Source: http://www.docstoc.com/docs/39576575/Thin-Film-Overvoltage-Protection-Devices---Patent-4809044
Timestamp: 2014-07-11 15:16:22
Document Index: 327852054

Matched Legal Cases: ['arts 289', 'art 295', 'arts 294', 'arts 289', 'arts 289', 'arts 289', 'arts 290']

Thin Film Overvoltage Protection Devices - Patent 4809044
United States Patent: 4809044
4,809,044
Solid-state overvoltage protection devices, preferably formed of deposited
thin film, chalcogenide, threshold switching materials, typically include
at least one elongated current conduction path through an elongated
cross-sectional area of the threshold switching material. The
cross-sectional area is formed with a length far exceeding the effective
width thereof for distributing the transient current produced by
overvoltage conditions over a relatively large area. In this manner, the
concentration of localized heating effects can be avoided.
06/936,553
257/3  ; 257/4; 257/741; 257/77
H02H 9/04&amp;nbsp(20060101); H01L 045/00&amp;nbsp(); H01L 029/44&amp;nbsp(); H01L 029/46&amp;nbsp(); H01L 029/86&amp;nbsp()
357/2,68,81,80,71,67,56
Pearson et al. &quot;Filamentary Conduction in Semiconductor Glass Diodes&quot;, Appl. Phys. Lett. vol. 14, No. 9, May 1969, pp. 280-282..
Assistant Examiner:  Lamont; John
filed Aug. 22, 1986 now abandoned.
1.  A solid-state overvoltage protection apparatus of the type having a plurality of spaced electrodes and a body of threshold switching material disposed therebetween, said body having
a high electrical resistance to provide a blocking condition for substantially blocking current therethrough at operating voltages below a first nominal voltage level and a lower electrical resistance at overvoltages above said nominal voltage level to
provide a conducting condition for conducting current therethrough, the improvement comprising in combination:
means for providing an elongated current conduction path of substantially uniform distance between said electrodes through said body confined to an elongated cross-sectional area of said body transverse to the direction of current flow, said
cross-sectional area having an effective length along the major dimension of said area which is at least about ten times greater than the maximum effective width of said area,
2.  An apparatus as in claim 1 wherein said plurality of electrodes each has an elongated surface portion adjacent to and in intimate electrical contact with said body, wherein one of said elongated surface portions is physically spaced
equidistantly along its length from and operatively disposed with respect to another of said elongated surface portions, thereby providing said elongated current conduction path of substantially uniform distance between said two associated elongated
3.  An apparatus as in claim 2 wherein said two associated elongated surface portions are generally non-coplanar.
4.  An apparatus as in claim 1 further including a highly thermally conductive substrate disposed below said threshold switching material.
5.  An apparatus as in claim 1 wherein said substrate has an insulating surface, thereby substantially preventing current flow between said electrode layers and said substrate.
6.  An apparatus as in claim 4 wherein said substrate is formed from a conductive metal.
7.  An apparatus as in claim 3 wherein said substrate is substantially formed from an insulating material selected from the group consisting of deposited diamond, quartz, sapphire, and single-crystal-semiconductor material.
8.  An apparatus as in claim 1 wherein said threshold switching material is a substantially amorphous semiconductor material.
9.  An apparatus as in claim 8 wherein said amorphous semiconductor material includes at least one chalcogenide element.
10.  An apparatus as in claim 1 wherein said threshold switching material has a threshold voltage value substantially equal to said first voltage level, and wherein said high electrical resistance in response to a voltage substantially above said
threshold voltage value very rapidly decreases in at least one portion of said path to said lower electrical resistance which is orders of a magnitude lower than said high electrical resistance to provide said conducting condition.
11.  An apparatus as in claim 1 wherein said plurality of electrodes each include a film of electrically conductive, non-single-crystal, phase-stable, non-switching carbon material in intimate electrical contact with said body.
12.  An apparatus as in claim 1 wherein said electrodes each include a thin film refractory material.
13.  An apparatus as in claim 12 wherein said refractory material is in direct contact with threshold switching material.
14.  An apparatus as in claim 1 wherein said threshold switching material and said plurality of electrodes are formed from vacuum deposited thin film materials.
15.  An apparatus as in claim 14 wherein said thin film materials are arranged in layers stacked one on top of the other such that when said current conduction path is formed in said apparatus between said plurality of electrodes said path
extends substantially vertically through said threshold switching material.
16.  An apparatus as in claim 14 wherein said electrodes are spaced substantially horizontally apart from one another, and that at least a portion of said threshold switching material extends generally between said layers such that when said
current conduction path is formed, it extends substantially horizontally through said material.
17.  An apparatus as in claim 14 wherein one of said electrodes includes a non-insulated part;  a first part of another of said electrodes is disposed above and horizontally displaced with respect to the nearest non-insulated part of said one of
said electrodes and said threshold switching material is disposed therebetween such that when said current conduction path forms in said apparatus, it extends substantially diagonally through said threshold switching material between said parts of said
18.  An apparatus as in claim 14 wherein at least one of said electrodes and said threshold switching material have been patterned to form a mesa structure.
19.  An apparatus as in claim 1, further comprising:
20.  An apparatus as in claim 19 wherein said opening defines said maximum effective width and limits said width to less than about five microns.
21.  An apparatus as in claim 1 wherein said elongated current conduction path is serpentine.
22.  An apparatus as in claim 1 wherein at least one of said electrodes is patterned to have a plurality of electrically interconnected sections spaced apart from one another at predetermined angles.
23.  An apparatus as in claim 22 wherein said one patterned electrode includes at least three leg sections interconnected at a common central node section with each such leg section extending outwardly therefrom in a direction different from the
other two leg sections.
24.  An apparatus as in claim 23 wherein said patterned layer includes a plurality of common node sections interconnected by at least one of said leg sections.
25.  An apparatus as in claim 24 having at least four common node sections, with each such common node sections being electrically and physically interconnected to two leg sections which each lead to another distinct common node section, thereby
Providing redundant electrical paths to each common node sections.
26.  An apparatus as in claim 22 wherein said plurality of leg sections each is associated with its own elongated current conduction path through said body of threshold switching material which is confined to a distinct elongated cross-sectional
area of said body thereunderneath transverse to the direction of current flow therethrough, each of said cross-sectional areas having an effective length along its major dimension which is at least about ten times greater than its maximum effective
width.  Description
This invention relates in general to solid-state overvoltage protection devices, and in particular to thin film semiconductor devices and structures utilizing substantially amorphous threshold switching material for suppression of high speed
The need to protect electronic circuitry from overvoltages, especially transient overvoltage conditions, is well known.  Most electronic components are only built to withstand the application of certain limited voltages across them, and will be
damaged or at least seriously malfunction if far higher voltages are applied.
There are many sources of transient overvoltages, such as lightning, electrostatic discharge (ESD), electromagnetic induction (EMI).  Failure of circuit components may also allow excess voltages to be applied across other circuit components.
