in a charge
balance around a gate, and a power conversion circuit which uses
the MOSFET.
Solution to Problem
[0012] [1] According to the present invention, there is provided a
MOSFET which includes:
[0013] 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
[0014] a gate electrode formed on a first main surface side of the
semiconductor base substrate by way of a gate insulation film,
wherein
[0015] 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
.rho.(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,
[0016] in a curve which expresses the average positive charge
density .rho.(x), a value of the average positive charge density
.rho.(0) when x=0 becomes a negative value and a value of the
average positive charge density .rho.(a) when x=a becomes a
positive value,
[0017] an area of a region surrounded by a curve which expresses
the average positive charge density .rho.(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
.rho.(x), a straight line when x=a, and the x axis,
[0018] 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,
[0019] 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 .rho.(x) becomes
0 as X.sub.m', 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 X.sub.0', and
[0020] 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
.rho..sub.0(x), assuming a depth position where the reference
average positive charge density .rho..sub.0(x) becomes 0 as
X.sub.m, 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 X.sub.0, a relationship of
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied.
[ Formula 1 ] .rho. ( x ) = q 2 w [ w n ( x ) N d ( x ) - w p ( x )
N a ( x ) ] ( 1 ) ##EQU00001##
[0021] (In the formula (1), W.sub.n(x) indicates a width of the
n-type column region at the predetermined depth position,
N.sub.d(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,
W.sub.p(x) indicates a width of the p-type column region at the
predetermined depth position, N.sub.a(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
W.sub.n(x)+W.sub.p(x)=2w.)
[0022] In this specification, "a depth at a predetermined depth
position of the super junction structure" means a depth at a
predetermined depth position of the super junction structure 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. "second main surface" means a main surface on a side
opposite to the first main surface. "average density of a positive
charge at the predetermined depth position of the n-type column
region" means average density of a positive charge generated by a
donor in the n-type column region when the n-type column region is
depleted by turning off the MOSFET, and "average density of a
negative charge at the predetermined depth position of the p-type
column region" means average density of a negative charge generated
in an acceptor of the p-type column region when the p-type column
region is depleted by turning off the MOSFET.
[0023] In this specification, "reference MOSFET" means a MOSFET
where a shape of the super junction structure (a p-type column
region and an n-type column region) is equal to the super junction
structure of the MOSFET according to the present invention, a
concentration gradient of a p-type dopant in the p-type column
region and a concentration gradient of an n-type dopant in the
n-type column region are equal to the corresponding concentration
gradients of the p-type dopant and the n-type dopant used in the
MOSFET according to the present invention, and dopant concentration
in the p-type column region is adjusted such that a total amount of
dopant in the p-type column region becomes equal to a total amount
of dopant in the n-type column region. On the other hand,
"reference average positive charge density .rho..sub.0(x)" means
average positive charge density of the reference MOSFET. The MOSFET
according to the present invention includes a MOSFET in a state
where a total amount of dopant in an n-type column region becomes
equal to a total amount of dopant in a p-type column region (that
is, a state where a charge balance is obtained) and a case where
the total amount of dopant in the n-type column region and the
total amount of dopant in the p-type column region are different
from each other (that is, a state where an irregularity occurs in a
charge balance).
[0024] [2] According to the MOSFET of the present invention, it is
preferable that the average positive charge density .rho.(x) be
expressed by an upward convex curve projecting in a right upward
direction.
[0025] In this specification, "average positive charge density
.rho.(x) at a predetermined depth position of the super junction
structure is expressed by an upward convex curve projecting in a
right upward direction" means not only the case where the average
positive charge density is expressed by a monotonous upward convex
curve projecting in a right upward direction but also a case where
the average positive charge density is expressed in a stepped shape
(a line (an envelope) which connects corner portions of steps
forming a monotonous upward convex curve projecting in a right
upward direction), and a case where the average positive charge
density is formed of a curve where a concave shape and a convex
shape are repeated (an envelope forming a monotonous upward convex
curve projecting in a right upward direction).
[0026] [3] According to the MOSFET of the present invention, it is
preferable that in a graph where a depth x of the predetermined
depth position of the super junction structure is taken on an axis
of abscissas and a width W.sub.p(x) at the predetermined depth
position of the p-type column region or a width W.sub.n(x) at the
predetermined depth position of the n-type column region is taken
on an axis of ordinates,
[0027] the width W.sub.p(x) at the predetermined depth position of
the p-type column region be expressed by a following formula (2),
and
[ Formula 2 ] Wp ( x ) = - [ - A exp ( - x .lamda. ) + B ] + W ( 2
) ##EQU00002##
(symbols A, B, .lamda. being constants)
[0028] the width W.sub.n(x) at the predetermined depth position of
the n-type column region be expressed by a following formula
(3).
[ Formula 3 ] Wn ( x ) = [ - A exp ( - x .lamda. ) + B ] + W ( 3 )
##EQU00003##
(symbols A, B, .lamda. being constants)
[0029] [4] According to the MOSFET of the present invention, it is
preferable that in a graph where a depth x at a predetermined depth
position of the super junction structure is taken on an axis of
abscissas and average density N.sub.a(x) of a negative charge at
the predetermined depth position of the p-type column region or
average density N.sub.d(x) 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 is
taken on an axis of ordinates,
[0030] the average density N.sub.a(x) 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 be
expressed by a following formula (4), and
[ Formula 4 ] Na ( x ) = - [ - A exp ( - x .lamda. ) + B ] + N ( 4
) ##EQU00004##
(symbols A, B, .lamda. being constants)
[0031] the average density N.sub.d(x) of the 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 be
expressed by a following formula (5).
[ Formula 5 ] N d ( x ) = [ - A exp ( - x .lamda. ) + B ] + N ( 5 )
##EQU00005##
(symbols A, B, .lamda. being constants)
[0032] [5] According to the MOSFET of the present invention, it is
preferable that in the case that 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 is brought about,
[0033] using the surface of the super junction structure on the
first main surface side as the reference,
[0034] 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 the average positive charge density at the
deepest depth position X.sub.0' 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 .rho.(X.sub.0'), and
[0035] assuming the reference average positive charge density at
the deepest depth position X.sub.0 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
.rho..sub.0(X.sub.0), a relationship of
.rho..sub.0(X.sub.0)>.rho.(X.sub.0') be satisfied.
[0036] [6] According to the MOSFET of the present invention, it is
preferable that in the case that 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 is brought about,
[0037] using the surface of the super junction structure on the
first main surface side as the reference,
[0038] 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 the depth position where the average
positive charge density .rho.(x) becomes 0 as X.sub.m', and
assuming the depth of the surface of the depletion layer on the
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 X.sub.1',
[0039] assuming the depth position where the reference average
positive charge density .rho..sub.0(x) becomes 0 as X.sub.m, and
assuming the shallowest depth position of the surface of the
depletion layer on the second main surface side when the depletion
layer extends most in the super junction structure by turning off
the reference MOSFET as X.sub.1, a relationship of
|X.sub.m-X.sub.m'|<|X.sub.1-X.sub.1'| be satisfied.
[0040] [7] According to the MOSFET of the present invention, it is
preferable that in the case that 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 is brought about,
[0041] using the surface of the super junction structure on the
first main surface side as the reference,
[0042] 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 the average positive charge density at a
depth X.sub.1' of the surface of the depletion layer on the second
main surface side at the shallowest depth position when the
depletion layer extends most in the super junction structure by
turning off the MOSFET as .rho.(X.sub.1'), and
[0043] assuming the reference average positive charge density at
the shallowest depth position X.sub.1 of the surface of the
depletion layer on the second main surface side when the depletion
layer extends most in the super junction structure by turning off
the reference MOSFET as .rho..sub.0(X.sub.1), a relationship of
.rho..sub.0(X.sub.1)<.rho.(X.sub.1') be satisfied.
[0044] [8] According to the MOSFET of the present invention, it is
preferable that in a case where the total amount of dopant in the
p-type column region is larger than the total amount of dopant in
the n-type column region, in a graph where a difference between the
total amount of dopant in the n-type column region and the total
amount of dopant in the p-type column region is taken on an axis of
abscissas, and a depth X.sub.1 of a surface of the depletion layer
on the 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 using the surface of the super junction
structure on the first main surface side as a reference is taken on
an axis of ordinates,
[0045] a curve which expresses the depth X.sub.1 be converged to a
predetermined value when a difference between the total amount of
dopant in the n-type column region and the total amount of dopant
in the p-type column region is increased.
[0046] [9] According to the MOSFET of the present invention, it is
preferable that 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, 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, assuming a depth of a
surface of the depletion layer on the 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', and
assuming a depth of a surface of the depletion layer on the second
main surface side at a shallowest depth position when the depletion
layer extends most in the super junction structure by turning off
the reference MOSFET as a,
[0047] an area of a region surrounded by a curve which expresses
the average positive charge density .rho.(x), a straight line when
x=a', and an x axis be set equal to an area of a region surrounded
by a curve which expresses the reference average positive charge
density .rho..sub.0(x), a straight line when x=a, and an x
axis.
[0048] [10] According to the MOSFET of the present invention, it is
preferable that 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, assuming a shallowest depth position of
a surface of the depletion layer on the second main surface side
when the depletion layer extends most in the super junction
structure by turning off the MOSFET as a, and assuming a depth of a
depth position where the average positive charge density .rho.(x)
becomes 0 in the super junction structure as d, a relationship of
0<d<a/2 be satisfied.
[0049] [11] According to the MOSFET of the present invention, it is
preferable that the semiconductor base substrate further include a
p-type base region formed on the n-type column region and the
p-type base region, and an n-type source region formed on a surface
of the base region,
[0050] the MOSFET further comprise, in a region where the n-type
column region is positioned as viewed in a plan view, a trench
formed so as to extend from a surface of the first main surface of
the semiconductor base substrate to a depth position deeper than a
deepest position of the base region and a portion of the source
region is exposed on an inner peripheral surface of the trench,
[0051] the gate insulation film be formed on the inner peripheral
surface of the trench, and
[0052] the gate electrode be embedded in the trench by way of the
gate insulation film.
[0053] [12] According to the MOSFET of the present invention, it is
preferable that the semiconductor base substrate further include: a
base region formed on a surface of the first main surface of the
semiconductor base substrate, a whole surface of the p-type column
region, and a portion of a surface of the n-type column region; an
n-type surface high concentration region formed on a surface of the
first main surface of the semiconductor base substrate and a
surface of the n-type column region such that the n-type surface
high concentration region is disposed adjacently to the base
region; and an n-type source region formed on a surface of the base
region, and
[0054] the gate electrode be formed on a surface of the base region
sandwiched between the source region and the n-type surface high
concentration region by way of the gate insulation film.
[0055] [13] According to the MOSFET of the present invention, it is
preferable that 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, assuming an axis along a depth direction
as an x axis, an x coordinate at a depth position of a lowermost
surface of the base region as -t, an x coordinate at a depth
position of a lowermost portion of the p-type column region as b,
and an x coordinate at a depth position where the average positive
charge density .rho.(x) becomes 0 as d, a relationship of
0<t+d<(t+b)/2 be satisfied.
[0056] [14] According to the present invention, there is provided a
power conversion circuit which includes at least:
[0057] a reactor;
[0058] a power source which supplies an electric current to the
reactor;
[0059] the MOSFET according to any one of claims 1 to 13 for
controlling an electric current supplied from the power source to
the reactor; and
[0060] a rectifier element which performs a rectifying operation of
the electric current supplied from the power source to the reactor
or an electric current from the reactor.
[0061] [15] According to the power conversion circuit of the
present invention, it is preferable that the rectifier element be a
fast recovery diode.
[0062] [16] According to the power conversion circuit of the
present invention, it is preferable that the rectifier element be a
built-in diode of the MOSFET.
[0063] [17] According to the power conversion circuit of the
present invention, it is preferable that the rectifier element be a
silicon-carbide Schottky barrier diode. [Advantageous effects of
the present invention]
[0064] According to the MOSFET of the present invention, the
relationship of |X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is
satisfied and hence, even when an irregularity occurs in a charge
balance so that the MOSFET becomes n-type dopant rich, when the
MOSFET is turned off, deviation of average positive charge density
.rho.(x) on a first main surface side (gate electrode side) is
small so that a region where an electric field is high minimally
approaches the gate electrode. Accordingly, an electric field in
the n-type column region around the gate electrode becomes
minimally high so that a change in potential of the n-type column
region around the gate becomes small easily. As a result, an
irregularity minimally occurs in switching characteristics when the
MOSFET is turned off.
[0065] According to the MOSFET and the power conversion circuit of
the present invention, the relationship of
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied and hence,
even when an irregularity occurs in a charge balance, an
irregularity in a switching loss when the MOSFET is turned off
minimally becomes large. Accordingly, it is possible to prevent the
occurrence of a defect in an operation of the power conversion
circuit (particularly, a circuit largely affected by a switching
loss at the time of turning off the MOSFET such as a current
critical type PFC or a current discontinuous type PFC) (see FIGS.
13A and 13B).
[0066] Further, the MOSFET according to the present invention
includes the semiconductor base substrate where the super junction
structure is formed of the n-type column region and the p-type
column region. Accordingly, in the same manner as the conventional
MOSFET 900, the MOSFET becomes a switching element having a low ON
resistance and a high withstand voltage.
[0067] In the MOSFET according to the present invention, the
average positive charge density .rho.(0) when x=0 takes a negative
value, and the average positive charge density .rho.(a) when x=a
takes a positive value. Accordingly, at a depth position in the
vicinity of a bottom portion of the p-type column region, a total
amount of dopant in the p-type column region becomes smaller than a
total amount of dopant in the n-type column region (becoming n-type
dopant rich). With such a configuration, a depletion layer
generated from the p-type column region when the MOSFET is turned
off minimally extends toward a second main surface side.
Accordingly, it is possible to provide a MOSFET where a breakdown
in a reach through mode minimally occurs so that a withstand
voltage is minimally lowered.
BRIEF DESCRIPTION OF DRAWINGS
[0068] FIG. 1 is a circuit diagram showing a power conversion
circuit 1 according to Embodiment 1.
[0069] FIG. 2 is a cross-sectional view showing a MOSFET 100
according to Embodiment 1.
[0070] FIG. 3 is a schematic view for explaining constants A, B and
.lamda. in a formula (2) and a formula (3).
[0071] In FIG. 3, the illustration of constitutional elements other
than the super junction structure 117 (n-type column regions 114
and p-type column regions 116) is omitted. FIG. 3 is a view
corresponding to a region surrounded by a chain line shown in FIG.
2.
[0072] FIG. 4 is a schematic view for explaining average positive
charge density .rho.(x) at a predetermined depth position of a
super junction structure when the super junction structure is
depleted by turning off the MOSFET 100 according to Embodiment 1.
