U.S. patent application number 09/776577 was filed with the patent office on 2001-09-27 for semiconductor device with reverse conducting faculty.
This patent application is currently assigned to NGK Insulators, Ltd.. Invention is credited to Iida, Katsuji, Imanishi, Yuichiro, Sakuma, Takeshi, Shimizu, Naohiro.
Application Number | 20010023963 09/776577 |
Document ID | / |
Family ID | 18555502 |
Filed Date | 2001-09-27 |
United States Patent
Application |
20010023963 |
Kind Code |
A1 |
Iida, Katsuji ; et
al. |
September 27, 2001 |
Semiconductor device with reverse conducting faculty
Abstract
A semiconductor device constructed as a reverse conducting
static induction thyristor including a thyristor section 114 formed
by an n.sup.- silicon substrate 101, p.sup.+ gate regions 102, 104
formed in one surface of the substrate, a p.sup.+ anode region 111
formed in the other surface of the substrate, a main diode section
134 having a cathode region formed by the silicon substrate and an
anode region 131 formed in the one surface of the substrate, and a
series arrangement 145 of diodes including plural p.sup.+ anode
regions 142, plural n.sup.+ cathode contact regions 143 formed in
the first surface of the substrate, and plural conductive layers
144 connecting these anode regions and cathode contact legions
successively. An anode and a cathode of the series arrangement of
diodes are connected to a cathode electrode 110 and an anode
electrode 113 of the thyristor section. Each of diodes in the
series arrangement has a breakdown voltage lower than that of the
thyristor section.
Inventors: |
Iida, Katsuji; (Nagoya City,
JP) ; Sakuma, Takeshi; (Nagoya City, JP) ;
Imanishi, Yuichiro; (Nagoya City, JP) ; Shimizu,
Naohiro; (Miura City, JP) |
Correspondence
Address: |
Stephen P. Burr
BURR & BROWN
P.O. BOX 7068
Syracuse
NY
13261-7068
US
|
Assignee: |
NGK Insulators, Ltd.
|
Family ID: |
18555502 |
Appl. No.: |
09/776577 |
Filed: |
February 2, 2001 |
Current U.S.
Class: |
257/355 ;
257/E27.052 |
Current CPC
Class: |
H01L 27/0817
20130101 |
Class at
Publication: |
257/355 |
International
Class: |
H01L 023/62 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 8, 2000 |
JP |
2000-30,502 |
Claims
What is claimed is:
1. A semiconductor device having reverse conducting faculty
comprising: a switching element including a semiconductor substrate
of a first conductivity type having first and second major
surfaces, a first main electrode region of the first conductivity
type formed in the first major surface of the semiconductor
substrate, a first main electrode connected to said first main
electrode region, a second main electrode region of a second
conductivity type formed in the second major surface of the
semiconductor substrate, a second main electrode connected to said
second main electrode region, a control electrode region of the
second conductivity type formed in the first major surface of the
semiconductor substrate for controlling a current passing between
the first and second main electrode regions, and a control
electrode connected to said control region; and a series
arrangement of a plurality of diodes connected between said first
main electrode and said second main electrode in an opposite
polarity to a current flowing between said first main electrode
region and said second main electrode region, each of said
plurality of diodes having a breakdown voltage lower than a
breakdown voltage of said switching element.
2. The semiconductor device according to claim 1, wherein said
series arrangement of a plurality of diodes is formed in said first
major surface of the semiconductor substrate in which said first
main electrode region is also formed.
3. The semiconductor device according to claim 1, wherein said
series arrangement of a plurality of diodes is formed on a separate
semiconductor substrate from said semiconductor substrate
semiconductor substrate which constitutes said switching
element.
4. The semiconductor device according to claim 1, wherein said
series arrangement of a plurality of diodes is formed as a diode
stack including first and second electrodes connected to said first
and second main electrodes of the switching element,
respectively.
5. The semiconductor device according to claim 4, wherein said
diode stack is beveled such that surface areas of successive diodes
in the diode stack are gradually decreased viewed in a direction in
which a current flows through the diode stack.
6. The semiconductor device according to claim 4, wherein each of a
plurality of diodes in the diode stack has p.sup.+-i-n.sup.+
structure.
7. The semiconductor device according to any one of claims 3-6,
wherein said switching device and series arrangement of a plurality
of diodes are installed in a common package.
8. The semiconductor device according to any one of claims 3-6,
wherein said switching element and said series arrangement of a
plurality of diodes are installed in separate packages.
9. The semiconductor device according to claim 1, wherein said
switching element is formed as a static induction thyristor whose
cathode region and cathode electrode are formed by said first main
electrode region and first main electrode, respectively, whose
anode region and anode electrode are formed by said second main
electrode region and second main electrode, respectively, and whose
gate region and gate electrode are formed by said control region
and control electrode, respectively.