Inductive surges are yet another source of overvoltage transients.
Lightning, ESD and inductive surges are all capable of producing very rapid high voltage transients.  An inductive surge produced by interrupting a running 115 volt motor can be as high as 1,000 volts or more, for example.  Electrostatic
discharges, such as those produced by a person walking on a wool rug on a dry winter day, can easily result in a charge of tens of thousands of volts.  Although such electrostatic discharges usually involve a relatively minor flow of current, they, like
inductive surges, are sufficient to destroy many types of microelectronic circuits.  Overvoltage transients caused by lightning can deliver by direct strikes large amounts of currents at tens of thousands to hundreds of thousands of volts.  By EMI,
lightning can generate high voltage transients in the megahertz frequency range and higher ranges.
Conventional means for dealing with relatively small overvoltages include shunting capacitors, breakdown diodes, varistors and inductive coils.  Breakdown diodes such as zener diodes when reverse biased beyond a certain threshold voltage conduct
large currents.  Like virtually all overvoltage protection devices, such a diode is placed ahead or &quot;upstream&quot; of or in parallel with a circuit element to be protected, and shunts excess voltage applied thereacross to a discharge path such as a neutral
line, D.C.  common line, chassis or ground.  However, such diodes are capable of only handling limited overvoltages without becoming permanently damaged themselves.
Varistors, which are typically made of pressed powders, act somewhat like zener diodes, in that they offer a high impedance at low voltages and a relatively low impedance at high voltages.  However, they are distinguished from zener diodes in
that their current characteristics are symmetrical rather than asymmetrical, and thus can offer protection against overvoltage in both directions.
Inductive coils or chokes, while unable to protect circuitry from low frequency or static overvoltages, do tend to filter out rapid voltage transients by presenting a large impedance.  Since they also present high impedance to high frequency
signals, they are inappropriate for protecting high frequency circuitry from high frequency overvoltages.  Such chokes are also normally relatively bulky and expensive.
Spark gaps are another form of overvoltage protection associated with higher power devices, and recently miniaturized forms of them have been developed for use on P.C.  boards and the like.  Spark gaps contain two opposing electrodes separated by
a gas, such as air, which has a desired breakdown, or sparking voltage.  When an overvoltage is applied across the spark gap, its nonconductive gas becomes ionized, forming a relatively low resistance path between its electrodes.  Although spark gaps
have beneficial uses, they usually are not very appropriate for use in solid-state circuitry because they are not solid-state devices and because they are usually fairly large, even in miniaturized form.  Also the time required for the operation of spark
gaps is usually too slow to provide full protection from extremely rapid transients.
Several types of integrated circuits, CMOS for example, are notoriously sensitive to static electricity, particularly before being inserted into a larger circuit on a P.C.  board.  Furthermore, the CMOS circuits themselves are typically unable to
handle any significant power, so that it is difficult and expensive to arrange on-chip protection by exclusively dedicating certain portions of the chip to such a protection function.  Thus, there is a definite need for extremely high speed and/or high
power protection that can be readily incorporated directly into all types of microelectronic circuitry, as an integral part thereof, to protect such circuitry at all times.
As a result of the nuclear age a new and very threatening source of overvoltage transients is made possible by the phenomenon known as the nuclear electromagnetic pulse or &quot;EMP&quot;.  EMP will be produced by the Compton electrons scattered by gamma
rays from a nuclear explosion colliding with air molecules of the upper atmosphere.  Theoretical studies have indicated that if a nuclear device were exploded at a high altitude above most of the earth&#39;s atmosphere a large EMP generated therefrom would
have sufficient intensity to induce a large current in conductors hundreds or thousands of miles away to destroy electronic equipment connected to or containg such conductors.
EMP is particularly difficult to protect against for three reasons: (1) the extremely rapid rise time; (2) the expected intensity, and (3) the ubiquitious presence, i.e., all conductors of any appreciable length not enclosed with a suitable
Faraday shield will act as an antenna, and thus be subject to severe electrical transients due to the EMP.  It has been estimated that EMP will produce an extremely high overvoltage within approximately one nanosecond or less and reach a peak field in
only about 10 nanoseconds, before trailing off in about one microsecond.  The peak field produced by a one-megaton warhead exploding in the upper atmosphere may be as high as 50,000 volts/meter.  Further details about the nature of EMP and the
inadequacies of conventional overvoltage protection devices to protect against them is found in &quot;Electromagnetic pulses: potential crippler,&quot; IEEE Spectrum, May, 1981, pp.  41-46.
Most conventional solid-state overvoltage protection devices are too slow or limited in their power handling capabilities to provide full protection against the effects of very close lightning strikes or EMP.  This is because such lightning
strikes and EMP can produce overvoltages two or three orders of magnitude or more above the normal operating voltages of the integrated circuits subjected to such transients, thus leading to enormous current surges capable of destroying virtually almost
all types of solid-state semiconductor protection devices.  As the energy content of such pulses is increased, the problem becomes more severe, and requires extremely rugged, high ampacity overvoltage protection devices, preferably incorporated at the
integrated circuit level, to handle any transients which reach such microelectronic circuits.  As the size of microelectronic circuit elements is reduced, the problem also becomes more severe since less energy is required to damage smaller devices.  To
avoid creating problems, overvoltage protection devices, when inserted into or included as part of the electronic circuit to be protected, must not impose undue insertion losses in the circuit, or decrease switching speeds or band width y adding
significant amounts of capacitance.
One class of overvoltage protection devices which has long held great potential for very high speed transient suppression applications are Ovonic threshold switching devices of the type first invented and announced by S. R. Ovshinsky in the
1960&#39;s.  U.S.  Pat.  Nos.  3,171,591 (1966) and 3,343,034 to S. R. Ovshinsky (1967) specifically teach that this type of threshold switching device is suitable for use as surge suppressors, such as for transient inductive pulses and the like.  Such
switches have been known since at least 1968 to have a switching speed of less than 150 picoseconds, see, e.g., S. R. Ovshinsky, &quot;Reversible Electrical Switching Phenomena in Disordered Structures&quot;, Physical Review Letters, Vol. 21, No. 20, Nov.  11,
1968, p. 1450(c).
R. Callarotti, et al., &quot;Transmission Line Protection With Thin Film Chalcogenide Glass Devices,&quot; Thin Solid Films, Vol. 90, pp.  379-384 (1982), suggest that an Ovonic threshold switch of a thin film of chalcogenide glass is well suited for
protecting a transmission line from EMP.  A detailed mathematical analysis is presented therein in support of this view.
In U.S.  patent application Ser.  No. 666,582 filed Oct.  30, 1984 by G. Cheroff et al., which is assigned to the assignee of the present invention, a number of overvoltage protection devices using Ovonic threshold switching materials are
proposed.  These devices include various electrical connectors with Ovonic threshold switches providing a path for shunting transients to the connector casings, and integrated circuits and printed circuit boards having a thin film of Ovonic threshold
material overlying the top wiring layer for providing protection for all conductors forming part of the top wiring layer.  These devices are intended for use in protecting against EMP, ESD and other high voltage transients.
