Note: Descriptions are shown in the official language in which they were submitted.
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1.
HIGH VOLTAGE DIELECTRICALLY
ISOLATED SOLID-STATE SWITCH
l'echnical Field
This învention relates to solid-state
structures and, in particular, to high voltage
solid-state structures useful in telephone switching
systems and many other applications.
Background of the Inv'ention
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In an article entitled "A Field Terminated
Diode" by Douglas F. Houston et al, published in
IEEE Trans'act'ions-on'E'l'e'ctron'Devic'es, Vol. ED-23,
No. 8, August 1976, there is described a discrete
s~lid-state high voltage switch that has a vertical
geometry and which includes a region which can be
pinched off to provide an "OF~" state or which can be
made highly conductive with dual carrier in~ection
to provide an "ON" state. One p~oblem with this
switch is that it is not easily manufacturable with
other like switching devices on a common substrate.
Another problem is that the spacing between the grids
and the cathode should be small to limit the magnitude
of the control grid voltage; however, this limits the
useful voltage range because it decreases grid-to-
cathode breakdown voltage. This limitation effectivelylimits the use of two of the devices with the cathode
of each coupled to the anode of the other to relatively
low voltageS- Such a dual device structure would be
useful as a high voltage bidirectional solid-state
switch. An additional problem is that the base region
should ideally be highly doped to avoid punch-
through from the anode to the grid; however, this
leads to a low voltage breakdown between anode and
cathode. Widening of the base region limits the
punch-through effect; however, it also increases the
resistance of the device in the "ON" state.
It is desirable to have a solid-state switch
which is easily integratable such that two or more
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switches can be simultaneously fabricated on a common
substrate and wherein each switch is capable of bilateral
blocking of relatively high voltages.
Summary of the Invention
In accordance with an aspect of the invention
there is provided a solid-state switching device comprising
a semiconductor body a bulk portion of which is of a first
conductivity type, a first region of the first conductivity
type, a second region of a second conductivity type
opposite that of the first conductivity type, a gate
region of the second conductivity type, the first, second,
and gate regions being mutually separated by portions of
the bulk portion, the resistivities of the first, second
and gate regions being lower than the resistivity of the
hulk portion, the parameters of the device being such
that, with a first voltage applied to the gate region, a
depletion region is formed in the semiconductor body which
substantially prevents current flow between the first and
second regions, and that, with a second voltage applied to
the gate region and with appropriate voltages applied to
the first and second regions, a relatively low resistance
current path is established between the first and second
regions by dual carrier injection, characterized in that
the first and second regions and the gate region each have
a surface contained on a first major surface of the
semiconductor body.
One embodiment of the present invention is a
structure comprising a semiconductor body whose bulk is of
one conductivity type and which has a major surface within
which semiconductor body is a localized first region which
is of the one conductivity type, and localized second and
third regions which are both of the opposite conductivity
type. The first, second and third regions are spaced
apart from each other, have separate electrode connections
thereto, and are of relatively low resistivity compared to
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the bulk of the semiconductor body. The structure is so
adapted that during operation there is dual carrier
injection and is further charactecized in that each o~ the
three regions has a portion which ~orms part of the major
sur~ace of the semiconductor body.
In a preferred embodiment the semiconductor body
is isolated from a semiconductor support by a dielectric
layer and a plurality of said bodies are formed in said
support and are separated from each other by at least a
dielectric layer. The first, second and third regions
serve as the anode, gate and cathode respectively, of the
structure.
The structure of the present invention, when
suitably designed, can be operated as a switch that is
characterized by a low impedance path between anode and
cathode when in the ON (conducting) state and a high
impedance path between anode and cathode when in the OFF
(blocking) state. The potential applied to the gate
region determines the state of the switch. During the O~
state there is dual carrier injection that results in the
resistance between anode and cathode being relatively low.
This structure, which is to be denoted as a gated
diode switch (GDS), when suitably designed, is
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capable in the OFF state of blocking relatively large
potential differences between anode and cathode
regions, independent of polarity, and is capable in
the ON state of conducting relatively large amounts of
current with a relatively low voltage drop between anode
and cathode.
Arrays of these GDSs can be fabricated on a
single integrated circuit chip together with other
high voltage circuit csmponents. ~he bilateral
blocking characteristic of the structure acilitates
its use in a bidirectional switch formed by two of the
structures of the present invention with the cathode of
each coupled to the anode of the other and the gates
being coupled together.
These and other novel features and advantages
of the present invention are better understood from
consideration of the following de*ailed description
taken in conjunction with the accompanying drawings.