In FIG. 4, the illustration of constitutional elements other than
the super junction structure 117 (n-type column regions 114 and
p-type column regions 116) and a buffer layer 113 is omitted (the
same definition being applied to FIG. 5B). With respect to two axes
on a left side of FIG. 4, an X axis on the left side is an axis
showing a depth when a depth position of a surface of the super
junction structure on a first main surface side (a depth position
of a lowermost surface of a base region) is used as a reference
(X=0). A position X.sub.0 on the X coordinate is a deepest depth
position of a surface of a depletion layer on a first main surface
side when the depletion layer extends most in the super junction
structure by turning off the MOSFET. A position X.sub.1 on the X
coordinate is a shallowest depth position of a surface of a
depletion layer on a second main surface side when the depletion
layer extends most in the super junction structure by turning off
the MOSFET. A position X.sub.2 on the X coordinate is a depth
position of a lowermost portion of the p-type column region 116
(the same definition being applied to FIGS. 5A and 5B, FIGS. 10A
and 10B and FIGS. 11A and 11B). With respect to two axes on the
left side of FIG. 4, an x axis on a right side is an axis showing a
depth x when 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 is used as a reference (x=0). A position -t on the x
coordinate is a depth position of a lowermost surface of a base
region 118, a position b on the x coordinate is a depth position of
a lowermost portion of a p-type column region 116, and a position a
on the x coordinate is a shallowest depth position of a surface of
a depletion layer on a second main surface side when the depletion
layer extends most in the super junction structure by turning off
the MOSFET (the same definition being applied to FIG. 5A to FIG.
9D).
[0073] FIG. 5A and FIG. 5B are views for explaining a MOSFET 800
according to Comparison example 1. FIG. 5A is a cross-sectional
view showing the MOSFET 800 according to Comparison example 1, and
FIG. 5B is a schematic view for explaining average positive charge
density .rho.(x) at a predetermined depth position of the super
junction structure when the super junction structure is depleted by
turning off the MOSFET 800 according to Comparison example 1.
[0074] FIG. 6A to FIG. 6D are graphs expressing changes when the
super junction structure is depleted by turning off the MOSFET 800
(reference MOSFET) according to Comparison example 1, that is, a
change in width W.sub.n(x) at a predetermined depth position of an
n-type column region with respect to a depth x, a change in width
W.sub.p(x) at a predetermined depth position of a p-type column
region with respect to the depth x, a change in average density
N.sub.d(X) of a positive charge at a predetermined depth position
of the n-type column region with respect to the depth x, a change
in average density N.sub.a(x) of a negative charge at a
predetermined depth position of the p-type column region with
respect to the depth x, a change in average positive charge density
.rho.(x) (reference average positive charge density .rho..sub.0(x))
and an electric field E(x) (E.sub.0(x)) at a predetermined depth
position of the super junction structure with respect to the depth
x. FIG. 6A is a graph expressing a change in width W.sub.n(x) at
the predetermined depth position of the n-type column region and a
change in width W.sub.p(x) at the predetermined depth position of
the p-type column region with respect to the depth x, FIG. 6B is a
graph expressing a change in average density N.sub.d(x) of a
positive charge at the predetermined depth position of the n-type
column region and a change in average density N.sub.a(x) of a
negative charge at the predetermined depth position of the p-type
column region with respect to the depth x, FIG. 6C is a graph
expressing a change in average positive charge density .rho.(x) at
the predetermined depth position of the super junction structure
with respect to the depth x, and FIG. 6D is a graph expressing a
change in an electric field E(x) at the predetermined depth
position of the super junction structure with respect to the depth
x (the same definition being applied to FIGS. 7A to 7D). In FIG. 6A
to FIG. 6D, 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 is used as a reference (x=0) (the same definition being
applied to FIG. 7A to FIG. 9D).
[0075] FIG. 7A to FIG. 7D are graphs expressing changes when the
super junction structure 117 is depleted by turning off the MOSFET
100 (reference MOSFET) according to Embodiment 1, that is, a change
in width W.sub.n(x) at a predetermined depth position of the n-type
column region 114 with respect to the depth x, a change in width
W.sub.p(x) at a predetermined depth position of the p-type column
region 116 with respect to the depth x, a change in average density
N.sub.d(x) of a positive charge at a predetermined depth position
of the n-type column region 114 with respect to the depth x, a
change in average density N.sub.a(x) of a negative charge at a
predetermined depth position of the p-type column region 116 with
respect to the depth x, and a change in average positive charge
density .rho.(x) (reference average positive charge density
.rho..sub.0(x)) and an electric field E(x) (E.sub.0(x)) at a
predetermined depth position of the super junction structure 117
with respect to the depth x.
[0076] FIG. 8A to FIG. 8D are graphs expressing changes when the
super junction structure is depleted by turning off an n-type
dopant rich MOSFET according to Comparison example 1, that is, a
change in width W.sub.n(x) at a predetermined depth position of an
n-type column region with respect to a depth x, a change in width
W.sub.p(x) at a predetermined depth position of a p-type column
region with respect to the depth x, a change in average density
N.sub.d(x) of a positive charge at a predetermined depth position
of the n-type column region with respect to the depth x, a change
in average density N.sub.a(x) of a negative charge at a
predetermined depth position of the p-type column region with
respect to the depth x, a change in average positive charge density
.rho.(x) and an electric field E(x) at a predetermined depth
position of the super junction structure with respect to the depth
x. FIG. 8A is a graph expressing a change in width W.sub.n(x) at
the predetermined depth position of the n-type column region and a
change in width W.sub.p(x) at the predetermined depth position of
the p-type column region with respect to the depth x, FIG. 8B is a
graph expressing a change in average density N.sub.d(x) of a
positive charge at the predetermined depth position of the n-type
column region and a change in average density N.sub.a(x) of a
negative charge at the predetermined depth position of the p-type
column region with respect to the depth x, FIG. 8C is a graph
expressing a change in average positive charge density .rho.(x) at
the predetermined depth position of the super junction structure
with respect to the depth x, and FIG. 8D is a graph expressing a
change in an electric field E(x) at the predetermined depth
position of the super junction structure with respect to the depth
x (the same definition being applied to FIG. 9A to FIG. 9D). A
position -t' on the x coordinate is a depth position of a lowermost
surface of a base region 118, a position b' on the x coordinate is
a depth position of a lowermost portion of a p-type column region
116, and a position a' on the x coordinate is a shallowest depth
position of a surface of a depletion layer on a second main surface
side when the depletion layer extends most in the super junction
structure by turning off the MOSFET (the same definition being
applied to FIG. 9A to FIG. 9D). Further, a relationship of
t'+b'=t+b is satisfied (the same definition being applied to FIG.
9A to FIG. 9D).
[0077] FIG. 9A to FIG. 9D are graphs expressing changes when a
super junction structure 117 is depleted by turning off an n-type
dopant rich MOSFET according to Embodiment 1, that is, a change in
width W.sub.n(x) at a predetermined depth position of an n-type
column region 114 with respect to the depth x, a change in width
W.sub.p(x) at a predetermined depth position of a p-type column
region 116 with respect to the depth x, a change in average density
N.sub.d(x) of a positive charge at a predetermined depth position
of the n-type column region 114 with respect to the depth x, a
change in average density N.sub.a(x) of a negative charge at a
predetermined depth position of the p-type column region 116 with
respect to the depth x, and a change in average positive charge
density .rho.(x) and an electric field E(x) at a predetermined
depth position of the super junction structure 117 with respect to
the depth x.
[0078] FIG. 10A and FIG. 10B are graphs expressing a change in
reference average positive charge density .rho..sub.0(X), a change
in average positive charge density .rho.(X), and respective changes
in electric fields E.sub.0(X), E(X) with respect to a depth X in
the reference MOSFET and the n-type dopant rich MOSFET of
Comparison example 1. FIG. 10A is a graph expressing a change in
reference average positive charge density .rho..sub.0(X) and a
change in average positive charge density .rho.(X) with respect to
the depth X in the reference MOSFET and the n-type dopant rich
MOSFET of Comparison example 1. FIG. 10B is a graph expressing
respective changes in the electric fields E.sub.0(X), E(X) with
respect to the depth X in the reference MOSFET and the n-type
dopant rich MOSFET of Comparison example 1.
[0079] FIG. 11A and FIG. 11B are graphs expressing a change in
reference average positive charge density .rho..sub.0(X), a change
in average positive charge density .rho.(X), and respective changes
in electric fields E.sub.0(X), E(X) with respect to a depth X in
the reference MOSFET and the n-type dopant rich MOSFET of
Embodiment 1. FIG. 11A is a graph expressing a change in reference
average positive charge density .rho..sub.0(X) and a change in
average positive charge density .rho.(X) with respect to the depth
X in the reference MOSFET and the n-type dopant rich MOSFET of
Embodiment 1. FIG. 11B is a graph expressing respective changes in
the electric fields E.sub.0(X), E(X) with respect to the depth X in
the reference MOSFET and the n-type dopant rich MOSFET of
Embodiment 1.
[0080] FIG. 12 is a graph expressing a relationship established in
the MOSFET 100 of Embodiment 1 between a charge balance which is a
difference between a total amount of dopant in the n-type column
region and a total amount of dopant in the p-type column region, a
deepest depth position X.sub.0 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, a depth
position X.sub.m when the average positive charge density .rho.(x)
becomes 0, and a depth X.sub.1 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.
[0081] FIG. 13A and FIG. 13B are graphs showing a relationship
between a charge balance, a switching loss Eoff when the MOSFET is
turned off, and a switching loss Eon when the MOSFET is turned on
in the MOSFET 100 of Embodiment 1 and the MOSFET 800 of Comparison
example 1. FIG. 13A is the graph showing the relationship
established in the MOSFET 800 according to Comparison example 1
between a charge balance and a switching loss Eoff when the MOSFET
800 is turned off, and between a charge balance and a switching
loss Eon when the MOSFET 800 is turned on and FIG. 13B is the graph
showing the relationship established in the MOSFET 100 according to
Embodiment 1 between a charge balance and a switching loss Eoff
when the MOSFET 100 is turned off, and between a charge balance and
a switching loss Eon when the MOSFET 100 is turned on.
[0082] FIG. 14 is a graph showing a result of a time transition
simulation of a drain-source voltage Vds, a drain current Id and a
gate-source voltage Vgs when the MOSFET is turned off in a power
conversion circuit using the MOSFET 800 according to Comparison
example 1. In FIG. 14, "p-type dopant rich" means that a total
amount of dopant in the p-type column region is larger than a total
amount of dopant in the n-type column region by 10%, "n-type dopant
rich" means that a total amount of dopant in the n-type column
region is larger than a total amount of dopant in the p-type column
region by 10%, and "Just" means that a total amount of dopant in
the n-type column region and a total amount of dopant in the p-type
column region are equal (the same definition being applied to FIG.
15). A power source voltage is 300V (the same definition being
applied to FIG. 15).
[0083] FIG. 15 is a graph showing a result of a time transition
simulation of a drain-source voltage Vds, a drain current Id and a
gate-source voltage Vgs when the MOSFET is turned off in a power
conversion circuit 1 according to Embodiment 1.
[0084] FIG. 16 is a schematic view showing a mode of the depletion
layer at a moment during a turn-off period in the MOSFET 800
according to Comparison example 1. In FIG. 16, the illustration of
a source region is omitted (the same definition being applied to
FIG. 17).
[0085] FIG. 17 is a schematic view showing a mode of the depletion
layer at a moment when the MOSFET is turned off in the MOSFET 100
according to Embodiment 1. FIG. 17 shows the mode of the depletion
layer at the same timing as FIG. 16.
[0086] FIG. 18A and FIG. 18B are cross-sectional views showing a
MOSFET 700 according to Comparison example 2 and a MOSFET 100A
according to a present invention example respectively. FIG. 18A is
the cross-sectional view showing the MOSFET 700 according to
Comparison example 2, FIG. 18B is the cross-sectional view showing
the MOSFET 100A according to the present invention example. FIG.
18A and FIG. 18B are schematic views and do not strictly reflect
sizes and shapes of the structures used in obtaining simulation
results shown in FIG. 19A and FIG. 19B.
[0087] FIG. 19A and FIG. 19B are views showing a simulation result
of equal potential lines when the MOSFET is turned off in the
MOSFET 700 according to Comparison example 2 and the MOSFET 100A
according to the present invention example respectively. FIG. 19A
is the view showing the simulation result of equal potential lines
when the MOSFET is turned off in the MOSFET 700 according to
Comparison example 2, and FIG. 19B is the view showing the
simulation result of equal potential lines when the MOSFET is
turned off in the MOSFET 100A according to the present invention
example. FIG. 19A is a view corresponding to a region surrounded by
a chain line shown in FIG. 18A, and FIG. 19B is a view
corresponding to a region surrounded by a chain line shown in FIG.
18B. In FIG. 19A and FIG. 19B, a bold black solid line indicates a
boundary between an n-type column region and a p-type column
region, a black fine solid line indicates equal potential lines
drawn for every 3V, and a white solid line indicates a boundary
between a region where an amount of a carrier becomes 5% compared
to the amount of the carrier at the usual time and other
regions.
[0088] FIG. 20A to FIG. 20C are views for explaining the MOSFET 102
according to Embodiment 2. FIG. 20A is a cross-sectional view
showing the MOSFET 102 according to Embodiment 2, FIG. 20B is a
graph expressing changes when a super junction structure 117 is
depleted by turning off the MOSFET, that is, a change in width
W.sub.n(x) at a predetermined depth position of an n-type column
region 114 with respect to a depth x and a change in width
W.sub.p(x) at a predetermined depth position of a p-type column
region 116 with respect to the depth x, and FIG. 20C is a graph
expressing a change in average density N.sub.d(x) of a positive
charge at a predetermined depth position of the n-type column
region with respect to the depth x, and a change in average density
N.sub.a(x) of a negative charge at a predetermined depth position
of the p-type column region with respect to the depth x.
[0089] FIG. 21 is a cross-sectional view of a MOSFET 200 according
to Embodiment 3.
[0090] FIG. 22 is a circuit diagram showing a power conversion
circuit 2 according to Embodiment 4.
[0091] FIG. 23 is a cross-sectional view showing a MOSFET 104
according to Modification 1.
[0092] FIG. 24 is a cross-sectional view showing a MOSFET 106
according to Modification 2.
[0093] FIG. 25 is a cross-sectional view showing a conventional
MOSFET 900. Symbol 912 indicates a low-resistance semiconductor
layer, symbol 913 indicates a buffer layer, symbol 915 indicates an
n-type semiconductor layer, symbol 930 indicates a source
electrode, and symbol 932 indicates a drain electrode.
DESCRIPTION OF EMBODIMENTS
[0094] Hereinafter, a MOSFET and a power conversion circuit
according to the present invention are described in accordance with
Embodiments shown in the drawings. The respective drawings are
schematic drawings, and do not always strictly reflect actual sizes
of the MOSFET and the power conversion circuit.