Ovonic threshold switching devices may be generally described for the purposes used herein as a switching device which has a bistable characteristic, including a threshold voltage and a minimum holding current.  Specifically, the device includes
a semiconductor material and at least a pair of electrodes in contact therewith, wherein the semiconductor material has a threshold voltage value and a high electrical resistance to provide a blocking condition for substantially blocking current
therethrough, and wherein the high electrical resistance in response to a voltage above the threshold voltage value very rapidly decreases in at least one path between the electrodes to a low electrical resistance which is orders of magnitude lower than
the high electrical reistance, which provides a conducting condition or path for conducting current through the semiconductor material.  The conducting condition or path is maintained in the device so long as at least a minimum holding current continues
to pass through the conducting path within the device.  When the current falls below this minimum current value, the device rapidly reverts to its high resistance blocking condition.  The voltage drop across the semiconductor material in a threshold
switch when in its conducting condition is a fraction of the voltage drop across the material when in its high electrical resistance blocking condition, as measured near the threshold voltage value of the switch.
Many different combinations of atomic elements when combined in the proper proportions and manner have been shown to produce a semiconductor material having the aforementioned threshold switching action.  Most commonly, chalcogenide glasses, such
as Te.sub.39 As.sub.36 Si.sub.17 Ge.sub.7 P.sub.1, are used.  Examples of such materials and threshold switching devices made therewith are found in the following list of U.S.  patents, all of which are assigned to the assignee of the present invention,
and all of which are hereby incorporated by reference:
______________________________________ 3,271,591 3,571,671  3,343,034 3,571,672  3,571,669 3,588,638  3,571,670 3,611,063  ______________________________________
Threshold switches are generally two terminal devices, and have been shown in a number of configurations, including one having a pair of electrodes arranged in the form of interleaving metallic fingers or combs (see FIG. 7 of U.S.  Pat.  No.
3,271,591 to S. R. Ovshinsky).  Since they exhibit symmetrical current-voltage (I-V) characteristics, have been applied typically in alternating current applications.  They are ambipolar devices, that is the current in the conduction path therein
consists of both holes and electrons.  They can have extremely high current densities.  If driven properly, threshold switches can have extremely fast switching speeds, such as into the nanosecond region and below, and make excellent surge suppression
devices.  Typically, a threshold switch is constructed of a thin film of preferably amorphous semiconductor material, and may be described as a semiconducting glass, although there are a number of other forms of threshold switches such as those described
in U.S.  Pat.  No. 3,715,634 to S. R. Ovshinsky.  Two terminal threshold devices, once turned on, cannot be turned off, except by reducing the current through the device below its minimum holding current for the requisite period of time, which is
typically well under one microsecond.
The aforementioned patents and patent application, while disclosing a number of useful structures and configurations for Ovonic threshold switching devices in a variety of applications, do not disclose how to optimize the design of such devices
for high power, extremely high speed applications.  In particular, the foregoing references do not specifically teach any method for avoiding localized concentrations of currents in the threshold switching material which have been known to be of such
intensity as to ablate the material or electrodes in contact therewith.  The patents also do not teach how to scale up the size of integrated threshold switching devices so that the devices may reliably be used to handle transient currents in excess of
several hundred milliamps, such as 5 amps, 10 amps or above.
Accordingly, the objects of the present invention are to provide an overvoltage protection device or apparatus which has at least several of the following attributes: (1) is capable of being scaled up to handle relatively large currents; (2) is
of a highly efficient thermal design to allow for dissipation of heat due to the shunting of current produced by extremely large overvoltages; (3) produces minimum insertion losses when in use, and has minimal capacitance; (4) has multiple current paths
for shunting current through a threshold switching material, including redundant interconnections to such current paths for increased reliability; (5) is capable of extremely high speed operation; and (6) presents minimum inductance when in use to
facilitate such high speed operation.
Another important object of the present invention is to provide and overvoltage device structure which is capable of confining a filamentary current into one or more selected elongated current conduction channels, as a means of obtaining a
structure well suited for handling large transient currents and dissipating any heat generated thereby.
In light of the foregoing objects, one aspect of the present invention provides a solid-state overvoltage protection apparatus of the type having a plurality of spaced electrodes and a body of threshold switching material disposed therebetween,
said body having a high electrical resistance to provide a blocking condition for substantially blocking current therethrough at operating voltages below a first nominal level and a lower electrical resistance at overvoltages above the first nominal
voltage level to provide a conducting condition for conducting current therethrough.  The improvement in this apparatus comprises in combination: means for providing an elongated current conduction path of substantially uniform distance between said
electrodes through the body confined to an elongated cross-sectional area of the body transverse to the direction of current flow, said cross-sectional area having an effective length along the major dimension of the area which is at least about ten
times greater than the maximum effected width of the area, whereby relatively large currents associated with overvoltages which may flow therethrough are distributable over said elongated area.
The plurality of electrodes in the above-described apparatus may each have an elongated surface portion adjacent to and in intimate electrical contact with the body of threshold switching material, wherein one of the elongated surface portions is
physically spaced equidistantly along its length from and operatively associated with another of said elongated surface portions, thereby providing said elongated current conduction path of substantially uniform distance between said two associated
elongated surface portions.  The apparatus may also include a highly thermally conductive substrate disposed below said threshold switching material.  The substrate may be formed from an electrically conductive metal covered with an insulating layer, or
may be formed from an insulating material.
The overvoltage protection devices of the present invention may have vertical, horizontal or diagonal current conduction paths through the body of threshold switching material.  At least one of the electrodes and the threshold switching material
may be patterned to form a mesa structure.  Alternatively the overvoltage protection devices or apparatus of the present invention may include a layer of insulating material having an elongated opening therein in which at least a portion of said
threshold switching material extends, and wherein one of said electrodes is disposed substantially in said opening, and another of said electrodes is disposed above said portion of said threshold switching material in said opening, such that when the
current conduction path is formed in the apparatus it extends vertically between the electrode portion through said portion of threshold switching material disposed in said opening.
The apparatus of the present invention may also have at least one of the electrodes formed from a thin film layer of conductive material patterned to have a plurality of electrically interconnected sections spaced apart from one another at
predetermined angles.  For example, such patterned electrode-forming layers may include at least three leg sections interconnected at a common central node section which each extend outwardly therefrom in a direction different from the other two leg
sections.  The patterned layer may further include a plurality of common node sections interconnected by at least one of said leg sections.
In the various devices of the present invention, at least a portion of said body of threshold switching material preferably has the property that it changes from a high resistance state to a low resistance state when the voltage applied across it
exceeds a given threshold voltage and maintains that low resistance state as long as a certain minimum maintanence voltage substantially less than the threshold voltage is maintained across it.  This characteristic of Ovonic threshold switching material
makes it ideally suited for handling high power transients, since it minimizes the generation of heat within an overvoltage protection device, and also acts to clamp the overvoltage to very low levels.