10. The semiconductor device according to claim 9, wherein said
semiconductor substrate is formed by an n.sup.- semiconductor
substrate, and said series arrangement of a plurality of diodes
comprises a plurality of recesses formed in the first major surface
of the n.sup.- semiconductor substrate, a plurality of p.sup.+
anode regions formed at bottom surfaces of said plurality of
recesses, a plurality of n.sup.- cathode regions formed by portions
of said first major surface of said n.sup.+ semiconductor substrate
situating between successive recesses, a plurality of conductive
layers formed on the first major surface of the n.sup.-
semiconductor substrate via insulating layers such that said
p.sup.+ anode regions and n.sup.+ cathode regions are successively
connected by said conductive layers, and a cathode conductive layer
for connecting an n.sup.+ cathode region of the outermost diode to
said anode electrode of the static induction thyristor.
11. The semiconductor device according to claim 10, wherein said
series arrangement of a plurality of diodes further comprises a
plurality of n.sup.+ cathode contact regions formed between said
n.sup.- cathode regions and said conductive layers.
12. The semiconductor device according to claim 10, wherein said
recesses, anode regions and cathode regions of the series
arrangement of a plurality of diodes are formed as ring-shape
surrounding said static induction thyristor.
13. The semiconductor device according to claim 12, wherein said
recesses and anode regions of the series arrangement of a plurality
of diodes are formed to serve as field limiting rings.
14. The semiconductor device according to claim 12, wherein said
recesses of the series arrangement of a plurality of diodes are
formed such that widths of successive recesses are increased toward
outside.
15. The semiconductor device according to claim 1, wherein said
switching element is formed as a reverse conducting static
induction thyristor including a thyristor section whose cathode
region and cathode electrode are formed by said first main
electrode region and first main electrode, respectively, whose
anode region and anode electrode are formed by said second main
electrode region and second main electrode, respectively, and whose
gate region and gate electrode are formed by said control region
and control electrode, respectively; and a main diode section
having an anode region connected to said cathode electrode of the
thyristor section and a cathode region connected to said anode
electrode of the thyristor section.
16. The semiconductor device according to claim 15, wherein said
semiconductor substrate is formed by an n.sup.- semiconductor
substrate; said main diode comprises a p.sup.+ anode region formed
in said first major surface of the n.sup.- semiconductor substrate,
said p.sup.+ anode region having a width longer than that of the
gate regions of the thyristor section; and said series arrangement
of a plurality of diodes comprises a plurality of recesses formed
in the first major surface of the n.sup.- semiconductor substrate,
a plurality of p.sup.+ anode regions formed at bottom surfaces of
said plurality of recesses, a plurality of n.sup.- cathode regions
formed by portions of said first major surface of said n.sup.-
semiconductor substrate situating between successive recesses, a
plurality of conductive layers formed on the first major surface of
the n.sup.- semiconductor substrate via insulating layers such that
said p.sup.+ anode regions and n.sup.+ cathode regions are
successively connected by said conductive layers, and a cathode
conductive layer for connecting an n.sup.+ cathode region of the
outermost diode to said anode electrode of the static induction
thyristor.
17. The semiconductor device according to claim 16, wherein an
anode region of the innermost diode of said series arrangement is
formed by said p.sup.+ anode region of said main diode.
18. The semiconductor device according to claim 16, wherein said
series arrangement of a plurality of diodes further comprises a
plurality of n.sup.+ cathode contact regions formed between said
n.sup.- cathode regions and said conductive layers.
19. The semiconductor device according to claim 16, wherein said
recesses, anode regions and cathode regions of the series
arrangement of a plurality of diodes are formed as ring-shape
surrounding said thyristor section and main diode.
20. The semiconductor device according to claim 19, wherein said
recesses and anode regions of the series arrangement of a plurality
of diodes are formed to serve as field limiting rings.
21. The semiconductor device according to claim 20, wherein said
recesses of the series arrangement of a plurality of diodes are
formed such that widths of successive recesses are increased toward
outside.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor device, and
more particularly to a semiconductor device having reverse
conducting faculty comprising a switching element including a
semiconductor substrate of a first conductivity type having first
and second major surfaces, a first main electrode region of the
first conductivity type formed on the first major surface of the
semiconductor substrate, a second major electrode region of a
second conductivity type formed on the second surface of the
semiconductor substrate, and a control electrode region of the
second conductivity type for controlling a current passing between
the first and second main electrode regions.
[0003] 2. Description of the Related Art
[0004] As a voltage supply source for a pulse laser and pulse
discharge device, there has been used a voltage supply source
generating a pulse having a high voltage and a large current. FIG.
1 shows an example of a known pulse generating circuit used as a
voltage supply source for use in a pulse laser. In this pulse
generating circuit, between output terminals 14a and 14b of a
charging circuit 14 including a DC power supply source 11, a switch
12 and a current limiting resistor 13, is connected a static
induction thyristor 15 (herein after abbreviated as SIThy). In
parallel with the SIThy 15, there are connected resonance coil 16
and capacitor 17. Furthermore, in parallel with the capacitor 17
are connected a capacitor 18 and a coil 19 having a large
inductance, and a discharge gap 20 is connected across the coil 19
as a load.