Embodiment 1
1. Structure and Operation of Power Conversion Circuit 1 According
to Embodiment 1
[0095] A power conversion circuit 1 according to Embodiment 1 is a
chopper circuit which is a constitutional element such as a DC-DC
converter or an inverter. The power conversion circuit 1 according
to Embodiment 1 includes, as shown in FIG. 1, a reactor 10, a power
source 20, a MOSFET 100 according to Embodiment 1, and a rectifier
element 30.
[0096] The reactor 10 is a passive element which can store energy
in a magnetic field generated by an electric current which flows
through the reactor 10.
[0097] The power source 20 is a DC power source which supplies an
electric current to the reactor 10. The MOSFET 100 controls an
electric current supplied from the power source 20 to the reactor
10. To be more specific, the MOSFET 100 is switched to assume an ON
state in response to a clock signal applied from a drive circuit
(not shown in the drawing) to a gate electrode of the MOSFET 100,
and makes the reactor 10 and a negative pole of the power source 20
electrically conductive with each other. The specific structure of
the MOSFET 100 is described later.
[0098] The rectifier element 30 is a fast recovery diode which
performs a rectifying operation of an electric current supplied
from the power source 20 to the reactor 10. To be more specific,
the rectifier element 30 is a lifetime-controlled PIN diode.
[0099] A positive pole (+) of the power source 20 is electrically
connected to one end 12 of the reactor 10 and a cathode electrode
of the rectifier element 30, and a negative pole (-) of the power
source 20 is electrically connected to a source electrode of the
MOSFET 100. A drain electrode of the MOSFET 100 is electrically
connected to the other end 14 of the reactor 10 and an anode
electrode of the rectifier element 30.
[0100] In such a power conversion circuit 1, when the MOSFET 100 is
in an ON state, an electric current path from a positive pole (+)
of the power source 20 to the negative pole (-) of the power source
20 through the reactor 10 and the MOSFET 100 is formed, and an
electric current flows through the electric current path. In this
case, electric energy of the power source 20 is stored in the
reactor 10.
[0101] When the MOSFET 100 is turned off, an electric current which
flows through the electric current path from the positive pole (+)
of the power source 20 to the negative pole (-) of the power source
20 through the reactor 10 and the MOSFET 100 is decreased and
becomes 0 soon. On the other hand, the reactor 10 generates an
electromotive force in a direction in which a change in an electric
current is obstructed (electric energy stored in the reactor 10 is
discharged) due to a self induction effect. An electric current
generated due to an electromotive force of the reactor 10 flows to
the rectifier element 30, and a forward electric current flows in
the rectifier element 30.
[0102] A sum of an amount of an electric current which flows
through the MOSFET 100 and an amount of an electric current which
flows through the rectifier element 30 is equal to an amount of an
electric current which flows through the reactor 10. A switching
period of the MOSFET 100 is short (possibly 100 nsec at maximum)
and hence, an amount of an electric current which flows through the
reactor 10 minimally changes during such a period. Accordingly, a
sum of an amount of an electric current which flows through the
MOSFET 100 and an amount of an electric current which flows through
the rectifier element 30 minimally changes in either case, that is,
an ON state, a turn-off period or an OFF state.
[0103] In such a power conversion circuit 1, a case is considered
where a MOSFET where side walls of the n-type column region 914 are
formed in a tapered shape narrowed toward a first main surface
side, and side walls of the p-type column region 916 are formed in
a tapered shape with a narrow bottom (for example, a conventional
MOSFET 900) is used as the MOSFET. In this case, when an
irregularity exists in a charge balance around the gate, there
arises a drawback that an irregularity in switching characteristics
when the MOSFET is turned off is liable to occur (see FIG. 14
described later).
[0104] Particularly, to consider a case where a circuit which is
largely affected by a switching loss at the time of turning off the
MOSFET such as a current critical type PFC or a current
discontinuous type PFC is used as a power conversion circuit, when
a MOSFET where side walls of the n-type column region are formed in
a tapered shape narrowed toward a first main surface side, and side
walls of the p-type column region are formed in a tapered shape
with a narrow bottom is used as the MOSFET (for example, when the
conventional MOSFET 900 is used), there arises a following
drawback. That is, as indicated by Eoff shown in FIG. 13A described
later, when an irregularity occurs in a charge balance when the
MOSFET is turned off, an irregularity in a switching loss becomes
large and hence, a defect may occur in an operation of the power
conversion circuit.
[0105] Accordingly, in the present invention, as the MOSFET, the
MOSFET 100 according to Embodiment 1 having the following structure
is used.
2. Structure of MOSFET 100 According to Embodiment 1
[0106] As shown in FIG. 2, the MOSFET 100 according to Embodiment 1
is a trench-gate-type MOSFET which includes a semiconductor base
substrate 110, a trench 122, a gate electrode 126, an interlayer
insulation film 128, a source electrode 130, and a drain electrode
132. A drain-source withstand voltage of the MOSFET 100 is 300V or
more, for example, 600V.
[0107] The semiconductor base substrate 110 has an n-type
low-resistance semiconductor layer 112, an n-type buffer layer 113
formed on the low-resistance semiconductor layer 112 and having a
lower dopant concentration than dopant concentration in the
low-resistance semiconductor layer 112, a super junction structure
117 formed of n-type column regions 114 and p-type column regions
116 which are formed on the buffer layer 113 such that the n-type
column region 114 and the p-type column region 116 are alternately
arranged along a horizontal direction, a p-type base region 118
formed on surfaces of the n-type column regions 114 and surfaces of
the p-type column regions 116, and n-type source regions 120 formed
on a surface of the base region 118. The buffer layer 113 and the
n-type column regions 114 are integrally formed with each other,
and the buffer layer 113 and the n-type column regions 114 form an
n-type semiconductor layer 115.
[0108] In the n-type column region 114 and the p-type column region
116, 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 (x=0), in a graph where a depth x at a
predetermined depth position of the super junction structure is
taken on an axis of abscissas and a width W.sub.n(x) at a
predetermined depth position of an n-type column region or a width
W.sub.p(x) at a predetermined depth position of a p-type column
region is taken on an axis of ordinates, the width W.sub.n(x) is
expressed by an upward convex curve projecting in a right upward
direction, and the width W.sub.p(x) is expressed by a downward
convex curve projecting in a right downward direction (see FIG.
7A). That is, the n-type column region 114 has a shape of a cup
with its open side down in cross section, while the p-type column
region 116 has a shape of a funnel in cross section.
[0109] To be more specific, in a graph where the depth x at the
predetermined depth position of the super junction structure is
taken on the axis of abscissas and the width W.sub.n(x) at the
predetermined depth position of the n-type column region 114 or the
width W.sub.p(x) at the predetermined depth position of the p-type
column region 116 is taken on the axis of ordinates, a width Wp(x)
of the p-type column region is expressed by a following formula
(2), and
[ Formula 2 ] Wp ( x ) = - [ - A exp ( - x .lamda. ) + B ] + W ( 2
) ##EQU00006##
(symbols A, B, .lamda. being constants)
[0110] a width Wn(x) of the n-type column region is expressed by a
following formula (3).
[ Formula 3 ] Wn ( x ) = [ - A exp ( - x .lamda. ) + B ] + W ( 3 )
##EQU00007##
(symbols A, B, .lamda. being constants)
[0111] Meanings of the constants W, A, B and .lamda. in the
above-mentioned formula (2) and formula (3) are described
hereinafter (see FIG. 3).
[0112] W indicates a positive constant which satisfies a formula of
W.sub.n(x)+W.sub.p(x)=2W. In other words, W indicates an average
pitch width (average column width) of the n-type column regions 114
and the p-type column regions 116.
[0113] The constant B indicates a difference between a width of the
p-type column region 116 and the average column width W when a
depth x is set to an infinite value. Accordingly, the width of the
p-type column region 116 becomes narrower than the average column
width W by B when the depth x is set to an infinite value. When a
boundary line is drawn between the n-type column region 114 and the
p-type column region 116 and divergence between the boundary line
and a line of the average column width W is observed, a divergence
width on one side becomes B/2. Accordingly, a total divergence
width becomes B by adding the divergence widths on both sides. In
this case, the n-type column region 114 is larger than the average
column width W by B. Further, although naturally induced from the
above-mentioned description, assuming a case where a depth x
becomes an infinite value, difference between the p-type column
region 116 and the n-type column region 114 becomes 2B.
[0114] The constant A is a coefficient of an exponential ex, and
indicates an amount of width by which a width of the p-type column
region 116 where the width of the p-type column region 116 when x=0
is set larger than the width of the p-type column region 116 when
the depth is set to an infinite value.
[0115] The constant .lamda. is also a scale for indicating a speed
of attenuation of the exponential. In the embodiment 1, the
constant .lamda. indicates a scale of a depth when a width of the
p-type column region 116 gradually approaches a width W-B of the
p-type column region 116 when the depth is set to an infinite
value. In the same manner, the constant .lamda. indicates a scale
of depth when a width of the n-type column region 114 gradually
approaches a width W+B of the n-type column region 114 when the
depth is set to an infinite value. As shown in FIG. 3, a tangent at
an arbitrary depth f is drawn on a boundary line between the n-type
column region 114 and the p-type column region 116, and an
intersecting point between the tangent and asymptotic line is
obtained. In this case, a depth of the intersecting point never
fails to become f+.lamda.. Further, to compare a divergence width
between the above-mentioned boundary line and the asymptotic line
at a depth a and a divergence width between the above-mentioned
boundary line and the asymptotic line at a depth f+.lamda., the
divergence width of the latter becomes 1/e of the divergence width
of the former (approximately 1/2.7).
[0116] Both of dopant concentration in the n-type column region 114
and dopant concentration in the p-type column region 116 are
respectively set to a fixed value regardless of depth.
[0117] All of the n-type column regions 114, the p-type column
regions 116, the source regions 120, the trenches 122 and the gate
electrodes 126 are formed in a stripe shape as viewed in a plan
view.
[0118] A thickness of the low-resistance semiconductor layer 112
falls within a range of 100 .mu.m to 400 .mu.m, for example, and
dopant concentration in the low-resistance semiconductor layer 112
falls within a range of 1.times.10.sup.19=.sup.-3 to
1.times.10.sup.20 cm.sup.-3, for example. A thickness of the n-type
semiconductor layer 115 falls within a range of 5 .mu.m to 120
.mu.m, for example. Dopant concentration in the n-type
semiconductor layer 115 falls within a range of
5.times.10.sup.13=.sup.-3 to 1.times.10.sup.16=.sup.-3, for
example. Dopant concentration in the p-type column region 116 falls
within a range of 5.times.10.sup.13=.sup.-3 to
1.times.10.sup.16=.sup.-3, for example. Using a depth position of
the first main surface of the semiconductor base substrate 110 as a
reference, a depth position of a deepest portion of the base region
118 falls within a range of 0.5 .mu.m to 4.0 .mu.m, for example,
and dopant concentration in the base region 118 falls within a
range of 5.times.10.sup.16 cm.sup.-3 to 1.times.10.sup.18
cm.sup.-3, for example. A depth position of a deepest portion of
the source region 120 falls within a range of 0.1 .mu.m to 0.4
.mu.m, for example, and dopant concentration in the source region
120 falls within a range of 5.times.10.sup.19 cm.sup.-3 to
2.times.10.sup.20 cm.sup.-3, for example.
[0119] The trench 122 is formed in a region where the n-type column
region 114 is positioned as viewed in a plan view such that the
trench 122 reaches a depth position deeper than the deepest portion
of the base region 118 from a surface of the first main surface of
the semiconductor base substrate 110 and a portion of the source
region 120 is exposed on an inner peripheral surface of the trench
122. A depth of the trench 122 is 5 .mu.m, for example.
[0120] The gate electrode 126 is embedded in the inside of the
trench 122 by way of a gate insulation film 124 formed on the inner
peripheral surface of the trench 122. The gate insulation film 124
is formed of a silicon dioxide film formed by a thermal oxidation
method and having a thickness of 100 nm, for example. The gate
electrode 126 is made of low-resistance polysilicon formed by a CVD
method or an ion implantation method.
[0121] The interlayer insulation film 128 is formed so as to cover
a portion of the source region 120, the gate insulation film 124
and the gate electrode 126. The interlayer insulation film 128 is
formed of a PSG film formed by a CVD method and having a thickness
of 1000 nm, for example.
[0122] The source electrode 130 is formed so as to cover the base
region 118, a portion of the source region 120, and the interlayer
insulation film 128. The source electrode 130 is electrically
connected with the source region 120. The drain electrode 132 is
formed on a surface of the low-resistance semiconductor layer 112.
The source electrode 130 is made of aluminum-based metal
(Al--Cu-based alloy, for example) formed by a sputtering method and
having a thickness of 4 .mu.m, for example. The drain electrode 132
is formed of a multi-layered metal film such as a Ti--Ni--Au film.
A total thickness of the multi-layered metal film is 0.5 .mu.m, for
example.
3. Average Positive Charge Density .rho.(x) at Predetermined Depth
Position of Super Junction Structure
[0123] To evaluate a charge amount of a positive charge in the
n-type column region 114 and a charge amount of a negative charge
in the p-type column region 116 when the MOSFET is turned off,
average positive charge density .rho.(x) at a predetermined depth
position of the super junction structure when the super junction
structure is depleted by turning off the MOSFET (hereinafter,
simply referred to as average positive charge density .rho.(x)) is
used.
[0124] 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, assuming a depth at a predetermined depth
position of the super junction structure 117 (hereinafter, simply
referred to as a depth x) as x, average positive charge density
.rho.(x) is expressed by a following formula (1).
[ Formula 1 ] .rho. ( x ) = q 2 W [ W n ( x ) N d ( x ) - W p ( x )
N a ( x ) ] ( 1 ) ##EQU00008##
[0125] (In the formula (1), W.sub.n(x) indicates a width of the
n-type column region 114 at the predetermined depth position,
N.sub.d(x) indicates average density of a positive charge at the
predetermined depth position of the n-type column region 114 when
the super junction structure 117 is depleted by turning off the
MOSFET, W.sub.p(x) indicates a width of the p-type column region at
the predetermined depth position, N.sub.a(x) indicates average
density of a negative charge at the predetermined depth position of
the p-type column region 116 when the super junction structure 117
is depleted by turning off the MOSFET, q indicates an elementary
charge, and W indicates a positive constant which satisfies
W.sub.n(x)+W.sub.p(x)=2W. See FIG. 4.)
[0126] In this embodiment, assuming that dopant concentration in
the n-type column region 114 and dopant concentration in the p-type
column region 116 are respectively set to a fixed value regardless
of depth, N.sub.a(x) and N.sub.d(x) become N.sub.0
(N.sub.a(x)=N.sub.d(x)=N.sub.0) and hence, average positive charge
density .rho.(x) is expressed by a following formula (6).