FIGS. 1 through 12 are cross-sectional side views of the several electrical devices of the present invention all of which can be configured as overvoltage protection devices to suppress high speed transients.  Most of these devices are
particularly well suited for forming high power overvoltage protection devices due to their highly efficient thermal design which makes it possible to control and quickly dissipate any heat which may be developed from handling high transient or even
moderate continuous overvoltages.  FIGS. 13 through 16 show various possible plan views of the electrical devices of the present invention, virtually all of which provide for efficient distribution of any heat generated in the device to highly thermal
conductive substrates and/or electrodes where it can be harmlessly dissipated.
The preferred embodiments of the devices of the present invention shown in FIGS. 1 through 12 are current-carrying solid-state semiconductor devices having a plurality of metallic electrode layers, a plurality of thin film layers of carbon
material associated with the electrodes, and a body or layer of semiconductor material in intimate electrical contact with the thin films of carbon material.  Each of the electrodes are in intimate electrical contact with its respective thin film of
carbon material such that current flows into one electrode through its layer of carbon material through the semiconductor material into the second layer of carbon material and from there into the second electrode.  (Although not preferred, the devices of
the present invention may also be constructed without the carbon films, provided that the remaining electrode layer in contact with the semiconductor layer is fully compatible with the semiconductor material.)
It is worth noting that our U.S.  patent application Ser.  No. 936,552 filed concurrently herewith and entitled &quot;Thin Film Electrical Devices With Amorphous Carbon Electrodes And Method Of Making Same&quot; discloses and broadly claims various aspects
of the subject matter of FIGS. 1 through 8B presented below.
current flows into one electrode through its layer of carbon material through the semiconductor material into the second layer of carbon material and from there into the second electrode.
The films of carbon material 36 and 40 are preferably deposited using DC magnetron sputtering.  Typical process parameters are a substrate temperature about 100 degrees C., a pressure of 0.5 pascal, a deposition rate of 200-300 angstroms per
TABLE I  ______________________________________ Reference  Exemplary Range of Typical  Numeral Material Thicknesses Thickness  ______________________________________ 42 molybdenum 1,500-25,000  5,000  40 a-carbon 100-2,000 1,000  38 Te.sub.39
As.sub.36 Si.sub.17 Ge.sub.7 P.sub.1  100-50,000  5,500  36 a-carbon 100-2,000 1,000  34 molybdenum 1,500-25,000  5,000  ______________________________________
about 0.6 micron thick for the typical thicknesses of layers 38-42 shown in Table 1.  After layer 70 has been deposited conventional photolithographic and etching techniques are used to create openings or vias 72 and 74 therein above mesa structures 62
of the bottom electrode and finger portion 152 of the top electrode.  In the illustrated embodiment, the transverse overlap 150 equals the width of smmaller finger portion 151.  The bottom and top electrodes also include larger contact portions 153 and
surface layer as indicated in Table II (thickness in angstroms):
Several hundred devices were made simultaneously on the substrate, which then was diced up to obtain individual devices inserted into DO-18 packages for testing.  Our prototype devices 146 demonstrated excellent long-term D.C.  stability, even
when operating at temperatures between 100.degree.  C. to 150.degree.  C. We attribute the success of these prototypes of device 146 to the use of amorphous thin film carbon as barrier layers to help stabilize the morphology of active layer 38, and to
the preferred fabrication and sealing of the central region of device 146 in a continuous partial vacuum.
the various directional electron beam sputtering or thermal evaporation steps.  Layers 34 through 42, and layer 148 were successively deposited as shown using for patterning four metal masks, each having an opening corresponding to the desired outline of
Then a suitable photoresist may be applied over an entire structure and patterned so that a central portion 138 of the semiconductor layer is not subject to etching, while the portions of the semiconductor layer 3B everywhere except under the electrodes
A preferred method for fabricating the structure 160 shown in FIG. 7 is disclosed in the partially completed structures of FIGS. 8A and 8B.  To create the partially completed structure 190 of FIG. BA a layer of insulating material 164 is
deposited on substrate 32, and a layer of photoresist is deposited thereover and patterned into a mask 1490.2 required for the etching step which immediately follows.  Layer 164 is then subject to a suitable anisotropic dry etch which removes the right
half of the layer, and in so doing leave angled face 166.  The anisotropy of the dry etching step is controlled so as to produce a rather steep angle 176 on the order of 45 to 90 degrees.  Then, the photoresist mask is removed.  Next, the electrode layer
34 is directionally sputtered onto the substrate 32 and patterned insulation 164 as shown by arrows 192 so as to deposit electrode material on top of the insulating layer 164 and on a surface portion 193 of the substrate 32, without depositing much if
each of the layers through suitable metal masks provided with opening therein, in the manner described with respect to FIG. 4B.
FIG. 9 shows a preferred embodiment of an all thin film electrical overvoltage protection device 220 of the present invention having multiple diagonal current conduction paths, which increases reliability by reducing the concentration of
localized heating effects therein.  A preferred method for constructing the structure 220 is illustrated in FIGS. 10 and 11.  As with the FIG. 7 structure, only one layer of thin film carbon need be deposited to make structure 220.  Structure 220
preferably has a highly thermally conductive substrate, which may be any suitable material or combination of materials, such as copper substrate 32&#39; which may be covered with an insulating layer 132, like in the FIG. 5 embodiment.  Over insulating layer
132 is bottom electrode layer 34, which is covered by thin film carbon layer 36.  An insulating layer 222 and conductive electrode forming layer 224 such as molybdenum are deposited one after the other on top of layer 36 and subsequently patterned as
shown in FIG. 10, using a patterned photoresist layer 229 and dry etching techniques, into a plurality of spaced mesa structures such as structures 226 and 228, which respectively include patterned electrodes 222a, 224a and 222 b, 224b of layers 222 and
224.  Between the adjacent mesa structures are channels, such as channel 230 between which is defined by opposed facing edges of mesa structures 226 and 228 and the portion of layer 36 therebetween.  After removing photoresist 229, the patterned
electrodes of layer 224 which remain are subjected to a wet etch with a suitable solvent that attacks only layer 224, to obtain the reduced size electrodes such as segments 224a and 224b shown in FIG. 9.
Next, a layer 232 of thin film carbon material, which may be 60 to 200 angstroms thick for example, is deposited over the mesa structures and channels, such that selected portions of layer 232 end up lining the bottom of the channels illustrated
by portion 232&#39; in channel 230, while other portions of layer 232, such as portion 232&quot;, end up on top of the mesa structures as shown in FIG. 11.  Thereafter, a layer 38 of threshold switching material is deposited over the discontinuous portions of
layer 232 and substantially fills the channels, as illustrated by portion 38a in channel 230.
