Note: Descriptions are shown in the official language in which they were submitted.
BACK~ROUND OF THE IN~ENTION
10 Field of the _ ention: -
The present invention pertains to semiconductor
switching devices, and'more particularly to two terminal
silicon thyristors.
De ~ he Prior Art:
In addition to the well known gating of thyristors
by means of a third terminal, it is known in the art that
thyristors may be switched from the forward blocking mode to
the forward conducting mode by rapidly increasing the vol-
tage across the thyristor. Thyristors which are designed to
be fired by such positive dv/dt are sometimes referred to in
the art as reverse switching rectifiers.
The thyristor device of the present invention dif-
fers from a reverse switching rectifier in that a rapidly
decreasing voltage (i.e., negative dv/dt) is used to initiate
~iring.
SUMMARY O~ THE INVENTION
The thyristor device of the present invention may
be fired by a collapsing voltage. The collapsing voltage
generates a displacement current which is used to effect
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firing of the thyristor. The word "firing" is used in this
specification to describe the dynamic process in which a
thyristor switches from the forward blocking mode to the
forward conducting mode. The expression "turn-on" is also
used in the art to describe this switching phenomenon.
In accordance with the present invention, the thy-
ristor device has an emitter region and an auxiliary emitter
region disposed in a body of semiconductor material. A
means for concentrating a displacement current, such as a
moat, is disposed in a base region between the main emitter
and the auxiliary emitter, whereby a collapsing voltage
concentrates a lateral flowing portion of the displacement
current. The concentrated displacement current causes
carrier emission from the auxiliary emitter into the base
region which generates a forward firing current.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a plan view of a thyristor embodiment
of the present invention;
Figure 2 is a cross-sectional view of the embodi-
ment of Figure l;
Figure 3 is a plan view of a first practicalembodiment of the present invention;
Figure 11 is a cross-sectional view of the embodi-
ment of Figure 3;
Figure 5 is a plan view of a second practical
embodiment of the present invention;
Figure 6 is a cross-sectional view of the embodi-
ment of ~igure 5;
Figures 7a and 7b are cross-sectional views of the
3o embodiment of Figure 1, schematically illustrating the
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firing process in tirne sequence;
Figure 8 is a graph illustrating the time sequence
of events described in Figures 7a and 7b;
Figure 9 is a simplified circuit model of the
thyristor of the present invention; and,
Figures 10, 11 and 12 are practical examples of
circuit applications of the thyristor of the present inven-
tion.
DESCRIPTION OF THE PREFERRED EMB~DIMENTS
Figures 1 and 2 il]ustrate a basic thyristor
embodiment 10 of the present invention comprising a body of
semiconductor material 12, which is preferably silicon. The
body 12 has four layers of alternate conductivity which, by
way of example, are designated with the letters "N" and "P".
Those skilled in the art will recognize that a complementary
device may be produced by interchanging the "N" and "P"
conductivity types in description which follows. Starting
from top surface 13 of body 12, a first layer consists of
two N-type emitter regions 14 and 16, which form PN junctions
15 and 17 with a second P-type layer or base region 18 as
shown. A third layer, 20 of N- ~pe conductivity lies beneath
base region 18, PN junction 19 being formed therebetween. A
fourth layer or anode emitter region 22 of P-type conductivity
lies beneath layer 20, PN junction 21 being formed there-
between. Electrodes 24 and 26 make ohmic contact with
regions 14 and 16 respectively at top major surface 13.
Electrode 28 makes ohmic contact with region 22 at bottom
major surface 29 as shown.
Base region 18 extends past emitter regions 14 and
3o 16 to surface 13 at at least three points. A first portion
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30 of base region 18 extends to surface 13 as shown, thereby
separating regions 14 and 16. A second portion 32 of base
region 18 extends to surface 13 on a side of region 14
separated from portion 30 as shown. A third portion 34 of
base region 18 extends to surface 13 on a side of region 16
separated from portion 30 as shown. Electrodes 24 and 26
make ohmic contact with portions 32 and 34 respectively at
surface 13.