[ Formula 6 ] .rho. ( x ) = q N 0 2 W [ W n ( x ) - W p ( x ) ] ( 6
) ##EQU00009##
[0127] An electric field E(x) at a predetermined depth position of
the super junction structure 117 (hereinafter, simply referred to
as an electric field E(x)) when the super junction structure 117 is
depleted by turning off the MOSFET expresses an electric field
generated by a positive charge (donor) at a predetermined depth
position of the n-type column region 114 and an electric field
generated by a negative charge (acceptor) at a predetermined depth
position of the p-type column region 116 when the super junction
structure 117 is depleted by turning off the MOSFET, and is
expressed by a following formula (7).
[ Formula 7 ] E ( x ) = .intg. 0 x .rho. ( x ) d x ( 7 )
##EQU00010##
(In the formula (7), .epsilon.: dielectric constant of a material
(for example, silicon) of a semiconductor base substrate)
[0128] To describe average positive charge density .rho.(x) of the
MOSFET 100 according to Embodiment 1, a MOSFET 800 according to
Comparison example 1 is described first.
[0129] The MOSFET 800 according to Comparison example 1 (see FIG.
5A and FIG. 5B) basically has substantially the same configuration
as the MOSFET 100 according to Embodiment 1. However, in the same
manner as the conventional MOSFET 900, the MOSFET 800 according to
Comparison example 1 differs from the MOSFET 100 according to
Embodiment 1 with respect to a point that side walls of an n-type
column region 814 are formed in a tapered shape narrowed toward a
first main surface side, and side walls of a p-type column region
816 are formed in a tapered shape with a narrow bottom.
[0130] In the MOSFET 800 according to Comparison example 1, in a
graph where a depth x is taken on an axis of abscissas and a width
W.sub.n(x) at a predetermined depth position of the n-type column
region 814 or a width W.sub.p(x) at a predetermined depth position
of the p-type column region 816 is taken on an axis of ordinates,
the width W.sub.p(x) is expressed by a straight line extending in a
right downward direction, and the width W.sub.n(x) is expressed by
a straight line extending in a right upward direction (see FIG.
6A).
[0131] Further, in the MOSFET 800 according to Comparison example
1, in the same manner as the MOSFET 100 according to Embodiment 1,
dopant concentration in the n-type column region 814 and dopant
concentration in the p-type column region 816 are set to a fixed
value regardless of depth (see FIG. 6B).
[0132] From the above, in the MOSFET 800 according to Comparison
example 1, average positive charge density .rho.(x) is expressed by
a straight line extending in a right upward direction (see FIG.
6C).
[0133] To observe the straight line which expresses average
positive charge density .rho.(x) in detail, 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,
assuming an axis along a depth direction as an x axis, an x
coordinate at a depth position of a lowermost surface of the base
region as -t, an x coordinate at depth position of a lowermost
portion of the p-type column region as b, an x coordinate at a
depth position where the average positive charge density .rho.(x)
becomes 0 as d, and an x coordinate at a shallowest depth position
of a surface of a depletion layer on a second main surface side
when the depletion layer extends most in the super junction
structure by turning off the MOSFET as a, the following (1) to (3)
are established.
[0134] (1) The x coordinated satisfies a relationship of d=a/2.
That is, a depth d at the predetermined depth position when the
average positive charge density .rho.(x) becomes 0 becomes a depth
half of the depth a of a surface of the depletion layer on the
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 (that is, a charge balance being taken at
the depth position where x=a/2).
[0135] (2) A value of the average positive charge density .rho.(0)
when x=0 becomes a negative value, and a value of the average
positive charge density .rho.(a) when x=a becomes a positive
value.
[0136] (3) An area S1 of a region surrounded by the straight line
expressing the average positive charge density .rho.(x), a straight
line when x=0, and the axis of abscissas (x axis) is equal to an
area S2 of a region surrounded by the straight line expressing the
average positive charge density .rho.(x), a straight line when x=a
and the axis of abscissas (x axis).
[0137] Further, in the MOSFET 800 according to Comparison example
1, an electric field E(x) becomes a downwardly convex quadratic
function having a peak when x=a/2 (see FIG. 6D). A state where the
electric field E(x) becomes a negative value means that an electric
field vector is directed in a direction that x approaches 0.
[0138] To the contrary, in the MOSFET 100 according to Embodiment
1, in a graph where the depth x at the deepest depth position of a
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 is used as a reference is taken on an axis
of abscissas, and a width W.sub.n(x) at the predetermined depth
position of the n-type column region or a width W.sub.p(x) at the
predetermined depth position of the p-type column region is taken
on an axis of ordinates, the width W.sub.n(x) is expressed by a
monotonous upward convex curve projecting in a right upward
direction, and the width W.sub.p(x) is expressed by a monotonous
downward convex curve projecting in a right downward direction (see
FIG. 7A).
[0139] Further, in the MOSFET 100 according to Embodiment 1,
average density N.sub.d(x) of a positive charge and average density
N.sub.a(x) of a negative charge are set to a fixed value regardless
of depth (see FIG. 7B). From these, in the MOSFET 100 according to
Embodiment 1, average positive charge density .rho.(x) is expressed
by a monotonous upward convex curve projecting in a right upward
direction (see FIG. 7C).
[0140] To observe the curve which expresses average positive charge
density .rho.(x) in detail, 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, assuming an axis along a
depth direction as an x axis, an x coordinate at a depth position
of a lowermost surface of the base region as -t, an x coordinate at
the depth position of a lowermost portion of the p-type column
region as b, an x coordinate at a depth position where the average
positive charge density .rho.(x) becomes 0 as d, and an x
coordinate at a shallowest depth position of the surface of the
depletion layer on a second main surface side when the depletion
layer extends most in the super junction structure by turning off
the MOSFET as a, the following relationships (1) to (4) are
satisfied.
[0141] (1) A relationship of 0<d<a/2 is satisfied. That is, a
depth d at a predetermined depth position when the average positive
charge density .rho.(x) becomes 0 is shallower than a depth half of
the depth a of a surface of the depletion layer on the second main
surface side at the shallowest depth position when the depletion
layer extends most in the super junction structure by turning off
the MOSFET (the depth position where a charge balance is obtained
is shallower than the depth position where a charge balance is
obtained in the MOSFET 800 according to Comparison example 1).
[0142] (2) A value of the average positive charge density .rho.(0)
when x=0 becomes a negative value, and a value of the average
positive charge density .rho.(a) when x=a becomes a positive
value.
[0143] (3) An area S3 of a region surrounded by a curve expressing
the average positive charge density .rho.(x), a straight line when
x=0 and the axis of abscissas (x axis) is equal to an area S4 of a
region surrounded by a curve expressing the average positive charge
density .rho.(x), a straight line when x=a and the axis of
abscissas (x axis).
[0144] (4) The relationship of 0<t+d<(t+b)/2 is
satisfied.
[0145] Further, an electric field E(x) becomes a downwardly convex
function having a peak when x=d (see FIG. 7D). In this case, the
electric field E(x) is sharply decreased when x<d, and the
electric field E(x) is gently increased when x>d.
4. Relationship Between Irregularity in a Charge Balance and
Switching Characteristic
(1) Irregularity in a Charge Balance and Deviation of Average
Positive Charge Density .rho.(x)
[0146] A relationship between irregularity in a charge balance and
switching characteristics in the MOSFET according to Embodiment 1
is described. First of all, deviation of average positive charge
density .rho.(x) when an irregularity occurs in a charge balance in
the MOSFET according to Embodiment 1 is described. For this
purpose, a reference MOSFET where a shape of the super junction
structure (p-type column regions and n-type column regions) is
equal and an irregularity does not occur in a charge balance, and a
MOSFET (n-type dopant rich MOSFET) where a shape of a super
junction structure (p-type column regions and n-type column
regions) is equal and an irregularity occurs in a charge balance so
that the MOSFET becomes n-type dopant rich are compared with each
other.
(1-1) Reference MOSFET in Comparison Example 1
[0147] The reference MOSFET in Comparison example 1 is a MOSFET
where a shape of the super junction structure (p-type column
regions and n-type column regions) is equal to a corresponding
shape in the MOSFET 800 according to Comparison example 1, and a
total amount of dopant in the n-type column region is equal to a
total amount of dopant in the p-type column region so that a charge
balance is obtained (see FIG. 6A to FIG. 6D).
[0148] To be more specific, the reference MOSFET according to
Comparison example 1 is the MOSFET having the configuration where
the shape of the super junction structure (p-type column regions
and n-type column regions) is equal to the corresponding shape in
the MOSFET 800 according to Comparison example 1, and dopant
concentration in the p-type column region is adjusted such that a
concentration gradient of a p-type dopant in the p-type column
region and a concentration gradient of an n-type dopant in the
n-type column region are equal to the corresponding concentration
gradients in the MOSFET 800 according to Comparison example 1, and
a total amount of dopant in the n-type column region and a total
amount of dopant in the p-type column region are equal (including a
case where a total amount of dopant in the n-type column region is
equal to a total amount of dopant in the p-type column region so
that the dopant concentration is not adjusted).
[0149] Average positive charge density in the reference MOSFET is
expressed as reference average positive charge density
.rho..sub.0(x).
(1-2) n-Type Dopant Rich MOSFET in Comparison Example 1
[0150] An n-type dopant rich MOSFET in Comparison example 1 is a
MOSFET having the configuration where a shape of the super junction
structure (p-type column regions and n-type column regions) is
equal to a corresponding shape in the MOSFET 800 according to
Comparison example 1, and a total amount of dopant in the n-type
column region 114 is set larger than a total amount of dopant in
the p-type column region (see FIG. 8A to FIG. 8D).
[0151] A width Wn(x) at a predetermined depth position of the
n-type column region 114, a width Wp(x) at a predetermined depth
position of the p-type column region 116, and average density Na(x)
of a negative charge at a predetermined depth position of the
p-type column region 116 of the n-type dopant rich MOSFET in
Comparison example 1 are equal to a width Wn(x) at a predetermined
depth position of the n-type column region 114, a width Wp(x) at a
predetermined depth position of the p-type column region 116, and
average density Na(x) of a negative charge at a predetermined depth
position of the p-type column region 116 of the reference MOSFET in
Comparison example 1 (see FIG. 8A and FIG. 8B). On the other hand,
average density Nd(x) of a positive charge at a predetermined depth
position of the n-type column region 114 of the n-type dopant rich
MOSFET in Comparison example 1 is set larger than average density
Nd(x) of a positive charge at a predetermined depth position of the
n-type column region 114 of the reference MOSFET in Comparison
example 1.
[0152] In the n-type dopant rich MOSFET in Comparison example 1, a
total amount of dopant in the n-type column region is set larger
than a total amount of dopant in the p-type column region and
hence, the n-type dopant rich MOSFET in Comparison example 1
differs from the reference MOSFET in Comparison example 1 with
respect to the manner of extension of a depletion layer. That is,
it is difficult for the depletion layer to extend in the n-type
column region 114, while it is easy for the depletion layer to
extend in the p-type column region 116. Accordingly, a relationship
of |-t'|<|-t| is satisfied.
[0153] Average positive charge density .rho.(x) and an electric
field E(x) of the n-type dopant rich MOSFET in Comparison example 1
and average positive charge density .rho.(x) and an electric field
E(x) of the reference MOSFET in Comparison example 1 exhibit
extremely similar graphs (see FIG. 8C and FIG. 8D).
[0154] An area S1' of a region surrounded by a straight line
expressing the n-type dopant rich average positive charge density
.rho.(x), a straight line when x=0, and an x axis is equal to an
area S2' of a region surrounded by a straight line expressing the
n-type dopant rich average positive charge density .rho.(x), a
straight line when x=a, and an x axis.
(1-3) Reference MOSFET in Embodiment 1
[0155] The reference MOSFET in Embodiment 1 is a MOSFET where a
shape of the super junction structure (p-type column regions and
n-type column regions) is equal to a corresponding shape in the
MOSFET 100 according to Embodiment 1, and a total amount of dopant
in the n-type column region is equal to a total amount of dopant in
the p-type column region so that a charge balance is obtained (see
FIG. 7A to FIG. 7D).
[0156] To be more specific, the reference MOSFET according to
Embodiment 1 is a MOSFET having the configuration where the shape
of the super junction structure (p-type column regions and n-type
column regions) is equal to the corresponding shape in the MOSFET
100 according to Embodiment 1, and dopant concentration in the
p-type column region is adjusted such that a concentration gradient
of a p-type dopant in the p-type column region and a concentration
gradient of an n-type dopant in the n-type column region are equal
to the corresponding concentration gradients in the MOSFET 100
according to Embodiment 1, and a total amount of dopant in the
n-type column region and a total amount of dopant in the p-type
column region are equal (including a case where a total amount of
dopant in the n-type column region is equal to a total amount of
dopant in the p-type column region so that the dopant concentration
is not adjusted) (see FIG. 7B). In Embodiment 1, the reference
MOSFET in Embodiment 1 is equal to the MOSFET according to
Embodiment 1.
[0157] In this case, average positive charge density in the
reference MOSFET is expressed as reference average positive charge
density .rho..sub.0(x).
(1-4) n-Type Dopant Rich MOSFET in Embodiment 1
[0158] An n-type dopant rich MOSFET in Embodiment 1 is a MOSFET
having the configuration where a shape of the super junction
structure (p-type column regions and n-type column regions) is
equal to a corresponding shape in the MOSFET 100 according to
Embodiment 1, and a total amount of dopant in the n-type column
region 114 is set larger than a total amount of dopant in the
p-type column region.
[0159] In the n-type dopant rich MOSFET in Embodiment 1, a total
amount of dopant in the n-type column region set larger than a
total amount of dopant in the p-type column region and hence, the
n-type dopant rich MOSFET in Embodiment 1 differs from the
reference MOSFET in Embodiment 1 with respect to the manner of
extension of a depletion layer. That is, it is difficult for the
depletion layer to extend in the n-type column region 114, while it
is easy for the depletion layer to extend in the p-type column
region 116. Accordingly, a relationship of |-t'|<|-t| is
satisfied.
[0160] A width Wn(x) at a predetermined depth position of the
n-type column region 114, a width Wp(x) at a predetermined depth
position of the p-type column region 116, and average density Na(x)
of a negative charge at the predetermined depth position of the
p-type column region 116 of the n-type dopant rich MOSFET in
Embodiment 1 are equal to a width Wn(x) at a predetermined depth
position of the n-type column region 114, a width Wp(x) at a
predetermined depth position of the p-type column region 116, and
average density Na(x) of a negative charge at the predetermined
depth position of the p-type column region 116 of the reference
MOSFET in Embodiment 1 (see FIG. 9A and FIG. 9B). On the other
hand, average density Nd(x) of a positive charge at a predetermined
depth position of the n-type column region 114 of the n-type dopant
rich MOSFET in Embodiment 1 is set larger than average density
Nd(x) of a positive charge at a predetermined depth position of the
n-type column region 114 of the reference MOSFET in Embodiment
1.
[0161] Curves expressing average positive charge density .rho.(x)
and an electric field E(x) of the n-type dopant rich MOSFET in
Embodiment 1 and curves expressing average positive charge density
.rho.(x) and an electric field E(x) of the reference MOSFET in
Comparison example 1 exhibit extremely similar (see FIG. 9C and
FIG. 9D).