A second layer 233 of photoresist is then deposited and patterned as shown in FIG. 11 in preparation for the etching of layers 38 and 232.  Both layers 232 and 3B are then patterned to provide openings above the mesa structures between the dotted
lines 238 in FIG. 11.  As shown in FIG. 9, this leaves portions of layer 232 on top of the mesa structures such as portions 232a and 232b respectively positioned on top of mesa structures 226 and 228 near channel 230.  Next an insulating layer 240 is
deposited and patterned as shown in FIG. 9 to open vias above the mesa structures such as vias 246 and 248 to permit electrical contact to be made between a subsequently deposited highly conductive top metallization layer 242 which is preferably
sputtered aluminum and the patterned upper electrodes such as electrodes 224a and 224b formed from layer 224.  Layer 242 is preferably patterned as shown to reduce stray capacitance between layer 34 and itself.  After this patterning, layer 242 includes
patterned sections above each mesa structure, such as segments 242a and 242b above mesa structures 226 and 228, which effectively each become the part of the upper electrode for the device below.  Layer 242 also preferably includes connecting traces 242c
and 242d as shown which electrically interconnect the sections above each mesa structure.  Layer 242 is preferably made of sputtered aluminum which is preferred for its high electrical and thermal conductivity, low cost and compatibility with other
semiconductor processes and materials.  However, other sufficiently conductive material may be used for layer 242.  To prevent diffusion of aluminum into layer 38, insulating layer 240 should be made from a material such as siliconoxynitride and have a
thickness which will prevent the aluminum from layer 242 from migrating into layer 38.
When structure 220 is to be placed in operation, high quality electrical connections are made to layers 242 and 34 using a conventional technique such as down-bonding to contact pads so that structure 220 is connected across or in parallel with a
circuit or circuit element to be protected from overvoltage conditions.  Accordingly, the upper electrodes made from layer 224 and its associated thin film carbon layer such as 232a are at one potential, while the bottom electrode layer 34 and its
associated thin film carbon layer 36 at the bottom of the channels such as thin film carbon portion 232&#39; at the bottom of channel 226 are at another potential.  When the voltage across carbon layers 232a and 232&#39; or 232b and 232&#39; exceed the threshold
voltage of switching layer 38 therebetween, a current filament is established between the carbon electrode layers 224a and 224b where indicated by dotted lines 250a and 250b.  Thus two filamentary current conduction paths will be established in channel
230.  Since carbon electrode layers 232a and 232b are skewed relative to and face away from the carbon electrode layer 232&#39;, the current filaments 250a and 250b should not expand into the central portion of the channel 230, but remain confined along the
edges of the channel as shown by dotted lines 250a and 250b.
The thicknesses of the various layers in the FIG. 9 device may be approximately the same as those given with respect to the device in FIG. 4A, for example.  Preferably, the insulating layer 222 used to produce the mesa structures should be
sufficiently thick and carbon layer 224 should be sufficiently thin so as to result in a significant elevational difference between the bottom of the channel, such as channel 230, and the top of the insulating layer, so as to assure discontinuities
between the upper and lower portions of the carbon layer in the manner depicted in FIG. 11.
Structure 260 shown in FIG. 12 is an alternative embodiment of the present invention constructing in a manner similar to that of structure 220 in FIG. 9, but has reduced inter-electrode capacitance, since its upper electrodes do not overlie its
lower electrodes.  The substrate 262 in FIG. 12 is an electrically insulating, thermally conductive substrate such as deposited amorphous diamond, sapphire, fused quartz or even single crystal silicon.  Deposited amorphous diamond is particularly
preferred due to its high thermal conductivity.  An insulating layer 222 is deposited directly on top of the substrate 262, and patterned with photoresist techniques to form mesa structure 222a and 222b and provide a channel 270 therebetween.  With the
patterned photoresist still on top of the patterned mesa structures, a layer 272 of highly conductive electrode material, such as aluminum, is deposited over the entire structure in a thickness substantially less than that of the insulating layer 222.
The photoresist is removed, and the portions of layer 272 remaining in the channel are thereafter patterned as shown by heavily attacking the layer with an isotrophic wet etch.  This ensures that no aluminum remains on the upper side walls of mesa
structure 38.
Next, an electrode-forming layer 224 is deposited over the patterned layers 222 and 272, and subsequently subjected to an isotropic wet etch to reduce its size to that shown in FIG. 12.  This leaves segments 224a and 224b on top of insulating
layer portions 222a and 222b respectively, while segment 224&#39; is positioned in channel 270 substantially below the upper surface of layer 222.  This elevational difference and etching ensures that upper electrode segments 224a and 224b are physically
distinct and electrically isolated from the lower electrode segments 224b&#39;.  Thereafter, layer 232 of thin film carbon is deposited to serve as the barrier layer for both the upper and lower electrodes.  Thereafter, the processing of structure 260 from
this point on is substantially similar to that of similar layers or parts of structure 220 in FIG. 11.  FIG. 12 shows that the patterned upper metallization layer segments 242a and 242b need not be interconnected, if this is not desired.  Improtant
advantages of the structures of FIGS. 11 and 12 is that they can be made with a reduced number of layers and aligned mask steps which have very large alignment tolerances.
It is preferable to provide an appreciable mass of thermally conductive material in intimate thermal contact with the upper and/or lower electrodes of the overvoltage protection devices of the present invention for maximum dissipation of any heat
which may be generated during device operation, especially for high energy applications.  This helps avoid localized concentrations of heat which may, in severe instances, result in ablation of the threshold switching material or electrode-forming layers
adjacent to a local hot spot.  Those in the art will appreciate that a relative thick, highly thermally conductive substrate, and a thick top metallization layer (whether patterned or continuous) will greatly help dissipate the heat.  For some
applications it may be sufficient to Provide such heat dissipation means in on one side of the devices of the present invention, such as the substrate side.  However, it is preferred, where possible, to provide for such heat dissipation means on all
electrode surfaces near Portions of the semiconductor body 38 where current filaments may form, particularly wherein threshold switching material having a high current density, such as above about 100 A/cm.sup.2, is used for layer 38.
Ovonic threshold switching material has extremely high current densities, on the order of 2.times.10.sup.4 A/cm.sup.2.  While these semiconductor materials are fairly rugged, radiation hard, and can withstand reasonable amounts of heating
effects, the contacts made with the semiconductor material are more suspect due to resistive heating effects.  In this regard, it it has been noted that most of the voltage drop across a threshold switching device made using such Ovonic switching
materials occurs across the contacts when the device is being driven hard.  Commonly, the voltage drop across a body of threshold switching material in an overvoltage protection device of this type is only approximately one tenth of a volt and is
somewhat independent of filament length.  This is due to the current conduction mechanism in Ovonic threshold switching materials being a plasma of both electrons and holes, wherein very little resistance to current flow is observed above the critical
holding voltage or field required to sustain such a plasma, once initiated.