[0005] Under a non-conductive condition of the SIThy 15, at first
the switch 12 is closed to charge the capacitor 17 through the
resistor 13 and coil 16. During this charging process, an impedance
of the coil 19 at a lower frequency is low, and thus the capacitor
18 is also charged through the coil 19. Now an output voltage of
the DC power supply source 11 is denoted by E. After charging the
capacitors 17 and 18 up to E, the SIThy 15 is turned-on by means of
a gate driving circuit 21. Then, charge stored in the capacitor 17
is discharged through the SIThy 15 in accordance with a resonance
characteristic determined by the coil 16 and capacitor 17, and the
capacitor 17 is charged in a reverse polarity to a polarity in
which the capacitor 17 is charged up to substantially -E. Charge
stored in the capacitor 18 is also discharged through the SIThy 15
and coil 19. Since an impedance of the coil 19 is very high for a
high frequency, the discharge is conducted very slowly. Therefore,
a voltage of about 2E will be applied across the discharge gap 20.
When a discharge occurs, charge stored in the capacitors 17 and 18
disappears by discharge at the discharge gap 20. And the switch 12
is closed to initiate the charging operation again.
[0006] In the above mentioned pulse generating circuit, if the
discharging operation is carried out correctly between the
discharge gap 20 when a voltage of -2E is applied across the
discharge gap, charge stored in a resonance circuit consisting of
the coil 16 and capacitor 17 disappears. Therefor, as shown by a
solid line in FIG. 2, no current flows through the SIThy 15 in the
reverse direction. However, if discharge does not occur correctly
due to any reason, a ringing current occurs in the resonance
circuit and a large current flows through the SIThy 15 in the
reverse direction as illustrated by a broken line in FIG. 2.
[0007] FIG. 3 is a graph showing a voltage across the anode-cathode
path of the SIThy 15. When discharge does not occurs correctly, a
reverse voltage is applied to the SIThy 15. In this case, a reverse
current flows from the cathode to the gate of the SIThy 15, and
this results in application of an excessive high reverse voltage
like as a reverse recovery phenomenon of the diode.
[0008] In order to protect the static induction thyristor from the
breakdown when the large reverse current flows through the
anode-cathode path of the thyristor, it has been proposed to flow
the reverse current through a diode connected in anti-parallel with
the static induction thyristor. The static induction thyristor
having such a diode is generally called a reverse conducting static
induction thyristor. In the reverse conducting static induction
thyristor, in order to make a wiring inductance as small as
possible, it has been proposed to form the diode by a common
semiconductor substrate together with the static induction
thyristor in a preliminary thesis issued for 1999 Conference of the
Electric Engineering Society by Shimizu et al., "4000V Class
Reverse Conducting SI Thyristor(1)". [0007] FIG. 4 is an equivalent
circuit of the above mentioned reverse conducting static induction
thyristor. A diode 32 is connected in anti-parallel with a static
induction thyristor (SIThy) 31 such that an anode of the diode is
connected to a cathode of the SIThy and a cathode of the diode is
connected to an anode of the SIThy. The anode of the diode 32 is
further connected to a gate of the SIThy 31 by means of a resistor
33, and the gate of the SIThy is connected to a gate driving
circuit (GC) 34 which controls the turn-on/turn-off of the SIThy.
When a main power supply source 35 is connected across the
anode-cathode path of the SIThy 31 as shown by a solid line in FIG.
4, a current I.sub.T flows through the SIThy, and when a voltage
supply source 36 is connected in a reverse polarity as depicted by
a broken line in FIG. 4, a current IR flows through the diode 32 to
protect the SIThy 31 from being breakdown.
[0009] FIG. 5 is a cross sectional view showing the structure of
the above mentioned known reverse conducting static induction
thyristor. In one major surface of an n.sup.- silicon substrate 41
there is formed a p.sup.+ gate regions 43, and p.sup.+ buried gate
regions 44 are formed within a channel region. A gate electrode 45
is provided on the gate region 42 via a conductive layer 45a. The
buried gate regions 43 are formed as a comb shape to be surrounded
by the gate region 42. Above the channel region, there are formed
n.sup.+ cathode regions 46 which are electrically connected to a
cathode electrode 47 via a conductive layer 47a. On the other major
surface of the silicon substrate 41, an anode electrode 52 is
provided via a conductive layer 52a. In this manner, a thyristor
section 44 is constructed by the gate region 42, buried gate
regions 43, channel region, cathode regions 46. Furthermore, a
diode section 49 is formed to surround the thyristor section 44 via
a separation band 48. The diode second includes a p.sup.+ anode
region 50 and a cathode region 41a formed by a part of the n.sup.-
silicon substrate 41. The anode region 50 is electrically connected
to the cathode electrode 47 of the static induction thyristor via a
conductive layer 47a and the cathode region 41a is connected to an
anode electrode 52 of the static induction thyristor by means of
n.sup.+ contact region 51 and conductive layer 52a.