[0162] An area S3' of a region surrounded by a curve expressing the
n-type dopant rich average positive charge density .rho.(x), the
straight line when x=0, and the x axis is equal to an area S4' of a
region surrounded by a curve expressing the n-type dopant rich
average positive charge density .rho.(x), the straight line when
x=a, and the x axis.
[0163] An area S4' of a region surrounded by a curve expressing
n-type dopant rich average positive charge density .rho.(x), a
straight line when x=a', and an x axis (see FIGS. 9A to 9D) is
equal to an area S4 of a region surrounded by a curve expressing
reference average positive charge density .rho.(x), the straight
line when x=a, and the x axis (see FIGS. 7A to 7D).
(1-5) Relationship Between Irregularity in a Charge Balance and
Deviation of Average Positive Charge Density .rho.(x)
[0164] A relationship between an irregularity in a charge balance
and deviation of average positive charge density .rho.(x) is
described. Using a surface of the super junction structure on a
first main surface side as a reference, in both the case of
Comparison example 1 and the case of Embodiment 1, a graph
indicating average positive charge density .rho.(X) in the n-type
dopant rich MOSFET is positioned on a left side of a graph
indicating reference average positive charge density .rho..sub.0(X)
(see FIG. 10A and FIG. 11A). Accordingly, in the case where an
irregularity occurs in a charge balance so that the MOSFET becomes
n-type dopant rich, a point (E(X.sub.m')) where an electric field
E(X) in the n-type dopant rich MOSFET is highest is positioned
closer to a source electrode side than a point (E(X.sub.m)) where
an electric field E(X) in the reference MOSFET is highest.
[0165] In the n-type dopant rich MOSFET according to Comparison
example 1 and in the reference MOSFET according to Comparison
example 1, to compare a straight line of the average positive
charge density .rho.(X) in the n-type dopant rich MOSFET with a
straight line of the reference average positive charge density
.rho..sub.0(X) using an upper surface of the super junction
structure (a lowermost surface of the base region) as a reference,
neither .DELTA.X.sub.0(=|X.sub.0-X.sub.0'|) nor
.DELTA.X.sub.m(=|X.sub.m-X.sub.m'|) largely changes (see FIG. 10A)
and hence, it is difficult to read the relationship between the
average positive charge density .rho.(X) and the reference average
positive charge density .rho..sub.0(X) from the graph. However, a
relationship of
.DELTA.X.sub.0(=|X.sub.0-X.sub.0'|)>.DELTA.X.sub.m(=|X.sub.m-X.sub.m'|-
) is satisfied. Further, it is understood from the graph that
.DELTA.X.sub.1(=|X.sub.1-X.sub.1'|) is smaller than .DELTA.X.sub.m.
Accordingly, a relationship of
.DELTA.X.sub.0>.DELTA.X.sub.m>.DELTA.X.sub.1 is
satisfied.
[0166] Further, in the electric field E(X), neither .DELTA.X.sub.0
nor .DELTA.X.sub.m largely changes and hence, a curve of the n-type
dopant rich MOSFET in Comparison example 1 and a curve of the
reference MOSFET in Comparison example 1 are minimally different
from each other in shape, and the curves have shapes translated
toward a source electrode side (see FIG. 10B).
[0167] On the other hand, in the n-type dopant rich MOSFET
according to Embodiment 1 and in the reference MOSFET according to
Embodiment 1, to compare a curve of average positive charge density
.rho.(X) in the n-type dopant rich MOSFET with a curve of reference
average positive charge density .rho..sub.0(X) using an upper
surface of the super junction structure (a lowermost surface of the
base region) as a reference, a relationship of
.DELTA.X.sub.0(=|X.sub.0-X.sub.0'|)<.DELTA.X.sub.m(=|X.sub.m-X.sub.m'|-
) is satisfied. Further, it is also understood from the graph that
.DELTA.X.sub.1(=|X.sub.1-X.sub.1'|) is larger than .DELTA.X.sub.m
(see FIG. 11A). Accordingly, a relationship of
.DELTA.X.sub.0<.DELTA.X.sub.m<.DELTA.X.sub.1 is satisfied.
Further, a relationship of .DELTA.X.sub.0<.DELTA.X.sub.m is
established in the electric field E(X) and hence, a gradient is set
steep within a range of X.sub.0<X<X.sub.m (see FIG. 11B).
[0168] In this manner, the MOSFET 100 according to Embodiment 1
satisfies a relationship of
.DELTA.X.sub.0(=|X.sub.0-X.sub.0'|)<.DELTA.X.sub.m
(=|X.sub.m-X.sub.m'|). Accordingly, even when an irregularity
occurs in a charge balance so that the MOSFET becomes n-type dopant
rich, deviation of average positive charge density .rho.(X) on a
first main surface side (gate electrode side) is small whereby a
region where an electric field E(X) is high minimally approaches
the gate electrode. Accordingly, an electric field E(X) in the
n-type column region around the gate electrode becomes minimally
high so that a change in potential of the n-type column region
around the gate becomes small easily. As a result, an irregularity
minimally occurs in switching characteristics when the MOSFET is
turned off.
[0169] In the n-type dopant rich MOSFET according to Embodiment 1
and the reference MOSFET according to Embodiment 1, using the
surface of the super junction structure on the first main surface
side as the reference, assuming the average positive charge density
at a depth X.sub.1 of the surface of the depletion layer on the
second main surface side at the shallowest depth position when the
depletion layer extends most in the super junction structure by
turning off the MOSFET as .rho.(X.sub.1), and assuming the
reference average positive charge density at the shallowest depth
position X.sub.1' of the surface of the depletion layer on the
second main surface side when the depletion layer extends most in
the super junction structure by turning off the reference MOSFET as
.rho..sub.0(X.sub.1), a relationship of
.rho..sub.0(X.sub.1)<.rho.(X.sub.1') is satisfied.
[0170] Using the surface of the super junction structure on the
first main surface side as the reference, assuming the average
positive charge density at the deepest depth position X.sub.0 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 n-type dopant rich MOSFET as
.rho.(X.sub.0') and assuming the reference average positive charge
density at the deepest depth position X.sub.0 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 .rho..sub.0(X.sub.0), a relationship of
.rho..sub.0(X.sub.0)>.rho.(X.sub.0') is satisfied.
(1-6) Relationship Between Configuration of Super Junction
Structure and Deviation of Average Positive Charge Density
.rho.(x)
[0171] It is confirmed by the following description that the MOSFET
100 according to Embodiment 1 satisfies the relationship of
.DELTA.X.sub.0(=X.sub.0-X.sub.0')<.DELTA.X.sub.m(=X.sub.m-X.sub.m')
as described above.
[0172] A width W.sub.p(x) at a predetermined depth position of a
p-type column region is expressed by a following formula (2), and a
width W.sub.n(x) at a predetermined depth position of an n-type
column region is expressed by a following formula (3).
[ Formula 2 ] Wp ( x ) = - [ - A exp ( - x .lamda. ) + B ] + W ( 2
) [ Formula 3 ] Wn ( x ) = [ - A exp ( - x .lamda. ) + B ] + W ( 3
) ##EQU00011##
[0173] To modify these formulae, the width W.sub.p(x) at a
predetermined depth position of a p-type column region and the
width W.sub.n(x) at a predetermined depth position of an n-type
column region are expressed by following formulae (8) and (9).
[ Formula 8 ] Wp ( x ) = - B [ - exp ( - x - .lamda.1 n A B .lamda.
) + 1 ] + W - .delta. ( 8 ) [ Formula 9 ] ( x ) = - B [ - exp ( - x
- .lamda. 1 n A B .lamda. ) + 1 ] + W + .delta. ( 9 )
##EQU00012##
[0174] By substituting W.sub.p(x) in the formula (8) and W.sub.n(x)
in the formula (9) for W.sub.p(x) and W.sub.n(x) in the formula
(6), by replacing x-.lamda. ln (A/B) with x (this operation
corresponding to the translation of the x coordinate) and by
arranging the formula (6), a following formula (10) is
obtained.
[ Formula 10 ] .rho. ( x ) = q N 0 W B [ - exp ( x .lamda. ) + 1 ]
( 10 ) ##EQU00013##
[0175] On the other hand, assume that dopant concentration in the
p-type column region 116 is changed as described below (formula
(11)).
[Formula 11]
Na(x)=(1+.delta.)N.sub.0 (11)
(wherein, |.delta.|<<1)
[0176] In the formula (11), .delta. expresses a width of a change
(for example, when dopant concentration in the p-type column region
116 is increased by 5%, .delta. becomes 0.05 (.delta.=0.05), and
when dopant concentration in the p-type column region 116 is
decreased by 5%, .delta. becomes -0.05 (.delta.=-0.05)). A
following formula (12) is obtained by substituting N.sub.a(x) in
the formula (11) for N.sub.a(x) in the formula (1).
[ Formula 12 ] .rho. ( x ) = q 2 W [ N 0 W n ( x ) - ( 1 + .delta.
) N 0 W p ( x ) ] ( 12 ) ##EQU00014##
[0177] In putting W.sub.p(x) in the formula (8) and W.sub.n(x) in
the formula (9) for W.sub.p(x) and W.sub.n(x) in the formula (12),
since |.delta.|<<1, it is possible to make the approximation
of exp(.delta.).apprxeq.1+.delta.. By further arranging the
approximated formula (12) by replacing x-.lamda. ln(A/B) with x, a
following formula (13) is obtained.
[ Formula 13 ] .rho. ( x ) = qN 0 W ( ( 1 + .delta. 2 ) B - .delta.
W 2 ) [ - exp ( - x - .lamda. .delta. W ( 2 + .delta. ) B .lamda. )
+ 1 ] ( 13 ) ##EQU00015##
[0178] As can be understood from FIG. 11A and FIG. 11B, .rho.(x)
becomes 0 (.rho.(x)=0) at a maximum electric field intensity point
X.sub.m and hence, the maximum electric field intensity point
X.sub.m appears when x=.lamda..delta.W/(2+.delta.)B. To compare the
formula (10) with the formula (13), the maximum electric field
intensity point X.sub.m is moved toward a source electrode side by
an amount of .lamda..delta.W/(2+.delta.)B when an irregularity
occurs in a charge balance (namely,
.DELTA.X.sub.m=.lamda..delta.W/(2+.delta.)B). By substituting
specific values in the formula in such a manner as .delta.=-0.05,
W=5 .mu.m, .lamda.=10 .mu.m, N.sub.0=3.times.10.sup.15 cm.sup.-3,
B=0.868 .mu.m, a moving amount .DELTA.X.sub.m of the maximum
electric field intensity point X.sub.m becomes 1.48 .mu.m
(.DELTA.X.sub.m=1.48 .mu.m). A value of B is calculated as follows.
An electric field E(X) is calculated based on .rho.(X). The maximum
electric field intensity E(X.sub.m) becomes -2.5.times.10.sup.5
(V/cm) (E(X.sub.m)=-2.5.times.10.sup.5 (V/cm)), and E(X.sub.0)
becomes 0 (E(X.sub.0)=0), and E(X.sub.1) becomes 0 (E(X.sub.1)=0).
Based on these values, the value of B is calculated.
[0179] 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 is a
point where .rho.(X) takes a minimum value. To compare the formula
(10) with the formula (13), such a position is translated toward a
source electrode side by (1+.delta./2)B-.delta.W/2 due to an
irregularity in a charge balance (that is,
.DELTA.X.sub.0=(1+.delta./2)B-.delta.W/2). To substitute the
above-mentioned numerical values in such a manner that
.delta.=-0.05, W=5 .mu.m, a moving amount .DELTA.X.sub.0 of the
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 becomes
0.971 .mu.m (.DELTA.X.sub.0=0.971 .mu.m).
[0180] Accordingly, .DELTA.X.sub.0<.DELTA.X.sub.m is
obtained.
[0181] That is, in the MOSFET 100 according to Embodiment 1, Wn(x)
and Wp(x) are exponentials expressed by the above-mentioned
formulae (2) and (3) and |.delta.|<<1 is established and
hence, average positive charge density .rho.(x) is expressed by a
following formula (14). Accordingly, even when an irregularity in a
charge balance occurs, a functional shape is not changed and, at
the same time, an irregularity in a charge balance can be absorbed
as a value of translational movement. That is, it is safe to say
that the MOSFET 100 according to Embodiment 1 ensure high robust
stability against an irregularity in a charge balance.
[ Formula 14 ] ( 1 + .delta. ) exp ( - x .lamda. ) = exp [ - ( x -
.delta. .lamda. .lamda. ) ] ( 14 ) ##EQU00016##
[0182] In Embodiment 1, the case is considered where a charge
balance becomes n-type dopant rich (a total amount of dopant in the
n-type column region becoming larger than a total amount of dopant
in the p-type column region). However, in the case where a charge
balance is deviated to p-type dopant rich, average positive charge
density .rho.(x) is positioned on a right side of reference average
positive charge density .rho..sub.0(x). Also in this case, there is
no possibility that the region where an electric field E(x) is high
approaches the gate electrode. Accordingly, an electric field E(x)
of the n-type column region around the gate electrode is minimally
increased and hence, a change in potential of the n-type column
region around the gate electrode becomes small easily. As a result,
an irregularity minimally occurs in switching characteristics when
the MOSFET is turned off. Also in this case,
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied.
(2) Irregularity in a Charge Balance and Deviations X.sub.0,
X.sub.m and X.sub.1
[0183] Next, using a surface of a super junction structure on the
first main surface side as a reference, a relationship between a
deepest depth position X.sub.0 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, a depth
position X.sub.m when the average positive charge density .rho.(x)
becomes 0, a depth X.sub.1 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, and a charge balance is described.
[0184] In a graph where a difference between a total amount of
dopant in the n-type column region and a total amount of dopant in
the p-type column region is taken on an axis of abscissas, and a
depth X using the surface of the super junction structure on the
first main surface side as a reference is taken on an axis of
ordinates, X.sub.0 and X.sub.m become straight lines which are
monotonously increased (see FIG. 12). When the difference between
the total amount of dopant in the n-type column region 114 and the
total amount of dopant in the p-type column region 116 is positive
(+), X.sub.1 becomes a straight line which is monotonously
increased. On the other hand, when the difference between the total
amount of dopant in the n-type column region 114 and the total
amount of dopant in the p-type column region 116 is negative (-),
X.sub.1 is converged to a predetermined value when the difference
between the total amount of dopant in the n-type column region 114
and the total amount of dopant in the p-type column region 116
becomes large. The predetermined value is set to substantially
equal to a value of a depth of a bottom of the p-type column region
116.
[0185] Further, when the MOSFET is n-type dopant rich, the
difference with respect to a value when a charge balance is
obtained is increased in order of X.sub.0, X.sub.m and X.sub.1 and
hence, it is understood that the relationship of
.DELTA.X.sub.0<.DELTA.X.sub.m<.DELTA.X.sub.1 is
satisfied.