Brie Description of the Drawings
FIG. 1 illustrates a structure in accordance
with one embodiment of the invention;
FIG. 2 illustrates a proposed electrical
circuit symbol for the structure of FIG. l;
FIG. 3 illustrates a bidirectional switch
circuit in accordance with another embodiment o the
invention;
FIG. 4 illustrates a structure in accordance
with another embodiment of the invention;
FIG. 5 illustrates a structure in accordance
with still another embodiment of the invention; and
~ IG. 6 illustrates a structure in accordance
with still another embodiment of the invention.
Detailed Description
Referring now to FIG. 1, there is illustrated
a structure 10 comprising a support member 12 having
a major surface 11 and a monocrystalline semiconductor
body 16 whose bulk is of one conductivity type and
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which is separated from support member 12 by a di-
electric layer 14. The monocrystalline semiconductor
body 16 has a portion that is common with surface 11.
A localized first anode region 18, which is
S of the one type conductivity, is included in body 16
and has a portion thereof that extends to surface 11.
A localized second gate region 20, which is of the
opposite conductivity, also is included in body 16
and has a portion thereof which extends to surface 11.
A localized third cathode region 24, which is of the
opposite type conductivity, is included in body 16
and has a portion which extends to surface 11. A
region 22, which is of the one type conductivity and
has a portion which extends to surface 11, encircles
region 24 and acts as a depletion layer punch-through
shield. rn addition it acts to inhibit inversion of
the portions of body 16 at or near surface 11 between
regions 20 and 24. Gate region 20 exists between
anode region 18 and region 22 and is separated from
both by bulk portions of body 16. The resistivities
of regions 18, 20, and 24 are low compared to that of
the bulk portions of body 16. The resistivity of
region 22 is intermediate between that of cathode
region 24 and that of the bulk portions of body 16.
Electrodes 28, 30, and 32 are conductors
which make low resistance contact to the surface
portions of regions 18, 20, and 24, respectively. A
dielectric layer 26 covers major surface 11 so as
to isolate electrodes 28, 30, and 32 from all regions
other than those intended to be electrically contacted.
An electrode 36 provides a low resistance contact to
support 12 by way of a highly doped region 34 which
is of the same conductivity type as support 12.
Advantageously, the support 12 and the body
16 are each of silicon and the support 12 may be
either of _ or _ type conductivity. Each of electrodes
28, 30 and 32 advantageously overlaps the semiconductor
region to which they make low resistance contact.
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Electrode 32 also overlaps region 22. This overlapping,
~hich is known as field plating, acilitates high
voltage operation because it increases the voltage at
which breakdown occurs.
In one illustrative embodiment, substrate 12
and body 16 and regions 18, 20, 22, 24 and 34 are of
n-, p-, p~, n+, p, n~ and n~ type conductivity,
respectively. Dielectric layer 14 is silicon dioxide
and electrodes 28, 30, 32, and 36 are all aluminum.
A plurality of separate bodies 16 can be
formed in a common support 12 to provide a plurality
of switches.
Structure 10 is typically operated as a
switch which is characterized by a low impedance path
between anode region 18 and cathode region 24 when in
the ON ~conducting) state and as a high impedance
between said two regions when in the OFF Cblocking~
state. The potential applied to gate region 20
determines the state of the suitch. Conduction between
anode region 18 and cathode region 24 occurs i the
potential of gate region 20 is below that of the
potential of anode region 18 and cathode region 24.
During the ON state holes are injected into body 16
from anode region 18 and electrons are injected into
body 16 from cathode region 24. These holes and
electrons can be in sufficient numbers to form a
plasma which conductivity modulates body 16. This
effectively lowers the resistance of body 16 such
that the resistance between anode region 18 and cathode
region 24 is relatively low when structure 10 is
operating in the ON state. This type of operation is
denoted as dual carrier injection. The type of
structure described herein is denoted as a gated
diode switch ~GDS).
Region 22 helps limit the punch-through of
a depletion layer formed during operation between
gate region 20 and cathode region 24 and helps inhibit
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formation of a surface inversion layer between those
t~o regions. In addition, it facilitates gate region
20 and cathode region 24 being relatively closely
spaced apart. This facilitates in relatively low
resistance between anode region 18 and cathode
region 22 during the ON state.
Substrate 12 is typically held at the
most positive potential level available. Conduction
between anode region 18 and cathode region 24 is
inhibited or cut off if the potential of gate region
20 is sufficiently more positive than that of anode
region 18 and cathode region 24. The amount of excess
positive potential needed to inhibit or cut off
conduction is a unction Or the geometry and impurity
concentration ~doping) le~els of structure 10. This
positive gate potential causes the portion of body 16
between gate region 20 and the portion of dielectric
layer 14 therebelow to be depleted such that the
potential of this portion of body 16 is more`positive
than that of anode region 18 and cathode region 24.