Since Ovonic switching materials have such high current carrying capacity, we believe that in high power devices employing such materials, care must be taken to minimize the resistive heating effects in the electrodes, lest such effects result in
the degradation or destruction of the device under severe operating conditions.  This can be done in part by minimizing the thicknesses and maximizing the cross-sectional areas of the barrier layer materials, such as thin film carbon and/or molybdenum,
both of which are more resistive than preferred metallic conductors such as aluminum.
In FIGS. 13 through 16, various plan views of overvoltage protection devices of the present invention are shown which provide additional protection against concentrated localized heating effects.  These embodiments have at least four important
advantages over conventional threshold switching devices.  Firstly, they avoid any concentrated localized heating effects by restricting the maximum width of a cross-sectional area of conductive filament in a body of threshold switching material to a
predetermined tolerable limit, which may be 25 microns, 10 microns, or even 5 microns or less, while providing for a much greater length such as 5, 10, 20, 50 or more times the width.  Secondly, they distribute the current paths over a relatively large
surface area, thereby making it possible to employ substantial masses of highly thermally conductive material as heat sinks to help distribute the heat harmlessly away from the filamentary current paths in the switching material.  Thirdly, they can be
scaled up in size to handle high power transients.  Finally, they provide redundant active sections, and redundant interconnections and electrodes to the various active sections of the device.  Such redundancy allows the overvoltage protection apparatus
to function even when one or more active sections of the device or its electrodes or interconnections have been open circuited by thermal stress.  Thus, the apparatus can stand up well under repeated very high current, high speed transients such as might
be generated by multiple lightning strikes or EMPs.
In FIG. 13, a partial plan view of an electrical device 280 of the present invention having a nonhorizontal elongated current conduction path arranged between interdigitated electrodes is shown.  The thin film structure 260 of FIG. 12 may be used
to construct the electrical device 280.  In particular a partial cross-section taken along line 12--12 of FIG. 13 would appear as shown in FIG. 12.  The structure 280 has a top metallization layer patterned into an upper electrode 242 which has a
plurality of finger-like sections, such as fingers 242a and 242b extending from a common connecting section 242c.  Connected to common section 242c and extending in the direction opposite the fingers is section 242d which may lead to a connection means,
such as a contact pad (not shown), for making an electrical connection to top electrode 242.  The bottom electrode 272 similarly has finger-like sections 272a and 272b, which are in the channels between the patterned mesa structures of insulating layer
222, and a common base section 272c to which its fingers are interconnected and a contact pad section 272d (partially shown) for making electrical connections thereto.  As shown in FIG. 13, the top electrode layer 242 is preferably formed above the
patterned insulating layer 222, while the bottom patterned electrode layer 272 is preferably formed adjacent to and in between the channels such as channel 270 defined by the side walls of insulating layer 222.
The width of the channels such as width 284 of channel 270 between insulating mesa structures 222a and 222b, should be sized for effective heat dissipation, such as approximately 5 to 50 microns or more in width.  As the channel width increases,
the average amount of heat being dissipated per unit area of the electrical device decreases.  The width of electrode layer 272 can also be increased correspondingly, thus increasing its ampacity so as to reduce resistance heating effects therein.
The electrical device 280 may also be constructed using the thin film structure 220 of FIG. 9.  In such a case, the bottom electrode layer would not have separate fingers, as does layer 272 shown in FIG. 13, but would consist of a solid plane of
material everywhere under the fingers of upper electrode layer 242.  (Upper electrode 242 could even be left unpatterned, if desired, for minimum inductance and maximum current-carrying and heating dissipating capacity.) Although only several fingers of
limited size are shown in FIG. 13, it will be appreciated that the length 286 and number of interdigitated fingers may be increased as desired, to increase current capacity of apparatus 280.
To use apparatus 280, one of the electrode layers 242 and 272 is connected to a conductor or electrical device to be protected, while the other of the electrode layers is connected to a discharge path such as DC common or ground to which current
due to overvoltage applied on the other conductor can be harmlessly shunted to.  The general location of the overall elongated current conduction path 288 of device 280 between the upper and lower electrodes 242 and 272 is indicated with stippling.  The
path 288 is made of contiguous transverse and longitudinal parts 289 through 299.  In operation, overvoltage will be substantially instanteously applied everywhere between the upper and lower electrodes, thus preparing all of the threshold switching
material 38 in the path 288 for being switched to its on-state.  At least one point such as point 300 in part 295 (which is arbitrarily chosen for purposes of this example) will switch to its on-state, forming a current filament.  The filament will
thereafter very rapidly spread in directions 302 and 304 along the path 288 until the size of the elongated filaments corresponds to the maximum current produced by the overvoltage condition.  For example, the elongated filament may only need to expand
to points 306 and 308 of parts 294 and 296 of the path 288 to handle a certain amount of current.  If the current is high enough, the filament may expand along the entire length of path 288.  The speed of filament growth or propagation along path 288 is
expected to be as fast as necessary to handle any transient overvoltage condition which is applied, and may well approach the speed of light.
The foregoing operation may be further understood by considering the FIG. 9 device.  The filamentary conduction path 250a shown in FIG. 9 between thin film carbon layers 232&#39; and 232a will grow along the perimeter of the mesa structure 226 as it
expands to handle whatever currents are imposed by overvoltage conditions experienced by device 306.  Similarly, filamentary conduction path 250b shown in FIG. 9 and associated with mesa structure 228 will expand as needed and substantially encircle the
perimeter of its mesa structure if necessary.  However, assuming that channel 230 has sufficient width, the filamentary path 250a will not expand directly across the channel to path 250b or vice versa.
Recent experiments we have done confirm that the size of the current filament in an Ovonic threshold switching device, once initiated, expands and contracts as required to maintain a current density of approximately 2.times.10.sup.4 amps per
square centimeter.  Thus, when apparatus 280 operates, for example, we expect that for small currents induced by overvoltage conditions, there may be a current filament only in one or two of the individual parts of path 288, and that it may not even be
necessary for this current filament to expand to fill the entire elongated current path of the device.  However, for larger currents applied across apparatus 300, it is expected that current filaments will extend to (or form in) two or more parts of path
288, such as contiguous 294 through 296, and will include however many of the parts 289-299 of path 288 that are required to handle the current surge.  The existence of a number of distinct interdigitated fingers provides a measure of redundancy for the
apparatus 280.  If one or more of the individual fingers or parts of the path 288 do not operate or have been open-circuited during a previous current surge, the remaining contiguously connected parts of the path 288 may well provide the necessary
The individual fingers and associated layers of device 280 are preferably formed simultaneously by successively depositing and patterning thin film layers as taught or shown in FIGS. 9 through 13.  Accordingly, the individual parts 289 through
299 of path 288 should have virtually identical current-voltage characteristics.  This is because each elongated surface portion of the upper and lower electrodes of each part of the path 288 adjacent to and in intimate electrical contact with the layer
38 of switching material will be physically spaced equidistantly along its length from and operatively disposed with respect to another of the elongated surface portions, thereby providing a highly uniform distance for current flow therebetween.  In this
respect, it is preferred as is shown in FIG. 13 to round the corners of current path 288 which respectively connect the transverse parts 289, 291, 293, 295 and 297 and horizontal parts 290, 292, 294, 296 and 298 thereof, in order to avoid high field
effects associated with rectangular corners, which might well change the I-V characteristics there.  Also, no appreciable additional cost is involved in forming any additional fingers, if greater ampacity is desired, since the thin films of material are
all patterned simultaneously over the same large area of the substrate 262.  Integrally forming multiple parts of a current conduction path such as path 288 with closely matched I-V characteristics helps ensure that the various parts of the path will
relatively equally share in handling large transient currents, instead of having one part of the current path attempt to handle it all, and possibly be destroyed while doing so.