(3) Relationship Between Irregularity in Charge Balance and
Switching Off Loss when MOSFET is Turned Off
[0186] As shown in FIG. 13A, in the MOSFET 800 according to
Comparison example 1, a switching loss Eoff when the MOSFET is
turned off falls within a range of approximately from 10 .mu.J to
15 .mu.J in the case of Just and in the case where the MOSFET 800
becomes p-type dopant rich. On the other hand, in the MOSFET 800
according to Comparison example 1, the switching loss Eoff is
increased as the MOSFET 800 becomes n-type dopant rich, and the
switching loss Eoff becomes 20 .mu.J in the case where the MOSFET
800 becomes n-type dopant rich of 5%, and the switching loss Eoff
becomes 30 .mu.J in the case where the MOSFET 800 becomes n-type
dopant rich of 10%. Accordingly, in the case where an irregularity
occurs in a charge balance and the MOSFET 800 becomes n-type dopant
rich, an irregularity in a switching loss when the MOSFET is turned
off is increased and hence, there is a possibility that a defect
occurs in an operation of a power conversion circuit (particularly,
a circuit largely affected by a switching loss at the time of
turning off the MOSFET such as a current critical type PFC or a
current discontinuous type PFC).
[0187] On the other hand, as shown in FIG. 13B, in the MOSFET 100
according to Embodiment 1, a switching loss Eoff when the MOSFET is
turned off falls within a range of from 10 .mu.J to 20.mu.J in all
cases of Just, n-type dopant rich and p-type dopant rich.
Accordingly, even in the case where an irregularity occurs in a
charge balance, an irregularity in a switching loss when the MOSFET
is turned off is not increased and hence, it is possible to avoid
occurrence of a defect in an operation of the power conversion
circuit. That is, it is safe to say that the MOSFET 100 according
to Embodiment 1 is a MOSFET which can reduce irregularities in a
switching loss when the MOSFET is turned off due to the occurrence
of an irregularity in a charge balance.
[0188] A switching loss when the MOSFET is turned on minimally
differs between all cases of Just, n-type dopant rich and p-type
dopant rich in either of the MOSFET 800 according to Comparison
example 1 and the MOSFET 100 according to Embodiment 1. Although a
switching loss when the MOSFET is turned on can be decreased by
decreasing gate resistance, a switching loss of the power
conversion circuit is largely dependent on a characteristic
(recovery characteristic) of a rectifier element (free wheel diode
or the like) and hence, the influence caused by a switching loss of
the MOSFET when the MOSFET is turned on is small thus minimally
contributing to a switching loss of the power conversion
circuit.
5. Manner of Operation and Waveform of MOSFET 100 when MOSFET 100
is Turned Off
[0189] In the power conversion circuit 1 according to Embodiment 1,
in the case where the MOSFET 800 according to Comparison example 1
is used in place of the MOSFET 100, the MOSFET 800 according to
Comparison example 1 is operated as follows.
(1) Drain Current Id
[0190] In the case where a total amount of dopant in the n-type
column region and a total amount of dopant in the p-type column
region are equal (hereinafter referred to as "in the case of
Just"), the MOSFET 800 is operated such that, during a period from
a point of time that a drain current Id starts to be decreased to a
point of time that the drain current Id becomes 0 for the first
time, a period where the drain current Id is temporarily increased
slightly appears (the MOSFET 800 being operated such that a hump
waveform slightly appears in a waveform of the drain current Id,
see Id (Just) in FIG. 14). The period from a point of time that a
drain current Id starts to be decreased to a point of time that the
drain current Id becomes 0 for the first time is approximately 0.02
.mu.sec (20 nsec).
[0191] In the case where an irregularity exists in a charge balance
such that a total amount of dopant in the n-type column region is
larger than a total amount of dopant in the p-type column region
(hereinafter referred to as "in the case of n-type dopant rich"),
the MOSFET 800 is operated such that, during a period from a point
of time that a drain current Id starts to be decreased to a point
of time that the drain current Id becomes 0 for the first time, a
period where the drain current Id is temporarily increased appears
(the MOSFET 800 being operated such that a large hump waveform
appears in a waveform of the drain current Id, see Id (n-type
dopant rich) in FIG. 14). With respect to the hump waveform, the
drain current Id is increased to a current value higher than a
current value in the case of Just and, at the same time, a period
until the drain current Id becomes 0 is largely prolonged compared
to the case of Just (approximately 0.02 .mu.sec (20 nsec) in the
case of Just and approximately 0.04 .mu.sec (40 nsec) in the case
of n-type dopant rich).
[0192] In the case where a total amount of dopant in the p-type
column region is larger than a total amount of dopant in the n-type
column region (hereinafter referred to as "in the case of p-type
dopant rich"), the MOSFET 800 is operated such that the drain
current Id is monotonously decreased (the MOSFET 800 being operated
such that no hump waveform appears in a waveform of the drain
current Id, see Id (p-type dopant rich) in FIG. 14).
(2) Drain-Source Voltage Vds
[0193] In the case of n-type dopant rich, the MOSFET 800 is
operated such that a drain-source voltage Vds is gently increased
to approximately 350V compared to the case of Just and, thereafter,
the drain-source voltage Vds is gently decreased and becomes stable
at a power source voltage (300V). A time until a drain-source
voltage Vds becomes stable from a point of time that the
drain-source voltage Vds starts to be increased is longer than a
corresponding time in the case of Just and is approximately 0.05
.mu.sec (50 nsec) (see Vds (n-type dopant rich) in FIG. 14).
[0194] In the case of p-type dopant rich, the MOSFET 800 is
operated such that a drain-source voltage Vds is steeply increased
to approximately 370V compared to the case of Just and, thereafter,
becomes stable at a power source voltage (300V) (see Vds (p-type
dopant rich) in FIG. 14). A time until a drain-source voltage Vds
becomes stable from a point of time that the drain-source voltage
Vds starts to be increased is approximately 0.02 .mu.sec (20
nsec).
(3) Gate-Source Voltage Vgs
[0195] In the case of n-type dopant rich, the MOSFET 800 is
operated such that a period during which a gate-source voltage Vgs
is temporarily increased slightly appears after completion of a
mirror period (see Vgs (n-type dopant rich) in FIG. 14). On the
other hand, in the case of Just and in the case of p-type dopant
rich, the MOSFET 800 is operated such that a gate-source voltage
Vgs minimally changes and is monotonously decreased (see Vgs
(p-type dopant rich) and Vgs (Just) in FIG. 14).
[0196] As can be understood from the above-mentioned (1) to (3), in
the MOSFET 800 according to the Comparison example 1, when an
irregularity exists in a charge balance around the gate (in case
such as Just being changed to n-type dopant rich or p-type dopant
rich), an irregularity in switching characteristics when the MOSFET
is turned off is increased. Particularly, an irregularity in a
drain current Id and an irregularity in a drain-source voltage Vds
are increased. When an irregularity occurs in a charge balance so
that the MOSFET800 becomes n-type dopant rich, an irregularity in
switching characteristics is particularly increased.
[0197] On the other hand, in the power conversion circuit 1
according to Embodiment 1, the MOSFET 100 according to Embodiment 1
is operated as follows.
(1) Drain Current Id
[0198] In all cases, that is, in the case of Just, in the case of
n-type dopant rich and in the case of p-type dopant rich, a
turn-off period becomes short. Further, in all cases, the MOSFET
100 is operated such that the drain current Id adopts a similar
waveform (see respective Id in FIG. 15). Particularly, in the case
of n-type dopant rich, a hump waveform minimally appears in a
waveform of the drain current Id and hence, the MOSFET 100 is
operated such that the waveform of the drain current Id becomes
similar to a waveform of the drain current Id in the case of Just
and the drain current Id in the case of p-type dopant rich.
(2) Drain-Source Voltage Vds
[0199] In all cases, that is, in the case of Just, in the case of
n-type dopant rich and in the case of p-type dopant rich, a
turn-off period becomes short. Further, in all cases, the MOSFET
100 is operated such that the drain-source voltage Vds adopts a
similar waveform (see respective Vds in FIG. 15). Although ringing
occurs in the case of p-type dopant rich, the ringing can be made
small by allowing the MOSFET 100 to include a mechanism to remove
ringing such as a snubber circuit.
(3) Gate-Source Voltage Vgs
[0200] In all cases, that is, in the case of Just, in the case of
n-type dopant rich and in the case of p-type dopant rich, the
MOSFET 100 is operated such that there is substantially no
difference in waveform of the gate-source voltage Vgs between these
cases (see respective Vgs in FIG. 15).
[0201] As can be understood from the above-mentioned (1) to (3), in
the MOSFET 100 according to Embodiment 1, even when an irregularity
exists in a charge balance around the gate (in case such as Just
being changed to n-type dopant rich or p-type dopant rich), an
irregularity in switching characteristics when the MOSFET is turned
off can be made small.
[0202] Next, the reason is explained why the MOSFET 100 according
to Embodiment 1 and the MOSFET 800 according to Comparison example
1 take the above-mentioned waveforms when the MOSFET is turned
off.
[0203] Firstly, in the MOSFET 800 according to Comparison example
1, when the MOSFET is turned off, a depletion layer extends to the
n-type column region 814 and the p-type column region 816 from the
PN junction between the p-type column region 816 (and the base
region 818) and the n-type column region 814. However, in the
MOSFET 800 according to Comparison example 1, although the
depletion layer extends to an area directly below the trench, the
depletion layer minimally extends to a drain electrode side and
hence, it is difficult to increase a distance between the region
which is not depleted in the n-type column region 814 and the gate
electrode 826 whereby it is difficult to decrease a feedback
capacitance Crss (see FIG. 16). Accordingly, the gate electrode 826
is liable to be affected by a change in potential of the n-type
column region 814 and hence, when an irregularity exists in a
charge balance around the gate, it is difficult to decrease an
irregularity in switching characteristics when the MOSFET is turned
off.
[0204] To the contrary, in the MOSFET 100 according to Embodiment
1, a depletion layer easily extends also to a drain electrode side
and hence, a distance between the region which is not depleted in
the n-type column region 114 and the gate electrode 126 can be
easily increased whereby a feedback capacitance Crss can be easily
made small (see FIG. 17). Accordingly, the gate electrode 126 can
be minimally affected by a change in potential of the n-type column
region 114 and hence, when an irregularity exists in a charge
balance around the gate, an irregularity in switching
characteristics when the MOSFET is turned off can be made
small.
[0205] The above-mentioned reason is explained from an aspect of
equal potential lines.
[0206] A MOSFET 700 according to Comparison example 2 is a MOSFET
having substantially the same configuration as the MOSFET 800
according to Comparison example 1 except for a point that a portion
of the MOSFET 700 which is brought into contact with the source
electrode is dug to a depth position of a lowermost portion of the
source region (see FIG. 18A). A MOSFET 100A according to a present
invention example is a MOSFET having substantially the same
configuration as the MOSFET 100 according to Embodiment 1 except
for a point that a portion of the MOSFET 100A which is brought into
contact with the source electrode is dug to a depth position of a
lowermost portion of the source region (see FIG. 18B).
[0207] In the MOSFET 700 according to Comparison example 2, when
the MOSFET is turned off, a state arises where a distance between
the equal potential lines on a bottom portion of a trench is narrow
(see FIG. 19A). This is because a distance between a non-depleted
region in an n-type column region 714 and a gate electrode 726 is
short. Accordingly, a potential gradient in the vicinity of the
bottom portion of the trench is increased and hence, the gate
electrode 726 is liable to be affected by a change in potential of
the n-type column region 714. Accordingly, when an irregularity
exists in a charge balance around the gate, it is difficult to
decrease an irregularity in switching characteristics when the
MOSFET 700 is turned off.
[0208] On the other hand, in the MOSFET 100A according to the
present invention example, when the MOSFET is turned off, a state
arises where a distance between the equal potential lines in the
vicinity of a bottom portion of a trench 122 is wide (see FIG.
19B). This is because a distance between a non-depleted region in
the n-type column region 114 and the gate electrode 126 is long.
Accordingly, a potential gradient in the vicinity of the bottom
portion of the trench 122 is decreased and hence, the gate
electrode 126 is minimally affected by a change in potential of the
n-type column region 114. Accordingly, even when an irregularity
exists in a charge balance around the gate, an irregularity in
switching characteristics when the MOSFET 100A is turned off can be
made small.
6. Advantageous Effects Acquired by MOSFET 100 and Power Conversion
Circuit 1 According to Embodiment 1
[0209] According to the MOSFET 100 and the power conversion circuit
1 of Embodiment 1, the relationship of |X.sub.0-X.sub.0'
|<|X.sub.m-X.sub.m'| is satisfied and hence, even when an
irregularity occurs in a charge balance so that the MOSFET 100
becomes n-type dopant rich, deviation of average positive charge
density .rho.(x) on a first main surface side (gate electrode side)
is small so that a region where an electric field E(x) is high
minimally approaches the gate electrode. Accordingly, an electric
field E(x) of the n-type column region 114 around the gate
electrode becomes minimally high so that a change in potential of
the n-type column region around the gate becomes small easily. As a
result, an irregularity minimally occurs in switching
characteristics when the MOSFET is turned off.
[0210] According to the MOSFET 100 and the power conversion circuit
1 of Embodiment 1, the relationship of
|X.sub.0-X.sub.0'|<|X.sub.m-X.sub.m'| is satisfied and hence,
even when an irregularity occurs in a charge balance, an
irregularity in a switching loss when the MOSFET is turned off
minimally becomes large. Accordingly, it is possible to prevent the
occurrence of a defect in an operation of the power conversion
circuit (particularly, a circuit largely affected by a switching
loss at the time of turning off the MOSFET such as a current
critical type PFC or a current discontinuous type PFC).
[0211] Further, the MOSFET 100 according to Embodiment 1 includes
the semiconductor base substrate 110 where the super junction
structure 117 is formed of the n-type column regions 114 and the
p-type column regions 116. Accordingly, in the same manner as the
conventional MOSFET 900, the MOSFET 100 becomes a switching element
having a low ON resistance and a high withstand voltage.
[0212] In the MOSFET 100 according to Embodiment 1, the value of
the average positive charge density .rho.(0) when x=0 takes a
negative value, and the value of the average positive charge
density .rho.(a) when x=a takes a positive value. Accordingly, at a
depth position in the vicinity of a bottom portion of the p-type
column region 116, a total amount of dopant in the p-type column
region 116 becomes smaller than a total amount of dopant in the
n-type column region 114 (becoming n-type dopant rich). With such a
configuration, a depletion layer generated from the p-type column
region 116 when the MOSFET is turned off minimally extends toward a
second main surface side. Accordingly, it is possible to provide a
MOSFET where a breakdown in a reach through mode minimally occurs
so that a withstand voltage is minimally lowered.