Disposed in portion 30 of region 18 is a barrier
or means for concentrating or channelling lateral flowing
current in region 18. What is meant by lateral flowing
current is, for example, a current which flows from the
portion of base region, 18 under emitter region 14 from rlght
to left in the view of Figure 2, and which continues to flow
through portion 30 of region 18 into the portion of region
18 under emitter region 16. A presently preferred current
` concentrating means is an etched moat 36. A suitable alter-
,;1
I native to a moat is a N-type conductivity region which may
-i be located in approximately the same position as moat 36.
. ~
An isthmus 38 lies in portion 30 of region 18
:, .
between portions of the moat 36 as shown in Figure 1, the
isthmus 38 carrying a concentrated lateral flowing current
near surface 13 in portion 30 of region 18. It is preferred
that region 18 be produced by diffusion so that the resisti-
vity in region 18 decreases as the distance from surface 13
decreases. Thus the isthmus 38 presents a relatively low
resistance path to lateral flowing current.
~;~ While the embodiment of Figures 1 and 2 is symme-
trical on elther side of moat 36, region 14 functions as a
30 main cathode emitter and region 16 functions as an auxiliary
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emitter. Connections to an external circuit are made at
cathode electrode 24 and anode electrode 28 as schematically
illustrated by the letters "K" and "A".
Now referring to ~igures 3 and 4, a presently pre-
ferred embodiment 110 is illustrated, which is particularly
suited to applications in the hundreds of amperes range.
Thyristor 110 is circular in general con~iguration; however~
similar numerals designate parts similar to those described
above in conjunction with basic thyristor device 10.
In particular, semiconductor body or wafer 112 has
four layers of alternate conductivity, wherein the top layer
comprises two N-type emitter regions 114 and 116. Auxiliary
emitter region 116 is ring-shaped and surrounds circular-
shaped main emitter region 114. Regions 114 and 116 form PN
junctions 115 and 117 respectively with P-type base region
118 disposed therebelow. N-type region 120 and P-type
region 122 lie beneath region 118, PN junctions 119 and 121
i interfacing the regions as shown. Electrodes 124, 126 and
128 make ohmic contact to regions 114, 116 and 122 respec-
20 tively. Base portions 130, 132 and 134 terminate at major
surface 113. Portion 130 separates region 114 from region
116.
Disposed in portion 130 of base region 118 between
regions 114 and 116 is a barrier or moat 136 similar in form
and function to the barrier 36 described above. Moat 136
circumscribes main emitter region 114 almost entirely, a
narrow isthmus 138 providing a path near surface 113 for
current flowing laterally through portion 130 of base region
118. As a means of reducing surface recombination, an
insulating layer 140, such as silicon dioxide, is disposed
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on surface 113 covering at least the portion of ~unctions
115 and 117 in the vicinity of isthmus 138. The insulating
layer 140 minimizes recombination of electrons injected by
emitter regions 114 and 116 into base region 118 under layer
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Portions 132 are hnown in the art as shorts or
shunts since they provide direct contact between cathode
electrode 124 and base region 118. Shunt portions 132 are
disposed throughout emitter region 114 in a regular pattern
known in the thyristor art, a preferred spacing between ad-
jacent shunts being 25 to 40 mils and a preferred shunt dia-
meter being 4 to 12 mils.
Semiconductor wafer 112 has a bevelled edge 141,
for which the angle of inclination and method of forming are
known in the art. Disposed on bevelled edge 141 is an insu-
lating and protective coating 142, a high temperature curing
silicon varnish being one of several suitable coating mate-
rials.
A second preferred embodiment 210 is illustrated
in Figures 5 and 6. The device 210 is fully analogous to
device 110, similar parts being designated by similar nume-
rals. However, device 210 differs from device 110 in that
main emitter region 214 and associated electrode 224 sur-
round an auxiliary emitter region 216 and associated elec-
trode 226.
For purposes of comparison, the generic structure
of Figure 2 is located in a portion of the cross-sections of
devices 110 and 210 designated by numeral 10'. The opera-
tion of the inventive devices 10, 110 and 210 is functionally
similar apart from geometrical differences.