This positive potential barrier inhibits the
conduction of holes from an~de region 18 to cathode
region 24. It essentially pinches off body 16 against
dielectric layer 14 in the bulk portion thereof below
gate region 20 and extending down to dielectric
layer 14. It also serves to collect electrons
emitted at cathode region 24 before they can reach
anode region 18. Control circuitry capable of
supplying thc needed gate potentials and absorbing
the electrons is illustrated and described in
copending Canadian Patent P~?plication Serial No. 342,165
which was filed in the names of A. R. ~lart~an, et al on
~ecemker 18, 1979.
During the ON state of structure 10, the
junction diode comprising body 16 and region 20 becomes
forward-biased. Current limiting means (not illustrated)
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are normally included to limit the conduction through
the forward-biased diode. One example of such
c.urrent limiting means is illustrated and described
in the above identified Canadian Patent ~p~llcation Serial
No. 342,165.
A proposed electrical symbol adopted for
this type o switch is illustrated in FIG. 2. The
anode, gate, and cathode electrodes of the GDS are
denoted as terminals, 28, 30, and 32, respectively.
One embodiment of structure 10 has been
fabricated with the following design. Support
member 12 is an n type silicon substrate, 18 to 22
mils thick, with an i~purity concentration of approx-
imately 2 x 10l3 impurities/cm3, and has a resistivity
greater than 100 ohm-centimeters. Dielectric layer
14 is a silicon dioxide layer 14 that is 2 to 4
microns thick. Body 16 is typically 30 to 50 microns
thick, approximately 430 microns long, 300 microns
wide, and is of p type conductivity with an impurity
concentration in the range of approximately 5-9 x 1013
impurities/cm3. Anode region 18 is of p~ type
conductivity, is typically 2 to 4 microns thick,
44 microns wide, 52 microns long, and has an impurity
concentration of approximately 1019 impurities/cm3.
Electrode 28 is typically aluminum, with a thickness
~ 25 of 1 1/2 microns, a width of 84 microns, and a
- length of 105 microns. Region 20 is of n+ type
conductivity and is typically 2 to 4 microns thick,
15 microns wide, 300 microns long, and has an impurity
concentration of approximately 1019 impuritiestcm3.
Electrode 30 is aluminum, 1 1/2 microns thick, 50
microns wide, and 210 microns long. The spa~ing
between adjacent edges of electrodes 28 and 30 and
between adjacent edges of electrodes 30 and 32 is
typically 40 microns in both cases. Region 22 is ~
type conductivity and is typically 3-6 microns thick,
64 microns wide, 60 microns long, and has an impurity
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concentration o approximately 1017 to 1018 impurities/cm3.
Cathode region 24 is n~ type conductivity and is
typically 2 microns thick, 48 microns wide, 44 microns
long, and has an impurity concentration of approximately
1019 impurities/cm3. Electrode 32 is aluminum, 1 1/2
microns thick, 104 microns wide, and 104 microns long.
The spacing between the ends of regions 18 and 22 and
the respective ends of region 16 is typically 55
microns. Region 34 is n~ type conductivity and is
typically 2 microns thick, 26 microns wide, 25 microns
long, and has an impurity concentration of 1019
impurities/cm3. Electrode 36 is aluminum which is 1 1/2
microns thick, 26 microns wide, and 26 microns long.
Structure 10, using the parameters denoted
above, has been operated as a gated diode switch ~GDS~
with 500 volts between anode and cathode. A layer of
silicon nitride tnot illustrated) was deposited by
chemical vapor deposition on top of silicon dioxide
layer 26 to provide a sodium barrier. Electrodes 28,
30, 32, and 36 were then formed and thereafter a
coating of radio frequency plasma deposited silicon
nitride ~not illustrated) was applied to the entire
surface of structure 10 except where electrical contact
is made. The layers of silicon nitride serve to help
prevent high voltage breakdown in the air between
adjacent electrodes.
Typically the anode had f250 volts applied
thereto, the cathode had -250 volts applied thereto,
and substrate 12 had ~280 volts applied thereto.
These applied potentials result in the anode and cathode
being essentially electrically isolated rom each
other and little or no current flow between anode and
cathode. The -250 volts can also be applied to the
anode and the ~250 volts applied to the cathode
without damage to structure 10. The anode and cathode
remain essentially electrically isolated from each
other and there is little or no current 1OW between
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anode and cathode. Thus, structure 10 bilaterally
blocks voltage be~ween anode and cathode. A
potential of ~280 ~olts applied to gate conductor
30 interrupted ~broke) 350 mA of current flow between
5 anode region 18 and cathode region 24. The ON
resistance of the GDS with 100 mA flowing between anode
and cathode is approximately 15 ohms and the voltage
drop between anode and cathode is typically 2.2 volts.