[0213] According to the MOSFET 100 and the power conversion circuit
1 of Embodiment 1, the average positive charge density .rho.(x) is
expressed by an upward convex curve projecting in a right upward
direction. With such a configuration, in a depth position around
the gate (a region where x is close to 0), the average positive
charge density .rho.(x) becomes smaller (larger on a negative side)
than a conventional MOSFET 900, and the difference between a charge
amount of a negative charge in the p-type column region 116 and a
charge amount of a positive charge in the n-type column region 114
becomes large compared to the conventional MOSFET 900. Accordingly,
the following advantages can be acquired.
[0214] (1) The n-type column region 114 around the gate becomes
easily depleted compared to the conventional MOSFET 900 and hence,
even when a drain voltage is increased, a potential of the n-type
column region 114 around the gate is minimally increased.
[0215] (2) A distance between a non-depleted region in the n-type
column region 114 and the gate electrode 126 becomes long compared
to the conventional MOSFET 900 and hence, a feedback capacitance
Crss (equal to a gate-drain capacitance Cgd) becomes small compared
to the conventional MOSFET 900. Accordingly, even when a potential
of the n-type column region 114 (the non-depleted region of the
n-type column region 114) is increased along with the increase of a
drain voltage which takes place when the MOSFET is turned off, the
gate electrode 126 is minimally affected by a change in potential
of the n-type column region 114. As a result, even when an
irregularity exists in a charge balance around the gate, an
irregularity in switching characteristics when the MOSFET is turned
off can be made small compared to the prior art.
[0216] Further, according to the MOSFET 100 of Embodiment 1, the
average positive charge density .rho.(x) is expressed as an upward
convex curve projecting in a right upward direction. With such a
configuration, at a depth position around the gate, average
positive charge density .rho.(x) becomes small (large on a negative
side) and hence, a charge amount of a negative charge in the p-type
column region 116 becomes larger than a charge amount of a positive
charge in the n-type column region. Accordingly, holes around the
gate can be easily extracted by a negative charge in the p-type
column region 116. As a result, an L-load avalanche breakdown
resistance can be increased.
[0217] According to the MOSFET 100 of Embodiment 1, a width Wp(x)
of the p-type column region is expressed by the following formula
(2), and
[ Formula 2 ] Wp ( x ) = - [ - A exp ( - x .lamda. ) + B ] + W ( 2
) ##EQU00017##
(symbols A, B, .lamda. being constants) a width Wn(x) of the n-type
column region is expressed by the following formula (3).
[ Formula 3 ] Wn ( x ) = [ - A exp ( - x .lamda. ) + B ] + W ( 3 )
##EQU00018##
(symbols A, B, .lamda. being constants)
[0218] With such a configuration, at the depth position around the
gate, the average positive charge density .rho.(x) becomes small
(large on a negative side) and hence, a charge amount of a negative
charge in the p-type column region 116 becomes larger than a charge
amount of a positive charge in the n-type column region.
Accordingly, holes around the gate can be easily extracted by a
negative charge in the p-type column region 116. As a result, an
L-load avalanche breakdown resistance can be increased.
[0219] In the MOSFET 100 according to Embodiment 1, since
.rho..sub.0(X.sub.0)>.rho.(X.sub.0') is satisfied, when the
MOSFET 100 is brought into 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, a first main surface side (gate
electrode side) becomes p-type dopant rich more easily.
Accordingly, even when an irregularity exists in a charge balance
around the gate, an irregularity in switching characteristics when
the MOSFET 100 is turned off can be made further small.
[0220] In the MOSFET 100 according to Embodiment 1, a relationship
of |X.sub.m-X.sub.m'|<|X.sub.1-X.sub.1'| is satisfied and hence,
a region where an electric field E(X) is high minimally reaches the
gate electrode. Accordingly, an electric field E(X) in the n-type
column region 114 around the gate electrode minimally becomes high
and hence, a change in potential of the n-type column region around
the gate becomes small easily. As a result, an irregularity
minimally occurs in switching characteristics when the MOSFET 100
is turned off.
[0221] In the MOSFET 100 according to Embodiment 1, when the MOSFET
100 becomes p-type dopant rich, a curve which expresses the depth
X.sub.1 is converged to a predetermined value when a difference
between a total amount of dopant in the n-type column region and a
total amount of dopant in the p-type column region is increased and
hence, extension of a depletion layer into a region deeper than the
bottom of the p-type column region becomes difficult. Accordingly,
it is possible to prevent the buffer layer 113 disposed in the
region deeper than the p-type column region 116 from being
completely depleted. As a result, even when the MOSFET 100 becomes
p-type dopant rich, it is possible to maintain an L-load avalanche
breakdown resistance. It is considered that the reason the
depletion layer minimally extends into the region deeper than the
bottom of the p-type column region is that only the n-type region
(buffer layer 113 and low-resistance semiconductor layer 112)
exists.
[0222] In general, in designing and manufacturing a MOSFET having a
super junction structure, the configuration where a withstand
voltage becomes highest when a charge balance is changed forms a
charge balance point, and such a charge balance point is regarded
as the configuration where a total amount of dopant in a p-type
column region and a total amount of dopant in an n-type column
region become equal to each other. In this case, the total amounts
of dopants become equal in the depleted regions in a p-type column
region and an n-type column region respectively.
[0223] Also in the MOSFET 100 according to Embodiment 1, the
configuration where a withstand voltage becomes highest is found by
changing a charge balance, and the configuration is regarded as a
charge balance point. Further, in the MOSFET 100 according to
Embodiment 1, when the MOSFET 100 becomes p-type dopant rich, a
curve which expresses the depth X.sub.1 is converged to a
predetermined value when a difference between a total amount of
dopant in an n-type column region and a total amount of dopant in a
p-type column region is increased. As can be also understood from
the above, in the MOSFET 100 according to Embodiment 1, the
configuration which is regarded as a charge balance point (the
configuration where a withstand voltage becomes highest) is
originally formed at a position where a depletion layer is disposed
at a slightly deeper position.
[0224] Accordingly, in a manufacturing process of the MOSFET, in a
state where a charge balance is deviated to n-type dopant rich,
even when an upper end X.sub.0 of the depletion layer moves in a
shallower direction, a distance from the gate electrode to the
upper end of the depletion layer is sufficiently long and hence,
the charge balance minimally affects the distribution of an
electric field around a gate whereby it is possible to maintain the
distribution of an electric field around the gate in a stable
state.
[0225] In a process control for mass production, a control of a
predetermined characteristic is performed such that an irregularity
in the predetermined characteristic is measured by measuring the
transition of the predetermined characteristic between wafers which
are flow products or the transition of the predetermined
characteristic between lots which are flow products, and the
irregularity is returned to Just when an irregularity width becomes
large. In the process control, an irregularity in a charge balance
is detected as lowering of a withstand voltage. When a withstand
voltage exhibits lowering tendency because of a gradual increase of
an irregularity width toward n-type dopant rich in a charge balance
as the transition of a predetermined characteristic between a
plurality of lots, this tendency is fed back to a dopant doping
step for returning the charge balance to Just.
[0226] With respect to several lots which indicate tendency where
an irregularity width is gradually increased toward n-type dopant
rich, to observe the inside of the device, a range where a
depletion layer is formed is moved to a shallower side. However, as
described above, the distribution of an electric field around the
gate is held stable and hence, switching characteristics is not
strongly affected by an irregularity in a charge balance. That is,
in the MOSFET 100 according to Embodiment 1, in a usual mass
production process control, by controlling a charge balance by
monitoring a withstand voltage, there is no possibility that an
irregularity in switching characteristics is increased within a
range of a width of such a control. That is, the MOSFET 100
according to Embodiment 1 has the structure where a dynamic
characteristic (switching characteristic) is automatically
controlled by controlling a static characteristic.
[0227] The MOSFET 100 according to Embodiment 1 is a
trench-gate-type MOSFET. With such a configuration, also in the
trench-gate-type MOSFET where the gate electrode and the drain
electrode are arranged close to each other so that a potential of
the n-type column region 114 around the gate is liable to be
increased compared to a planar-gate-type MOSFET, an irregularity in
switching characteristics when the MOSFET is turned off can be made
small compared to the prior art.
[0228] In the MOSFET 100 according to Embodiment 1, assuming a
shallowest depth position of a surface of the depletion layer on
the second main surface side when a depletion layer extends most in
the super junction structure 117 by turning off the MOSFET as a,
and a depth of a depth position where the average positive charge
density .rho.(x) becomes 0 in the super junction structure 117 as
d, a relationship of 0<d<a/2 is satisfied (see FIGS. 7A to
7D). Accordingly, the difference between a charge amount of a
negative charge in the p-type column region 116 and a charge amount
of a positive charge in the n-type column region 114 becomes large
at a depth position around the gate and hence, the n-type column
region 114 around the gate becomes easily depleted. Accordingly,
even when an irregularity exists in a charge balance around the
gate, an irregularity in switching characteristics when the MOSFET
is turned off can be made further more small.
[0229] According to the MOSFET 100 of Embodiment 1, assuming an
axis along a depth direction as an x axis, an x coordinate at a
depth position of a lowermost surface of the base region 11 as -t,
an x coordinate at a depth position of a lowermost portion of the
p-type column region 116 as b, and an x coordinate at a depth
position where the average positive charge density .rho.(x) becomes
0 as d, a relationship of 0<t+d<(t+b)/2 is satisfied (see
FIGS. 7A to 7D). Accordingly, with such a configuration, the
difference between a charge amount of a negative charge in the
p-type column region 116 and a charge amount of a positive charge
in the n-type column region 114 becomes large at a depth position
around the gate and hence, the n-type column region 114 around the
gate becomes easily depleted. As a result, even when an
irregularity exists in a charge balance around the gate, an
irregularity in switching characteristics when the MOSFET is turned
off can be made further more small.
[0230] In the power conversion circuit 1 according to Embodiment 1,
the rectifier element is a fast recovery diode and hence, a
turn-off period is short whereby, when the MOSFET is turned off, a
potential of the n-type column region 114 is minimally increased
along with the increase of a drain voltage. Accordingly, a
potential of the gate electrode is also minimally increased. As a
result, even when an irregularity exists in a charge balance around
the gate, an irregularity in switching characteristics when the
MOSFET is turned off can be made small compared to the prior
art.
Modification
[0231] A MOSFET according to Modification (not shown in the
drawing) basically has substantially the same configuration as
MOSFET 100 according to Embodiment 1. However, the MOSFET according
to Modification differs from MOSFET 100 according to Embodiment 1
with respect to a point that a case is considered where an
irregularity occurs not in a dopant concentration in the n-type
column region but occurs in a width W.sub.p(x) at a predetermined
depth position of a p-type column region and a width W.sub.n(x) at
a predetermined depth position of an n-type column region. That is,
in the MOSFET according to Modification, a width W.sub.p(x) at the
predetermined depth position of the p-type column region is
narrowed by .delta. as expressed by a following formula (15), and
the width W.sub.n(x) at the predetermined depth position of the
n-type column region is narrowed by .delta. as expressed by a
following formula (16).
[ Formula 15 ] Wp ( x ) = - B [ - exp ( - x - .lamda.1 n A B
.lamda. ) + 1 ] + W - .delta. ( 15 ) [ Formula 16 ] Wn ( x ) = - B
[ - exp ( - x - .lamda. 1 n A B .lamda. ) + 1 ] + W + .delta. ( 16
) ##EQU00019##
In this case, average positive charge density .rho.(x) is expressed
by a following formula (17).
[ Formula 17 ] .rho. ( x ) = q N 0 W ( B + .delta. ) [ - exp ( - x
+ .lamda. .delta. B .lamda. ) + 1 ] ( 17 ) ##EQU00020##
[0232] In this case, average positive charge density .rho.(x) is 0
(.rho.(x)=0) at a maximum electric field intensity point X.sub.m
and hence, the maximum electric field intensity point X.sub.m
appears when x=.lamda..delta.W/B. To compare the formula (10) with
the formula (17), the maximum electric field intensity point
X.sub.m is moved toward a source electrode side by an amount of
x=.lamda..delta.W/B when an irregularity occurs in a charge
balance. To substitute numerical values in such a manner that W=5
.mu.m, .lamda.=10 .mu.m, N.sub.0=3.times.10.sup.15 cm.sup.-3,
B=0.868 .mu.m in the formula, a moving amount .DELTA.X.sub.m of the
maximum electric field intensity point X.sub.m becomes 1.44 .mu.m
(.DELTA.X.sub.m=1.44 .mu.m).
[0233] 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 is a
point where .rho.(x) becomes minimum. Accordingly, to compare the
formula (10) with the formula (17), such a position is translated
toward a source electrode side by (B+.delta.) due to an
irregularity in a charge balance. To substitute the above-mentioned
numerical values such as W=5 .mu.m in the formula, a moving amount
.DELTA.X.sub.0 of the 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 becomes 0.44 .mu.m (.DELTA.X.sub.0=0.44 .mu.m).
[0234] Accordingly, in this case also, a relationship of
.DELTA.X.sub.0<.DELTA.X.sub.m is obtained.
[0235] In this manner, the MOSFET and the power conversion circuit
according to Modification differ from the MOSFET 100 according to
Embodiment 1 with respect to the point that the case is considered
where an irregularity occurs not in the dopant concentration in the
n-type column region but occurs in a width W.sub.p(X) at a
predetermined depth position of the p-type column region and in a
width W.sub.n(x) at a predetermined depth position of the n-type
column region. However, in the same manner as the MOSFET 100
according to Embodiment 1, a relationship of
|X.sub.0-X.sub.0'|.ltoreq.|X.sub.m-X.sub.m'| is satisfied and
hence, even when an irregularity occurs in a charge balance so that
the MOSFET becomes n-type dopant rich, deviation of average
positive charge density .rho.(x) on a first main surface side (gate
electrode side) is small whereby a region where an electric field
E(x) is high minimally approaches the gate electrode. Accordingly,
an electric field E(x) of the n-type column region 114 around the
gate electrode becomes minimally high so that a change in potential
of the n-type column region around the gate becomes small easily.
As a result, an irregularity minimally occurs in switching
characteristics when the MOSFET is turned off.
Embodiment 2
[0236] A MOSFET 102 according to Embodiment 2 basically has
substantially the same configuration as MOSFET 100 according to
Embodiment 1. However, MOSFET 102 according to Embodiment 2 differs
from MOSFET 100 according to Embodiment 1 with respect to a point
that average density N.sub.d(x) of a positive charge at a
predetermined depth position of an n-type column region and average
density N.sub.a(x) of a negative charge at a predetermined depth
position of a p-type column region when a super junction structure
is depleted by turning off the MOSFET are changed in place of
changing of a width W.sub.n(x) at a predetermined depth position of
the n-type column region and a width W.sub.p(x) at a predetermined
depth position of the p-type column region. That is, in the MOSFET
102 according to Embodiment 2, in a graph where a depth x is taken
on an axis of abscissas, average density N.sub.d(x) of a positive
charge at the predetermined depth position of the n-type column
region 114 and average density N.sub.a(x) of a negative charge at
the predetermined depth position of the p-type column region 116
are taken on an axis of ordinates, average density N.sub.d(x) of a
positive charge is expressed by a monotonous upward convex curve
projecting in a right upward direction, and average density
N.sub.a(x) of a negative charge is expressed by a monotonous
downward convex curve projecting in a right downward direction (see
FIG. 20C). The width of the n-type column region 114 and the width
of the p-type column region 116 are set to a fixed value regardless
of depth (see FIG. 20A and FIG. 20B).