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~ or ease of illustration, the operation of device
13 will now be described diagrammatically with the aid of
Figures 7a, 7b and 8.
At t=0 a forward blocking voltage VAK = VO exists
across device 10 with the polarity shown at terminals "A"
and "K". At some later time t=tl, the voltage is caused to
decrease sharply by means of external control~ causing a
displacement current id to flow from cathode electrode 24 to
; anode electrode 28 along t~e solid line paths shown in
Figure 7a. The current id is proportional to the capaci-
tance of forward blocking junction 19 and the rate of de-
crease in voltage. Some of the current id is channelled
through isthmus 38 and then through a portion of base region
;~ 18 under auxiliary emitter region 16 as shown. A voltage
drop "idr" therefore exists in base region 18 under aux~-
liary emitter region 16, wherein "r" is the resistance along
the current path, which is proportional to the resistivity
of the base region 18. When a voltage drop of greater than
about 0.7 volts exists under auxiliary emitter region 16,
electrons are emitted from region 16 as shown. These elec-
trons are replenished at the interface between metal elec-
trode 26 and base region 18 where a positive current of
holes flows from auxiliary emitter electrode 26 into base
region 18 as illustrated by the dashed line.
At t=t2 the voltage VAK has dropped from an initial
value of VO to about 0.25 VO as shown in Figure 8. By this
time the electron emission from the edge of region 16 has
caused localized lowering of junction resistance in PN
``I junction 19, which allows a localized forward firing current
if to flow. The current if flows through isthmus 38 and
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then through base region 18 under main emitter region 14 to
cathode electrode 24, thus producing a voltage drop ("ifr")
having the polarity shown in Figure 7b. When ifr exceeds
about 0,7 volts, electrons are emitted from the edge of
emitter region 14 as shown. Those skilled in the art will
recognize that such electron emission will fire the thyristor.
Device embodiments 110 and 210 function similarly
to device embodiment 10 as just described. Both devices 110
and 210 are fired by a collapsing voltage which produces a
displacement current and resulting emission from an auxi-
liary emitter.
In the case of thyristor device 110~ which is
illustrated in Figures 3 and 4, the aforementioned displace-
ment current flows from the cathode electrode 124 through
the shunts 132 into base region 118. A portion of the
displacement current then flows laterally (i.e., radially
outward) until it encounters moat 136, which causes a con-
centrated current to flow through isthmus 138. The con-
centrated current continues laterally outward similarly as
' 20 described above in conjunction with the discussion of device
10 in Figure 7a. A voltage drop occurs under auxiliary
emitter region 116 of device 110 in a portion of base region
~; 118 in the vicinity of isthmus 138. Electron emission then
occurs from auxiliary emitter region 116 through a portion
of PN junction 117 nearest isthmus 138. The junction resis-
tance of a portion of PN junction 119 under the isthmus 138
is lowered as a result of the electron emission, which
causes a localized forward firing current to flow from anode
electrode 128 through the low resistance portion of PN junc-
tion 119 and then through the shunts 132 to cathode electrode
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124. The firing current produces a voltage drop in base
region under main emitter region 114, causing electron
emission from region 114 which fires the thyristor 110.
The positioning of shunts 132 is important. The
nearest shunt 132 to isthmus 138 must be spaced at a dis-
tance far enough from isthmus 138 so that the voltage drop
under emitter region 114, as measured from the nearest shunt
132 to the isthmus 138, iS sufficiently high to cause elec-
tron emission from the edge o~ emitter region 114 nearest
the isthmus 138.
Again referring to Figures 5 and 6, thyristor
device 210 functions in an analogous manner to device 110
discussed above. Briefly, when the voltage VAK collapses, a
displacement current flows from cathode electrode 224 to
anode electrode 228. Some of the displacement current flows
laterally through isthrnus 238 causing electron emission from
auxiliary emitter region 216. The electron emission from
region 216 lowers the junction resistance of PN junction 219
;; in the vicinity of the emission, which permits a forward
20 firing current to flow from anode electrode 228 to cathode
- electrode 224 causing electron emission from emitter region
214 and a resulting firing of the device 210. The shunts
232 of device 210 serve the same purpose as shunts 132 of
device 110 discussed above.