[0010] In the above explained reverse conducting static induction
thyristor, when a reverse voltage is applied across the
anode-cathode main current path, the diode section 49 is made
conductive to prevent the thyristor section 44 from the breakdown.
However, when the known reverse conducting thyristor is used in the
above mentioned pulse generating circuit shown in FIG. 1, the
static induction thyristor is often broken by the ringing current
generated in the resonance circuit by failure of discharge. In
order to investigate a mechanism of such a phenomenon, the
inventors have conducted a detailed analysis about the influence of
the application of the reverse voltage across the anode-cathode
path of the reverse conducting static induction thyristor.
[0011] FIGS. 6, 7 and 8 are graphs showing the operation of the
static induction thyristor used in the pulse generating circuit
upon occurrence of discharge failure. FIG. 6 represent a variation
of a current I.sub.ak flowing through the anode-cathode path, FIG.
7 shows a variation of a gate current I.sub.g and FIG. 8 denotes a
variation of a gate voltage V.sub.g. In these figures, A represents
a case in which a pulse duration t.sub.w is long, and B shows a
case in which a pulse duration t.sub.w is long. When the current
I.sub.ak is larger than 3000 A and the pulse duration t.sub.w is
longer than several tens .mu.s, breakdown of the reverse conducting
static induction thyristor does not occur. However, when a pulse
duration t.sub.w is set to a shorter value within a range from
several hundreds ns to several .mu.s, the reverse conducting static
induction thyristor might be broken. In this case, a breakdown
point situates in the static induction thyristor section and no
abnormal phenomenon occurs in the diode section. From these
phenomena, it is assumed that the breakdown of the reverse
conducting static induction thyristor depends on an inclination of
a raising portion of the current I.sub.ak. In the longer pulse
duration shown in FIG. 6A, an inclination of a reverse current
i.sub.r (d.sub.ir/dt) is about 0.5 KA/.mu.s, and in the shorter
pulse duration illustrated in FIG. 6B, an inclination of the
reverse current is about 3 KA/.mu.s. Furthermore, as depicted in
FIG. 8B, when the breakdown of the reverse conducting static
induction thyristor due to discharge failure occurs, a remarkable
variation appears in the gate voltage V.sub.g immediately after a
reverse voltage peak.
[0012] Next the performance of the diode upon an occurrence of an
abruptly increase in the current flowing through the diode is
analyzed. FIGS. 9 and 10 show a forward current IF flowing through
the anode-cathode path of the diode 32 shown in FIG. 4 and a
forward voltage drop V.sub.F appearing across the anode-cathode
path of the diode when the diode is operated by a pulse. A denotes
a case of a smaller inclination and B represents a case of s larger
inclination. From these graphs it can be understood that there is
an intimate correlation between the inclination of the raising
portion of the current I.sub.F and a transient on-voltage (forward
recovery voltage) V.sub.FP as shown in FIG. 11. That is to say, for
the diode having the breakdown voltage of 4000 V, when the
inclination of the current I.sub.F (d.sub.IF/dt) is about 500
A/.mu.s, the forward recovery voltage V.sub.FP is lower such as
about 70 V, but when the inclination of the current (d.sub.IF/dt)
is high such as 1000A/.mu.s and 2000A/.mu.s, the forward recovery
voltage V.sub.FP is becomes higher such as about 100 V and 170 V,
respectively.
[0013] FIG. 12 is a graph showing a relationship between the
forward recovery voltage V.sub.FP and the breakdown voltage of the
diode for the inclination d.sub.IF/dt of 2000 A/.mu.s. In
accordance with the increase in the diode breakdown voltage, the
forward recovery voltage V.sub.FP becomes higher. When the diode
has a breakdown voltage of 4000 V, the forward recovery voltage
V.sub.FP is about 170 V. In the reverse conducting static induction
thyristor, the breakdown voltage of the diode section should be not
lower than the breakdown voltage of the thyristor section, and
therefore the diode section should have the breakdown voltage of
several thousands volts. The diode section having such a high
breakdown voltage also has a high forward recovery voltage
V.sub.FP. In other words, the higher the breakdown voltage of the
diode section is, the forward pulse current hardly flows through
the diode section.
[0014] In the manner explained above, in the known reverse
conducting static induction thyristor having a breakdown voltage
of, for instance 4 KV, when a large reverse current is to flow
immediately after conducting a large forward current, the
protection diode section could not be made conductive, and a large
amount of carriers stored in the channel regions in FIG. 5 flow
abruptly in the reverse direction from the cathode region 46 to the
buried gate region 43. Particularly, in a region denoted by X in
FIG. 5, i.e. in a vicinity of the buried gate region 43 into which
the gate current is supplied much more abruptly than the central
gate region 42, an excessive amount of carriers are generated and
there is produced a filamentation between the channel regions by
the diode reverse recovery phenomenon of the diode section and the
thyristor section 44 might be destroyed.