In FIG. 14, there is shown a plan view for yet another overvoltage protection device 380 of the present invention.  Although this device may be implemented using a number of the structures shown in FIGS. 1 through 12, it will be explained as
though it is constructed using the structure 118 of FIG. 4B.  Device 380 may include a large rectangular, planar bottom electrode 34 as shown in FIG. 14 and a top electrode formed of electrode layer 42 and top metallization layer 110 patterned in the
form of a cross as shown.  The top electrode includes a central node section 381 indicated by dashed lines and four leg sections 382 through 385 which each extend radially outwardly from the common central node section 382 as shown.  A cross-section of
leg section 385 taken along lines 4&#39;--4&#39; in FIG. 16 would appear similar to structure 118 shown in FIG. 4B, including having a channel 104 formed by an elongated opening 104 in the insulating layer 102.  The width 108 of the channel 104 in leg section
385 may be 2 to 25 microns, with 2 to 10 microns being preferred.  The length of the individual leg sections 382 through 385 is preferably 10 times, and may be 20 to 100 times or more than the width of the channels such as channel 104 in leg section 385. The channel or current path of device 380 is shown in dashed line extending across through the central node section and under the leg sections 382 through 384, and forms a smaller cross-like pattern.
In operation device 380 may, for example, begin to conduct at point or location 388 in channel 390 of leg section 384 when a relatively small overvoltage condition is applied to the device.  As the current increases, the current filament will
expand in all directions filling the width of the channel 390 of threshold switching material beneath leg section 384, quickly contacting the side walls of the channel adjacent either side of location 388 and continuing to expand longitudinally along the
channel as indicated by arrows 392 and 393.  When the expanding current filament reaches point or location 396, it has the opportunity to continue to expand in three orthogonal directions indicated by arrows 398, 400 and 402.  Thus, the expanding current
filament is bifurcated or split into the channels filled with threshold switching material below leg sections 382, 383 and 385.  The expansion of the current filament into these separate channels allows the current to be distributed over a much larger
area than would be possible if the threshold device simply consisted of a large pancake of threshold switching material disposed between two large pancake electrodes on either side thereof.  Thus, concentration of localized heating effects, which are
expected to impose a fundamental limitation on the maximum effective size of the pancake design, are substantially or completely avoided by the structure 380.
Referring now to FIG. 15, an overvoltage protection apparatus 420 is shown in plan view therein.  The apparatus 420 includes an upper electrode layer 422 and a continuous planar bottom electrode layer 424, both patterned as shown.  The top
electrode 422 is provided with a contact pad section 426 and redundant interconnecting traces 427 and 428 leading from contact pad 426 to the main portion of the patterned layer 422.  The top metal layer 422 includes central node sections 431 through 438
which have leg sections extending outwardly therefrom at least three orthogonal directions.  The leg sections include horizontal leg sections 441 through 449 and vertical leg sections 451 through 458.  Together the central node section 431 through 438
and the leg sections 441 through 458 form a rectangularly arranged, highly conductive grid of plural horizontal lines and plural vertical lines intersecting one another for distributing current due to overvoltage conditions over a fairly wide area.  This
interconnected grid provides redundant conductive paths in top electrode layer 422 to reach any particular point of conduction thereunderneath.
The apparatus 420 shown in FIG. 15 can be implemented using a number of the structures in FIGS. 1 through 12.  For example, the FIG. 1 structure may be used with the bottom electrode layer 34 in FIG. 1 serving as the bottom electrode 424 in the
FIG. 17 apparatus, and the top electrode layer 42 in FIG. 1 being patterned so as to form the top electrode layer 422 shown in FIG. 17.  In such an embodiment, the layer 38 of threshold switching material would not need to be patterned in the vicinity of
the various parts of the current path, although it could be if desired.  The same is true for the upper layer 40 of thin film carbon material, since due to its thinness, it would have relatively high resistivity or could readily be made so by adjusting
selected deposition process parameters.  Even when layers 38 and 40 are continuous (i.e., not patterned) as shown, the width of the elongated current conduction path beneath the leg and node sections of patterned layer 422 will be substantially confined
to the area directly under the leg and node sections due to a voltage and/or current a micron or several microns away from the edges of the patterned areas of the upper electrode that is insufficient to sustain the conduction process required for lower
resistance in any threshold switching material.  One benefit of not patterning upper carbon layer 40 is that it provides a barrier to help prevent contamination of layer 38 by subsequent processing steps, until a passivating layer is placed thereover.
For more efficient distribution of current (as well as any heat generated during operation of the device), it is preferred to also provide a layer of relatively thick aluminum over the electrode layer 422 which may be patterned if desired like
layer 422.
By properly sizing the widths and lengths of the leg sections of the apparatus 420, concentration of localized heating effects can be substantially avoided or eliminated.  It is preferred to have the length of the leg sections in the FIG. 15
embodiment be at least 5 times and preferably 10 or 20 times or more that of the width of the leg sections.  Thus, for example, if the leg sections were 10 microns wide, the rectangular area 460 in the upper lefthand corner of FIG. 17 enclosed by leg
sections 441, 444, 451 and 452 would be 100 microns by 100 microns or 200 microns by 200 microns or more.
In FIG. 16 there is shown one more embodiment of an overvoltage protection device of the present invention.  The device 480 therein has a top metallization layer patterned into a generally rectangular electrode 482, a bottom electrode 484, and a
serpentine conduction channel 486 of threshold switching material disposed therebetween.  The device 480 may be implemented using a number of the structures shown in cross-section in FIGS. 1 through 12.  For example, the FIG. 4A device could be used to
implement device 480 by simply patterning the insulating layer 102 so that opening 102 forms a serpentine shaped channel 486 as shown in dotted lines in FIG. 16.  As with the other embodiments, the FIG. 16 device beneficially limits the current
conduction or the threshold material to the area under the patterned electrode layer 482, thus preventing deleterious localized heating effects when the device is placed in operation.
The variety of configurations available for the overvoltage protection devices of the present invention, coupled with their small size and generally planar construction, allows the devices to be tailored to have a preselected impedance by
controlling resistance, capacitance and inductance.  If desired, capacitance and/or inductance can be minimized as pointed out with respect to a number of the illustrated embodiments.  Thus, the devices of the present invention when appropriately scaled
in size are particularly well suited for use in connection with microelectronic circuit applications, where large capacitances are to be avoided.  Similarly, the devices of the present invention are very well suited for extremely high speed operation, on
account of their very low inductance values.  In this regard, it is preferred to make electrical connections to the devices of the present invention with a minimum number of turns or angles so as to not unnecessarily increase the apparent inductance of
the overvoltage protection device.