Referring now to FIG. 3, there is illustrated
a bidirectional switch combination comprising two
GDSs (GDSl and GD~2) in accordance with the prese-nt
invention with electrode 28 ~the anode electrode of
GDSl) electrically coupled to electrode 32a ~the
cathode electrode of GDS2~, and electrode 32 ~the
cathode electrode of GDSl) electrically coupled to
electrode 28a ~the anode electrode of GDS2). This
s~itch combination is capable o~ conducting signals
from electrodes 28 and 32a to electrodes 28a and 32
or ~ice versa. The bilateral blocking characteristic
of structure 10 facilitates this bilateral switch
combination. Two separate bodies 16 can be formed
in a common support 12 and the appropriate electrical
connections can be made to form the above-described
bidirectional switch. A plurality of separate bodies
16 can be formed in a common support 12 to form an
array of switches.
Referring now to FIG. 4, there is illustrated
a structure 100. Structure 100 is very similar to
structure 10 and all components thereof which are
essentially identical or very similar to those of
structure 10 are denoted by the sa~e reerence number
with the addition of one "0" at the end. The basic
difference between struct~res 100 and 10 is the
elimination from structure 100 of semiconductor region
like region 22 of FIG. l. Appropriately increasing
the spacing of region 240 from region 200 provides
sufficient protection against depletion layer punch-
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through to reglon 240 and facilitates the use of
structure lnO as a high voltage swi*ch.
Referring now to FIG. 5, there is illustrated
a structure 1000. Structure 1000 is very similar to
structure 10 and all components thereof which are
essentially identical or similar to those of structure
10 are denoted by the same reference number with the
addition of two "Os" at the end. The basic difference
between structures 1000 and 10 is the use of a semi-
conductor guard ring region 40 encircling region2400 and being separated therefrom by portions o
region 1600. In addition, there is no equivalent of
a semiconductor region like region 22 of FIG. 1.
Guard ring 40 provides protection against inversion
of body 1600, particularly between gate region 2000
and cathode region 2400. Guard ring 40 is of the
same conductivity as body 1600 but o lowe2 resistivity.
The protection afforded is adequate in particular
instances. As is illustrated by the dashed lines,
region 40 can be extended so as to contact region
2400. Typically the impurity concentration of region
40 is 1019/cm3.
Referring now to FIG. 6, there is illustrated
a structure lO,000. Structure 10,000 is very similar
to structure 10 and all components which are
essentially the same or very similar are denoted by the
same reference number with the addition of three "Os"
at the end. The main difference between structure
10,000 and structure 10 is the use of a semiconductor
` 30 guard ring region 400 which encircles cathode region
2400. Guard ring 400 is similar to guard ring region
40 of structure 1000 of FIG. 5. The dashed line portion
of guard ring 400 illustrates that it can be extended
so as to contact cathode region 24,000. The combi-
nation of region 22,000 and guard ring 400 provides
protection against inversion of portions of region
18,000 at or near surface 11,000, particularly between
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gate region 20,000 and cathode region 24,000, and
provides protection against depletion layer punch-
through to cathode region 2400. Guard ring 400 is
of the same conductivity as region 22,000, but is
o lower resistivity. This type of dual protection
structure encircling cathode region 24,000 is the
preferred protection structure.
The embodiments described herein are intended
to be illustrative o the general principles of the
invention. Various modifications are possible
consistent with the spirit of the invention. For
example, for the designs described, support members
12, 120, 1200, and 12,000 can alternatively be
p-type conductivity silicon, gallium arsenide,
sapphire, a conductor, or an electrically inactive
material. If regions 12, 120, 1200 and 12,000 are
electrically inactive materials then dielectric
layers 14, 140, 140Q, 14,000 can be eliminated. Still
further, bodies 16, 160, 1600, 16,000 can be fabricated
as air isolated type structures. This allows for the
elimination of support members 12, 120, 1200, and
12,000 and dielectric layers 14, 140, 1400, and
14,000. Further, the electrodes can be doped poly-
silicon, gold, titanium, or other types of conductors.
Still further, the impurity concentration levels,
spacings between different regions, and other
dimensions of the regions can be adjusted to allow
significantly different operating voltages and
currents than are described. Additionally, other
types of dielectric ~aterials, such as silicon
nitride, can be substituted for silicon dioxide. Still
further, the conductivity type of all regions within
the dielectric layer can be reversed provided the
voltage polarities are appropriately changed in the
manner well known in the art. It is to be appreciated
that the structure of the prese~t invention allows
alternating or direct current operation.
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