[0015] It should be noted that the above explained problem occurs
not only in the reverse conducting static induction thyristor, but
also in a semiconductor switching device such as normal type
thyristor, gate turn-off (GTO) SCR and insulated gate bipolar
transistor (IGBT).
SUMMARY OF THE INVENTION
[0016] The present invention has for its object to provide a novel
and useful semiconductor device, in which the above mentioned
problem of the known reverse conducting static induction thyristor,
and even if a high reverse voltage is applied to a switching
element abruptly, a protection diode can be brought into conductive
and the switching element can be effectively protected from the
breakdown.
[0017] According to the invention, a semiconductor device having
reverse conducting faculty comprises:
[0018] a switching element including a semiconductor substrate of a
first conductivity type having first and second major surfaces, a
first main electrode region of the first conductivity type formed
in the first major surface of the semiconductor substrate, a first
main electrode connected to said first main electrode region, a
second main electrode region of a second conductivity type formed
in the second major surface of the semiconductor substrate, a
second main electrode connected to said second main electrode
region, a control electrode region of the second conductivity type
formed in the first major surface of the semiconductor substrate
for controlling a current passing between the first and second main
electrode regions, and a control electrode connected to said
control region; and
[0019] a series arrangement of a plurality of diodes connected
between said first main electrode and said second main electrode in
an opposite polarity to a current flowing between said first main
electrode region and said second main electrode region, each of
said plurality of diodes having a breakdown voltage lower than a
breakdown voltage of said switching element.
[0020] Upon practicing the semiconductor device according to the
present invention, it is preferable that said series arrangement of
a plurality of diodes is formed in said first major surface of the
semiconductor substrate in which said first main electrode region
is also formed. Such a structure is particularly suitable for a
high frequency pulse circuit in which inductance of wiring has to
be reduced as far as possible. According to the invention, said
series arrangement of a plurality of diodes may be formed on a
separate semiconductor substrate from said semiconductor substrate
semiconductor substrate which constitutes said switching element,
or said series arrangement of a plurality of diodes may be formed
as a diode stack including first and second electrodes connected to
said first and second main electrodes of the switching element,
respectively.
[0021] In the latter two cases, it is preferable that said
switching device and series arrangement of a plurality of diodes
are installed in a common package in view of a reduction of wiring
inductance. However, according to the invention, said switching
device and series arrangement of a plurality of diodes may be in
separate packages.
[0022] In a preferable embodiment of the semiconductor device
according to the present invention, said switching element is
formed as a static induction thyristor whose cathode region and
cathode electrode are formed by said first main electrode region
and first main electrode, respectively, whose anode region and
anode electrode are formed by said second main electrode region and
second main electrode, respectively, and whose gate region and gate
electrode are formed by said control region and control electrode,
respectively.
[0023] In another preferable embodiment of the switching device
according to the invention, said switching element is formed as a
reverse conducting static induction thyristor including
[0024] a thyristor section whose cathode region and cathode
electrode are formed by said first main electrode region and first
main electrode, respectively, whose anode region and anode
electrode are formed by said second main electrode region and
second main electrode, respectively, and whose gate region and gate
electrode are formed by said control region and control electrode,
respectively; and
[0025] a main diode section having an anode region connected to
said cathode electrode of the thyristor section and a cathode
region connected to said anode electrode of the thyristor
section.
[0026] In these preferable embodiments, said series arrangement of
a plurality of diodes are preferably formed as field limiting rings
surrounding said static induction thyristor. In this case, a
plurality of diodes of said series arrangement may be preferably
formed such that breakdown voltages of the diodes are gradually
increased toward outside.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 is a circuit diagram showing an example of a known
pulse generating circuit using a static induction thyristor;
[0028] FIGS. 2 and 3 are graphs illustrating the operation of the
thyristor shown in FIG. 1;
[0029] FIG. 4 is a schematic diagram depicting a principal
structure of a known reverse conducting static induction
thyristor;
[0030] FIG. 5 is a cross sectional view showing a detailed
structure of the known reverse conducting thyristor;
[0031] FIGS. 6A, 6B; 7A, 7B and 8A, 8B are signal waveforms
explaining the operation of the known reverse conducting static
induction thyristor;
[0032] FIGS. 9A and 9B are signal waveforms representing a forward
recovery characteristic of the known reverse conducting static
induction thyristor;
[0033] FIGS. 10A and lOB are signal waveforms denoting a transient
turn-on voltage of the known reverse conducting static induction
thyristor;
[0034] FIG. 11 is a graph expressing a relationship between an
inclination of a current and a forward recovery voltage of a
diode;
[0035] FIG. 12 is a graph showing a relationship between a
breakdown voltage and a forward recovery voltage of a diode;
[0036] FIG. 13 is a schematic diagram illustrating a principal
structure of the semiconductor device according to the
invention;
[0037] FIG. 14 is a schematic diagram depicting another principal
structure of the semiconductor device according to the
invention;
[0038] FIG. 15 is a cross sectional view showing a detailed
structure of a first embodiment of the semiconductor device
according to the invention;
[0039] FIG. 16 is an enlarged cross sectional view of a part of the
semiconductor device shown in FIG. 15;
[0040] FIG. 17 is a cross sectional view showing a detailed
structure of a second embodiment of the semiconductor device
according to the invention;
[0041] FIG. 18 is an enlarged cross sectional view of a part of the
semiconductor device shown in FIG. 17;
[0042] FIG. 19 is a cross sectional view illustrating a detailed
structure of a third embodiment of the semiconductor device
according to the invention;
[0043] FIG. 20 is a cross sectional view depicting a detailed
structure of a fourth embodiment of the semiconductor device
according to the invention; and
[0044] FIG. 21 is a graph representing test results of the
semiconductor device according to the invention in comparison with
the known reverse conducting thyristor.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] Now the present invention will be explained in detail with
reference to several embodiments shown in the accompanying
drawings.