[0237] 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 density N.sub.d(x) of a
positive charge at the predetermined depth position of the n-type
column region 114 or average density N.sub.a(x) of a negative
charge at the predetermined depth position of the p-type column
region 116 is taken on an axis of ordinates,
[0238] average density N.sub.d(x) of a positive charge at the
predetermined depth position of the n-type column region 114 is
expressed by the following formula (4), and
[ Formula 4 ] Na ( x ) = - [ - A exp ( - x .lamda. ) + B ] + N ( 4
) ##EQU00021##
(symbols A, B, .lamda. being constants)
[0239] average density N.sub.a(x) of a negative charge at a
predetermined depth position of the p-type column region is
expressed by the following formula (5).
[ Formula 5 ] N d ( x ) = [ - A exp ( - x .lamda. ) + B ] + N ( 5 )
##EQU00022##
(symbols A, B, .lamda. being constants)
[0240] In this manner, the MOSFET 102 according to Embodiment 2
differs from the MOSFET 100 according to Embodiment 1 with respect
to the point that the average density N.sub.d(x) of a positive
charge at the predetermined depth position of the n-type column
region and the average density N.sub.a(x) 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
are changed in place of changing of the width W.sub.p(x) at a
predetermined depth position of the p-type column region and the
width W.sub.n(x) at the predetermined depth position of the n-type
column region. However, in the same manner as the MOSFET 100
according to Embodiment 1, the relationship of
|X.sub.0-X.sub.0'|.ltoreq.|X.sub.m-X.sub.m'| is satisfied and
hence, even when an irregularity occurs in a charge balance so that
the MOSFET becomes n-type dopant rich, deviation of average
positive charge density .rho.(x) on a first main surface side (gate
electrode side) is small so that a region where an electric field
E(x) is high minimally approaches the gate electrode. Accordingly,
an electric field E(x) of the n-type column region 114 around the
gate electrode becomes minimally high so that a change in potential
of the n-type column region around the gate becomes small easily.
As a result, an irregularity minimally occurs in switching
characteristics when the MOSFET is turned off.
[0241] The MOSFET 102 according to Embodiment 2 has substantially
the same configuration as the MOSFET 100 according to Embodiment 1
with respect to points other than the point where the MOSFET 102
according to Embodiment 2 is characterized in that the average
density N.sub.d(x) of a positive charge at the predetermined depth
position of the n-type column region and the average density
N.sub.a(x) 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 are changed in place of changing
of the width W.sub.n(x) at a predetermined depth position of the
n-type column region and the width W.sub.p(x) at a predetermined
depth position of the p-type column region. Accordingly, the MOSFET
102 according to Embodiment 2 acquires advantageous effects
corresponding to the advantageous effects which the MOSFET 100
according to Embodiment 1 acquires among all advantageous effects
which the MOSFET 100 according to Embodiment 1 possesses.
Embodiment 3
[0242] MOSFET 200 according to Embodiment 3 basically has
substantially the same configuration as MOSFET 100 according to
Embodiment 1. However, MOSFET 200 according to Embodiment 3 differs
from MOSFET 100 according to Embodiment 1 with respect to a point
that the MOSFET 200 according to Embodiment 3 is not a
trench-gate-type MOSFET but is a planar-gate-type MOSFET. That is,
in the MOSFET 200 according to Embodiment 3, as shown in FIG. 21, a
semiconductor base substrate 210 has: a base region 218 which is
formed on a surface of a first main surface of the semiconductor
base substrate 210, a whole surface of a p-type column region 216
and on a portion of a surface of an n-type column region 214; an
n-type surface high concentration region 219 formed on a surface of
a first main surface of the semiconductor base substrate 210 and on
a surface of the n-type column region 214 in a state where the
n-type surface high concentration region 219 is disposed adjacently
to a base region 218; and an n-type source region 220 formed on a
surface of the base region 218. A gate electrode 236 is formed on
the surface of the base region 218 sandwiched between source
regions 220 and the n-type surface high concentration regions 219
by way of gate insulation films 234. A depth position of the n-type
surface high concentration region 219 falls within a range of 1.0
.mu.m to 4.0 .mu.m, and dopant concentration in the n-type surface
high concentration region 219 falls within a range of
1.times.10.sup.14 cm.sup.-3 to 1.times.10.sup.16 cm.sup.-3.
[0243] As described above, the MOSFET 200 according to Embodiment 3
differs from the MOSFET 100 according to Embodiment 1 with respect
to the point where the MOSFET 200 according to Embodiment 3 is not
a trench-gate-type MOSFET but is a planar-gate-type MOSFET.
However, in the same manner as the MOSFET 100 according to
Embodiment 1, a relationship of
|X.sub.0-X.sub.0'|.ltoreq.|X.sub.m-X.sub.m'| is satisfied and
hence, even when an irregularity occurs in a charge balance so that
the MOSFET becomes n-type dopant rich, deviation of average
positive charge density .rho.(x) on a first main surface side (gate
electrode side) is small so that a region where an electric field
E(x) is high minimally approaches the gate electrode. Accordingly,
an electric field E(x) of the n-type column region 114 around the
gate electrode becomes minimally high so that a change in potential
of the n-type column region around the gate becomes small easily.
As a result, an irregularity minimally occurs in switching
characteristics when the MOSFET is turned off.
[0244] The MOSFET 200 according to Embodiment 3 has substantially
the same configuration as the MOSFET 100 according to Embodiment 1
with respect to points other than the point where the MOSFET 200
according to Embodiment 3 is not a trench-gate-type MOSFET but is a
planar-gate-type MOSFET and hence, the MOSFET 200 according to
Embodiment 3 acquires advantageous effects corresponding to the
advantageous effects which the MOSFET 100 according to Embodiment 1
acquires among all advantageous effects which the MOSFET 100
according to Embodiment 1 possesses.
Embodiment 4
[0245] A power conversion circuit 2 according to Embodiment 4
basically has the same configuration as the configuration of the
power conversion circuit 1 according to Embodiment 1. However, the
power conversion circuit 2 according to Embodiment 4 differs from
the power conversion circuit 1 according to Embodiment 1 with
respect to a point where the power conversion circuit is a full
bridge circuit. That is, as shown in FIG. 22, the power conversion
circuit 2 according to Embodiment 4 includes four MOSFETs 100 (100a
to 100d) as the MOSFET, and also includes built-in diodes of the
respective MOSFETs as rectifier elements.
[0246] As described above, the power conversion circuit 2 according
to Embodiment 4 differs from the power conversion circuit 1
according to Embodiment 1 with respect to the point where the power
conversion circuit is a full bridge circuit. However, in the same
manner as the power conversion circuit 1 according to Embodiment 1,
a relationship of |X.sub.0-X.sub.0'|.ltoreq.|X.sub.m-X.sub.m'| is
satisfied and hence, even when an irregularity occurs in a charge
balance so that the MOSFET becomes n-type dopant rich, deviation of
average positive charge density .rho.(x) on a first main surface
side (gate electrode side) is small whereby a region where an
electric field E(x) is high minimally approaches a gate electrode.
Accordingly, an electric field E(x) of an n-type column region 114
around the gate electrode becomes minimally high so that a change
in potential of the n-type column region around the gate becomes
small easily. As a result, an irregularity minimally occurs in
switching characteristics when the MOSFET is turned off.
[0247] In the power conversion circuit 2 according to Embodiment 4,
the average positive charge density .rho.(x) is expressed by an
upward convex curve projecting in a right upward direction.
Accordingly, as described above, even when a potential of the
n-type column region 114 (non-depleted region of the n-type column
region 114) is increased along with the increase of a drain voltage
which takes place when the MOSFET is turned off, the gate electrode
126 is minimally affected by a change in potential of the n-type
column region 114. Accordingly, a phenomenon referred to as "false
turn-on (erroneous ON)" minimally occurs.
[0248] The phenomenon referred to as "false turn-on" is a
phenomenon where, in a circuit where two or more MOSFETs are
connected to each other, when either one of the MOSFETs is turned
on, the other MOSFETs are also erroneously turned on due to a
change in potential.
[0249] Further, according to the power conversion circuit 2 of
Embodiment 4, the rectifier element is the built-in diode of the
MOSFET and hence, it is unnecessary to prepare a rectifier element
as an additional part.
[0250] The power conversion circuit 2 according to Embodiment 4 has
substantially the same configuration as the power conversion
circuit 1 according to Embodiment 1 with respect to points other
than the point where the power conversion circuit is a full bridge
circuit. Accordingly, the power conversion circuit 2 according to
Embodiment 4 acquires advantageous effects corresponding to the
advantageous effects which the power conversion circuit 1 according
to Embodiment 1 acquires among all advantageous effects which the
power conversion circuit 1 according to Embodiment 1 possesses.
[0251] Although the present invention has been described based on
the above-mentioned embodiments, the present invention is not
limited to the above-mentioned embodiments. The present invention
can be carried out in various modes without departing from the gist
of the present invention, and, for example, the following
modifications are also conceivable.
[0252] (1) The numbers, materials, shapes, positions, sizes and the
like of the constitutional elements described in the
above-mentioned respective Embodiments are provided only for an
exemplifying purpose, and can be changed within a scope where
advantageous effects of the present invention are not impaired.
[0253] (2) In the above-mentioned Embodiments 1 and 2, a width
W.sub.n(x) at a predetermined depth position of the n-type column
region and a width W.sub.p(x) at a predetermined depth position of
the p-type column region or the average density N.sub.d(x) of a
positive charge at a predetermined depth position of the n-type
column region and the average density N.sub.a(x) of a negative
charge at a predetermined depth position of the p-type column
region are expressed in the form of an exponential. However, the
present invention is not limited to such a configuration. Provided
that the relationship of
|X.sub.0-X.sub.0'|.ltoreq.|X.sub.m-X.sub.m'| is satisfied, the
width W.sub.n(x) and the width W.sub.p(x) or the average density
N.sub.d(x) and the average density N.sub.a(x) may be expressed in
other forms besides an exponential.
[0254] (3) In the above-mentioned embodiment 1, a width W.sub.n(x)
at the predetermined depth position of the n-type column region is
expressed by a monotonous upward convex curve projecting in a right
upward direction, and a width W.sub.p(x) at a predetermined depth
position of the p-type column region is expressed by a monotonous
downward convex curve projecting in a right downward direction.
However, the present invention is not limited to such a
configuration. For example, the width W.sub.n(x) at a predetermined
depth position of the n-type column region may be expressed by a
stepped shape (a line (envelope) which connects corner portions of
steps forming a monotonous upward convex curve projecting in a
right upward direction), and the width W.sub.p at a predetermined
depth position of the p-type column region may be expressed by a
stepped shape (a line (envelope) which connects corner portions of
steps forming a monotonous downward convex curve projecting in a
right downward direction) (see FIG. 23). Further, the width
W.sub.n(x) at a predetermined depth position of the n-type column
region may be expressed by a curve where a concave shape and a
convex shape are repeated (an envelope made by the concaves and the
convexes forming a monotonous upward convex curve projecting in a
right upward direction), and the width W.sub.p at a predetermined
depth position of the p-type column region may be expressed by a
curve where a concave shape and a convex shape are repeated (an
envelope made by the concaves and the convexes forming a monotonous
downward convex curve projecting in a right downward direction)
(see FIG. 24).
[0255] (4) In the above-mentioned Embodiment 2, average density
N.sub.d(x) of a positive charge at a predetermined depth position
of the n-type column region is expressed by a monotonous upward
convex curve projecting in a right upward direction. However, the
present invention is not limited to such a case. For example,
average density N.sub.d(x) of a positive charge at a predetermined
depth position of the n-type column region may be expressed by a
stepped shape (a line which connects corner portions of steps
forming a monotonous upward convex curve projecting in a right
upward direction) or may be expressed by a curve where a concave
shape and a convex shape are repeated (an envelope made by the
concaves and the convexes forming a monotonous upward convex curve
projecting in a right upward direction).
[0256] (5) In the above-mentioned Embodiment 2, average density
N.sub.a(x) of a negative charge at a predetermined depth position
of the p-type column region is expressed by a monotonous downward
convex curve projecting in a right downward direction. However, the
present invention is not limited to such a case. For example,
average density N.sub.a(x) of a negative charge at a predetermined
depth position of the p-type column region may be expressed by a
stepped shape (a line which connects corner portions of steps
forming a monotonous downward convex curve projecting in a right
downward direction) or may be expressed by a curve where a concave
shape and a convex shape are repeated (an envelope made by the
concaves and the convexes forming a monotonous downward convex
curve projecting in a right downward direction).
[0257] (6) In the above-mentioned respective embodiments, the
n-type column regions 114, the p-type column regions 116, the
trenches 122, and the gate electrodes 126 are formed in a stripe
shape as viewed in a plan view. However, the present invention is
not limited to such a configuration. The n-type column regions 114,
the p-type column regions 116, the trenches 122, and the gate
electrodes 126 may be formed in a circular shape (in a columnar
shape as viewed stereoscopically), a quadrangular frame shape, a
circular frame shape, a circular grid shape or the like as viewed
in a plan view.
[0258] (7) In the above-mentioned respective embodiments, a DC
power source is used as the power source. However, the present
invention is not limited to such a configuration. An AC power
source may be also used as the power source.
[0259] (8) A chopper circuit is used as the power conversion
circuit in the above-mentioned respective Embodiments 1 to 3, and a
full bridge circuit is used as the power conversion circuit in the
above-mentioned Embodiment 4. However, the present invention is not
limited to such configurations. As the power conversion circuit, a
half bridge circuit, a three-phase AC converter, a
non-insulation-type full bridge circuit, a non-insulation-type half
bridge circuit, a push-pull circuit, an RCC circuit, a forward
converter, a fly-back converter, a PFC circuit (a current critical
type PFC circuit, a current discontinuous type PFC circuit, a
current continuous type PFC or other PFC circuits) and other
circuits can be used.
[0260] (9) A PIN diode is used as the rectifier element in the
above-mentioned Embodiments 1 to 3, and a built-in diode of the
MOSFET is used as the rectifier element in the above-mentioned
Embodiment 4. However, the present invention is not limited to such
configurations. As the rectifier element, a JBS, a MPS, or other
fast recovery diodes, a silicon-carbide Schottky barrier diode or
other diodes may be used.
[0261] (10) In the above-mentioned Embodiment 4, only a built-in
diode of the MOSFET is used as the rectifier element. However, the
present invention is not limited to such a configuration. When a
recovery loss of the built-in diode is excessively large, a
rectifier element provided as an additional part may be connected
to the MOSFET in parallel.
* * * * *