As a practical example of the invention, a working
embodiment has been made having the basic geometry of thy-
ristor device 110 shown in Figures 3 and ll. The working
embodiment had the following dimensions: overall diameter
equals 33 mm, diameter of cathode electrode 124 equals 22
mm, inside diameter of auxiliary emitter electrode 126
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equals 28 mm, outside diameter of auxiliary emitter elec-
trode 126 equals 32 mm, width of auxiliary emltter region
equals 2 mm, width of moat 136 equals 1 mm, spacing between
moat edge and adJacent emitter regions 114 and 116 equals
0.5 mm, width of isthmus 138 measured from the ends of moat
equals 2 mm, peripheral overlap of electrode 126 onto base
region 118 equals 0.25 mm. In addition, the working embo-
diment employed a hexagonal arrangement of shunts 132, each
shunt having approximately a 0.2 mm diameter and a 0.9 mm
10 spacing between nearest neighboring shunts. However, the
shunts 132 were arranged so that the distance from the edge
of emitter region 114 to the nearest shunt 132 (i.e., dis-
tance "X" in Figure 4) was approximately 4.5 mm.
A better understanding of the dynamics of thyris-
tor firing has lead to the above-described structure and
presently preferred dimensions. As discussed in more detail
above, it is necessary to produce a critical voltage of
greater than about 0.7 volts under an emitter region in
order to cause the desired electron emission from the emitter.
20 The distance "X" must therefore be long enough to produce a
voltage drop in excess of 0.7 volts, and most preferably
greater than about 1.0 volts. The working embodlment des-
cribed above had a sheet resistivity of about 600 ohms/cm2
in base region 118. The distance "X" was therefore selected
to be 4.5 mm so that the forward firing current passing .
laterally through base region 118 would produce the desired
voltage drop of greater than about 1.0 volts. Similar r
considerations affected the choice of the width of the
auxiliary emitter region 116. The displacement current,
30 being concentrated as it passes through isthmus 138, has a
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magnitude sufficient to produce the critical voltage drop in
a distance of 2 mm given a shee~ resistivity of 600 ohms/cm2.
Now referring to Figure ~, a simplified circuit
model 310 of the inventive thyristor device is shown, which
by way of analogy further describes its operation. When the
voltage VAK collapses~ a displacement current flows from
; capacitor C which fires thyristor Tl, which in turn fires
the main thyristor T2.
Figure 10 shows a general circuit application for
the inventive thyristor device 310. Initially no current
flows through the circuit, the switch 350 being open-cir-
cuited. When the switch 350 closes, the thyristor 310 is
` turned-on by the collapsing voltage, provided that the
conduction state impedance of the switch 350 is greater than
that of the thyristor 310. The switch may take any form,
such as mechanical, electro-mechanical or solid-state elec-
tronic.
Figure 11 shows an application having parallel
operation of thyristors 310 and a conventional three-termi-
nal thyristor 360. By applying a signal to the gate 362 in
the usual manner known in the thyristor art, the three-
terminal thyristor 360 is fired, which causes all thyristors
i 310 to fire.
Series-parallel operation is also contemplated, as
! illustrated in Figure 12. Parallel banks of thyristors 310
i are connected in series as shown. Each bank has a control
device 370 in parallel, the device 370 being capable of
firing its entire bank. For example, a light activated
switch may be used as a control device 370, which would
enable simultaneous firing of all parallel banks. A light
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activated switch has the advantage of enabling "gating" of
each bank without regard to the voltage level at each bank.
These and other advantages of the thyristor device
of the present invention will be appreciated by those skilled
in the art. ~or example, it may be advantageous in certain
applications to have conventional three-terminal thyristors
constructed with the turn-on capability described herein.
If gate failure then occurs in such a device, firing will
still take plaee under collapsing voltage coneitions.
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