[0046] FIG. 13 is a schematic diagram showing a principal structure
of the semiconductor device according to the invention. As
explained above, according to the invention, the switching element
may be formed not only by a static induction thyristor, but also by
another semiconductor switching element such as SCR and IGBT. For
the sake of explanation, hereinafter the switching element is
formed by a static induction thyristor.
[0047] According to the invention, between an anode A and a cathode
K of a static induction thyristor 61, is connected a series
arrangement 63 of a plurality of diodes 62 is connected such that
an anode of the series arrangement of diodes 63 is connected to the
cathode K of the static induction thyristor 61 and a cathode of the
series arrangement of diodes is connected to the anode A of the
static induction thyristor, each of said plurality of diodes 62
having a breakdown voltage which is lower than a breakdown voltage
of the static induction thyristor 61. That is to say, the series
arrangement 63 of diodes 62 is connected in parallel with the
static induction thyristor in a reverse polarity. The breakdown
voltages of these diodes 62 are set such that a sum of breakdown
voltages of respective diodes becomes not less than the breakdown
voltage of the static induction thyristor 61.
[0048] For instance, now it is assumed that all the diodes 62 have
the same characteristics and the static induction thyristor 61 has
the breakdown voltage of 4000 V, then eight diodes each having a
breakdown voltage of 500 V may be connected in series. In this
case, as can be understood from FIG. 12, the forward recovery
voltage V.sub.FP of each diodes becomes about 10 V. Even if eight
diodes are connected in series, a total forward recovery voltage of
the series arrangement of diodes becomes 80 V(=10.times.8) which is
sufficiently lower than the forward recovery voltage of about 170 V
of a single diode having a breakdown voltage of 4000 V. When a
reverse current is to be flown abruptly through the anode-cathode
path of the static induction thyristor 61, the series arrangement
63 of diodes 62 are conducted. Therefore, the reverse current
flowing through the static induction thyristor 61 is reduced and a
high reverse voltage is not applied to the static induction
thyristor. In this manner, the static induction thyristor 61 can be
effectively protected against the breakdown.
[0049] FIG. 14 is a schematic diagram illustrating another
principal structure of the semiconductor device according to the
invention. In this structure, the static induction thyristor is
constructed as the reverse conducting static induction thyristor
having a static induction thyristor section 61 and a diode section
64 connected in parallel with the static induction thyristor
section 61 in a reverse polarity. Furthermore, a series arrangement
62 of a plurality of diodes 62 is connected in parallel with the
reverse conducting static induction thyristor.
[0050] The semiconductor device according to the invention having
the above explained principal structures may be embodied in various
ways. For instance, the series arrangement 63 of a plurality of
diodes 62 may be formed on a same semiconductor substrate of the
static induction thyristor 61 as a single unit. Particularly, when
the semiconductor device is used in a high frequency circuit, it is
necessary to make a stray inductance of wiring conductors as small
as possible, and therefore it is preferable to construct the static
induction thyristor 61 and series arrangement 63 of a plurality of
diodes 62 in a single common semiconductor substrate. Furthermore,
in the semiconductor device according to the invention, the series
arrangement 63 of a plurality of diodes 62 may be constructed
separately from a semiconductor substrate on which the static
induction thyristor 61 is formed. In this case. the series
arrangement 63 of diodes is preferably arranged within a package in
which the static induction thyristor 61 is arranged, but it may be
provided outside the package. This type of structure may be
advantageously applied to the known semiconductor devices such as
the static induction thyristor 61 shown in FIG. 13 and the reverse
conducting static induction thyristor having the static induction
thyristor 61 and the single diode 64 illustrated in FIG. 14.