The foregoing discussion of the various preferred embodiments of the present invention have all included two layers of thin film carbon as barrier layers between the threshold switching material and the more conductive electrode layers, such as
electrode layers 34 and 42 in FIG. 1.  The use of such thin film carbon layers is preferred when a overvoltage protection device having long-term highly stable device characteristics is desired.  In particular the thin film carbon layers are believed to
provide a superior barrier layer for direct current applications of thin film threshold switching devices using amorphous chalcogenide switching material of the type invented by S. R. Ovshinsky.  However, it is well known that overvoltage protection
devices made with such threshold switching material but without such barrier layers of thin film carbon, work quite satisfactorily especially for alternating current applications.  Accordingly, it is to be appreciated that all of the overvoltage
protection devices and apparatus of the present invention may be made without such thin film carbon layers.  In such embodiments the electrode layers, such as layers 34 and 42 in FIG. 1, would directly contact the threshold switching material.
Although the amorphous chalcogenide threshold switching materials are preferred for use in the embodiments of the present invention, any other suitable threshold switching material may be used, provided it can be suitably deposited or otherwise
incorporated into the devices of the present invention.
Substantially amorphous molybdenum has been disclosed above as a preferred material for making the intermediate electrode layers, such as electrode layers 34 and 42 in FIG. 1 of the illustrated threshold switching devices.  Any number of other
conductive materials may be utilized, provided they are compatible with the threshold switching material and other materials being used in the device.  For example, overvoltage protection devices made with amorphous chalcogenide threshold switching
materials may also utilize electrode layers made of tantalum, graphite, niobium, refractory metal oxides, carbides and sulphides.  Preferably, such materials are deposited using vacuum techniques in a substantially disordered generally amorphous
condition so that there is no tendency for the amorphous chalcogenide semiconductor material to assume a crystalline-like state from being in contact with such electrodes.
In each embodiment of the present invention shown in the Figures, a passivating layer of any suitable insulating material may be and is preferably deposited over the structure to provide for protection against environmental contamination and/or
unintended electrical contact with other devices or circuits.  It is preferred to use a a material for this passivating layer which has a good thermal conductivity so that a heat sink may be placed in intimate physical contact therewith to provide
additional heat dissipating capability.  A disk or thin sheet of aluminum or silver or suitable silicon-based liquid or synthetic oil material could be used as the heat sink for example.  These and other suitable heat sink designs are well known to those
in the art of designing transient overvoltage protection devices, and thus need not be described further here.
Each overvoltage apparatus of the present invention may be deposited directly on top of an existing microelectronic circuit and connected thereto so as to provide overvoltage protection to a conductor or a circuit thereon.  The thin film devices
of the present invention can be manufactured simultaneously by the dozens or thousands of units on top of a single integrated circuit (IC) chip or crystalline wafer containing many such chips.  Thus, the devices of the present invention can be
economically made an integral part of IC chips by the chip manufacturer.  Alternatively, several dozens or hundreds or more devices of the present invention can be simultaneously mass-produced on a single large area (e.g., 100 to 1000 cm.sup.2)
substrate, such as thin sheets of stainless steel with (or without) a thin layer of insulating material, synthetic plastic web materials, or glass, using batch processing techniques for making integrated solid-state devices.  The substrate may be
subsequently diced into groups of devices or individual devices and one or more for packaging in conventional fashion in cannisters or chip carriers, so they may be sold as discrete devices for use in the electronic industry.  Such packages may also be
provided with conventional heat sinks to improve the ability of the package devices to dissipate heat.  Special packages could also be designed to allow such devices to be readily incorporated into electrical connectors and the like, as taught in
aforementioned U.S.  patent application Ser.  No. 666,582.
Having thus described several preferred embodiments of the present invention, it is recognized that those skilled in the art may make various modifications or additions to the preferred embodiments chosen to illustrate the present invention
without departing from the spirit and the scope of the present contribution to the art.  For example, one or more of the structures of the present invention may be incorporated into all thin film electronic arrays or in hybrid crystalline/thin film
electronic arrays to protect solid-state circuit components therein.  In such instances, thin film electrode layer 34 may be placed on top of or replaced by a bottom electrode-forming layer which also forms part of a diode, isolation device, or other
addressing means constructed on or in the thin film structure or crystalline structure below.  The &quot;electrode&quot; or &quot;electrode layer&quot; as used in the claims below is meant to include such electrode-forming layers.  Therefore, it is to be understood that
within the scope of the appended claims the inventions can be practiced otherwise than have specifically been described above.
Thin film overvoltage protection devices, Pryor, et al., Roger W. Pryor, Napoleon P. Formigoni, Stanford R. Ovshinsky, Application number 06 936-553, Active Solid-State Devices (E.G. Transistors Solid-State Diodes), Conversion Devices, Energy Conversion Devices, memory device, memory cell, memory elements, J. Non-Cryst, chalcogenide glass, chalcogenide glasses, Thin film, Bipolar Transistors
This invention relates in general to solid-state overvoltage protection devices, and in particular to thin film semiconductor devices and structures utilizing substantially amorphous threshold switching material for suppression of high speedtransients.BACKGROUND OF THE INVENTIONThe need to protect electronic circuitry from overvoltages, especially transient overvoltage conditions, is well known. Most electronic components are only built to withstand the application of certain limited voltages across them, and will bedamaged or at least seriously malfunction if far higher voltages are applied.There are many sources of transient overvoltages, such as lightning, electrostatic discharge (ESD), electromagnetic induction (EMI). Failure of circuit components may also allow excess voltages to be applied across other circuit components. Inductive surges are yet another source of overvoltage transients.Lightning, ESD and inductive surges are all capable of producing very rapid high voltage transients. An inductive surge produced by interrupting a running 115 volt motor can be as high as 1,000 volts or more, for example. Electrostaticdischarges, such as those produced by a person walking on a wool rug on a dry winter day, can easily result in a charge of tens of thousands of volts. Although such electrostatic discharges usually involve a relatively minor flow of current, they, likeinductive surges, are sufficient to destroy many types of microelectronic circuits. Overvoltage transients caused by lightning can deliver by direct strikes large amounts of currents at tens of thousands to hundreds of thousands of volts. By EMI,lightning can generate high voltage transients in the megahertz frequency range and higher ranges.Conventional means for dealing with relatively small overvoltages include shunting capacitors, breakdown diodes, varistors and inductive coils. Breakdown diodes such as zener diodes when reverse biased beyond a certain threshold voltage conductlarge curre
Overcurrent and Overvoltage Protection devices in Pad-Mounted NITINOL Thin Film Stents and Devices