[0051] FIGS. 15 and 16 are cross sectional views illustrating a
first embodiment of the semiconductor device according to the
invention having the principal structure shown in FIG. 14, in which
the series arrangement 63 of a plurality of diodes 62 is connected
in parallel with the switching element, i.e. static induction
thyristor 61 as well as the single diode 64. A p.sup.+ gate region
(control region) 102 is formed substantially at a center area of
one major surface of an n.sup.- silicon substrate 101, a plurality
of first ring-shaped recesses 103 are formed to surround the gate
region 102, and gate regions 104 are formed along the recesses 103.
Furthermore, buried gate regions 105 are formed in channel regions
formed by parts of the silicon substrate 101 surrounded by
successive gate regions 102 and 104. On the gate regions 102 and
104 are formed conductive layers 106 and a gate electrode 107 is
provided on the conductive layer at the central gate region 102. In
surface portions of the channel regions, there are formed n.sup.+
cathode regions (one major electrode region) 108 and the cathode
regions are connected to a cathode electrode 110 via conductive
layers 109. In the other major surface of the silicon substrate
101, there are formed p.sup.+ anode regions (the other major
electrode region) 111, which are connected to an anode electrode
113 via a conductive layer 112. In this manner, the thyristor
section 114 is constructed.
[0052] In the first major surface of the silicon substrate 101,
there is formed a single ring-shaped second recess 121 and a
plurality of ring-shaped p.sup.+ regions 122 are formed on a bottom
of the second recess to construct a separation band 123.
[0053] Outside the separation band 123, there is formed a p.sup.+
anode region 131 of a main diode 134, said anode region having a
wider width than the remaining p.sup.+ regions 104 in the thyristor
section 114. The anode region 131 is connected to the cathode
electrode 110 via a conductive layer 132 of the thyristor section
114. A cathode region of the main diode section 134 is formed by a
bulk of the n.sup.- silicon substrate 101 situating under the anode
region 131. In the second major surface of the silicon substrate
101 at a portion corresponding to the anode region 131, there is
formed an n.sup.+ emitter region 133, which is connected to the
anode electrode 113 of the thyristor section 144. In this manner,
the main diode section 134 serving to protect the thyristor section
from the breakdown is formed such that the main diode section 134
is connected in parallel with the thyristor section 114 in a
reverse polarity.
[0054] The structure of the thyristor section 114, separation band
123 and main diode section 134 is identical with that of the known
reverse conducting static induction thyristor. According to the
present invention, in the first major surface of the silicon
substrate 101, there are formed a plurality of ring-shaped third
recesses 141 are formed to surround the main diode section 134, and
in bottoms of these third recesses there are formed p.sup.+ anode
regions 142. As clearly illustrated in FIG. 16, each of the anode
regions 142 is formed at a position deflected toward one side of
the third recess 141. Between successive third recesses 141 there
are formed n.sup.+ cathode contact regions 143, and the successive
cathode regions 143 are connected to adjacent p.sup.+ anode regions
142 successively to constitute a series arrangement of a plurality
of diodes by means of conductive layers 144. In this manner, a
series arrangement section 145 is formed. Although not shown in
FIG. 15, exposed portions of the first major surface of the silicon
substrate 101 are covered with insulating layers 146 such as
silicon oxide layers as shown in FIG. 16.
[0055] As illustrated in FIG. 15, an anode region of the innermost
diode in the series arrangement section 145 is formed by the anode
region 131 of the main diode section 34, and a cathode of the
outermost diode is connected to the anode electrode 113 of the
static induction thyristor by means of the outermost cathode
contact region 143 and conductive layers 147 and 112. In this
manner, there is formed the series arrangement section 145, in
which the anode of the innermost diode is connected to the cathode
electrode 110 of the thyristor and the cathode of the outermost
diode is connected to the anode electrode 113. The main diode
section 134 is connected in parallel with the series arrangement
section 145 of a plurality of diodes.
[0056] In the semiconductor device of the present embodiment, each
of the plural diodes of the series arrangement section 145 has a
breakdown voltage which is sufficiently lower than a breakdown
voltage of the main diode section 134, and therefore its forward
recovery voltage is sufficiently lower than that of the main diode
section and energy loss due to the forward conduction becomes also
very small. When a reverse voltage is applied to the thyristor
section 114, at first a plurality of diodes in the series
arrangement section 145 are made conductive, and then the main
diode section 134 is made conductive. In this manner, the thyristor
section 134 can be effectively prevented from the breakdown.
[0057] FIGS. 17 and 18 are cross sectional views illustrating a
second embodiment of the semiconductor device according to the
invention. In the present embodiment, the switching element is
formed by the static induction thyristor without the main diode,
and portions similar to those of the first embodiment are denoted
by the same reference numerals used in FIGS. 15 and 16. The
ring-shaped recesses 141 formed in the series arrangement section
145 of a plurality of diodes are formed such that successive
recesses viewed from the internal one have increasing widths so
that the successive diodes have increasing breakdown voltage. Then,
the p.sup.+ anode regions 142 of these diodes serve optimally as a
field limiting ring.
[0058] Also in the first embodiment illustrated in FIGS. 15 and 16,
the anode regions of a plurality of diodes of the series
arrangement section 145 serve as the field limiting ring to some
extent, but they do not optimally operate as the field limiting
ring, because the third recesses 141 have identical width. In the
present embodiment, the third recesses 141 formed in the series
arrangement section 145 of a plurality of diodes have widths which
are successively increased toward outside such that the p.sup.+
anode regions 142 of the diodes work optimally as the field
limiting ring. Such a structure may be easily realized by a known
field limiting ring designing method.
[0059] FIG. 19 is a cross sectional view showing a third embodiment
of the semiconductor device according to the invention. Like as the
second embodiment, in the present embodiment, the switching element
is formed by the static induction thyristor without a main diode.
That is to say, in the first embodiment, the series arrangement
section 145 of a plurality of diodes is provided in addition to the
main diode section 134, but in the present embodiment the main
diode section is not provided. In the present embodiment, a p.sup.+
anode region of the innermost diode within the series arrangement
145 of a plurality of diodes is constituted by a p.sup.+ region 122
within the separation band 123. The remaining structure of the
present embodiment is identical with the above mentioned second
embodiment.
[0060] FIG. 20 is a cross sectional view of a fourth embodiment of
the semiconductor device according to the invention. In this
embodiment, the switching element is formed by the reverse
conducting thyristor. In the embodiments so far explained, the
series arrangement of diodes is formed on the silicon substrate on
which the thyristor is also formed. In the present embodiment, only
the thyristor section 114, separation band 123 and main diode
section 134 are formed on the silicon substrate 101, and a series
arrangement 152 of a plurality of diodes 151 is provided separately
from the silicon substrate 101. It should be noted that in FIG. 20,
a reference numeral 161 denotes a field limiting ring section.
[0061] In this case, the series arrangement of diodes may be formed
on a semiconductor wafer like as the previous embodiment, but in
the present embodiment, the series arrangement 152 of a plurality
of diodes 151 is constructed by a diode stack. Furthermore, in the
present embodiment, respective diodes 151 is constructed to have
P.sup.+i-n.sup.+ structure and the series arrangement 152 is
beveled such that a surface area of the diodes 151 is gradually
decreased from the anode side to the cathode side. An anode
electrode 153 of the series arrangement 152 of a plurality of
diodes 151 is connected to the cathode electrode 110 of the
thyristor section 114 and a cathode electrode 154 is connected to
the anode electrode 113 of the thyristor section 114. The series
arrangement 152 of a plurality of diodes 151 is preferably
installed within a common package together with the silicon
substrate 101 of the reverse conducting thyristor, but it may be
installed in a separate package.
[0062] FIG. 21 is a graph representing a breakdown rate of the
semiconductor device according to the invention in comparison with
the known reverse conducting static induction thyristor. In the
known reverse conducting static induction thyristor having only the
main diode, the breakdown occurs in almost all samples when a width
of a pulse voltage applied to the thyristor is not longer than 0.1
.mu.s, i.e. when an inclination of a reverse current is not smaller
than about 100 KA/.mu.s as shown by a curve A, and the breakdown
occurs in a substantially half number of the samples at about 12
KA/.mu.s. In the semiconductor device according to the invention,
no breakdown occurs in all samples even when the reverse current
having an abrupt inclination not smaller than 100 KA/.mu.s flows as
depicted by a curve B. In this test, a peak value of the forward
current is set to 4000 A.
[0063] It should be noted that the present invention is not limited
to the embodiments explained above, but many alternations and
modifications may be conceived by a person skilled in the art
within the scope of the invention defined by claims. For instance,
in the above embodiments, the switching element formed on the
semiconductor substrate is constituted by the static induction
thyristor, but according to the invention, the switching element
may be formed by any other switching element such as the gate
turn-off (GTO) SCR and insulated gate bipolar transistor (IGBT).
The number of diodes within the series arrangement for use in the
pulse generating circuit may be determined at will taking into
account of breakdown voltage of the thyristor as well as breakdown
voltages of respective diodes.
[0064] As stated above in detail, in the semiconductor device
according to the present invention, since the series arrangement of
a plurality of diodes is connected in parallel with the switching
element in a reverse polarity and each of these diodes has a
breakdown voltage lower than a breakdown voltage of the switching
element, when a current flowing through the switching element is
abruptly decreased and a large reverse voltage is applied to the
switching element, the series arrangement of diodes is positively
conducted and the switching element can be effectively prevented
from breakdown.
[0065] Moreover, in the above embodiment in which the anode regions
of a plurality of diodes are constructed to serve as the field
limiting rings, the series arrangement of diodes can be formed as a
substantially same size as a conventional semiconductor device
having the field limiting rings. Therefore, an increase in
manufacturing cost can be restricted and an increase in cost of the
semiconductor device according to the invention can be limited.
* * * * *