Metal Oxide Varistor With Laterally Spaced Electrodes

Abstract

A metal oxide varistor having an alpha in excess of 10 in the current density range of from 10.sup..sup.-3 to 10.sup.2 amperes per square centimeter is formed with laterally spaced electrodes adjacent a first surface. A spaced third electrode may be associated with the first surface or a second surface. To improve the current carrying capacity of the varistor body the conduction gap between the electrodes may extend along the first surface an extended distance greater than the width of the surface. The conduction gap width may be varied continuously or in discrete steps.

Description

My invention is directed to a circuit component including a metal oxide varistor having laterally spaced electrodes.

It can be generally stated that the current which flows between two spaced points is directly related to the potential difference between the points. For most known substances current conduction therethrough is equal to the applied potential difference divided by a constant, which has been defined by Ohm's law to be its resistance. There are, however, a few known substances which have been observed to exhibit non-linear resistances and which require resort to the following equation (1) to relate quantitatively current and voltage:

(1) I = (V/C).sup. alpha

Where V is the voltage between two points separated by a body of the substance under consideration, I is the current flowing between the two points, C is a constant, and alpha is an exponent greater than 1. There are many known electrical circuits in which it is quite desirable to incorporate one or more functional elements having non-linear or exponential resistance characteristics. For example, the non-linear resistance properties of silicon carbide have been widely utilized in commercial silicon carbide varistors. Typically silicon carbide varistors exhibit an alpha of no more than 6.

It has been recently appreciated that varistors having alphas in excess of 10 within the current density range of 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter may be made from bodies which are comprised of metal oxides. The metal oxide body may be formed predominantly of zinc oxide with small quantities of one or more other metal oxides being present. Metal oxide varistors having alphas in excess of 10 are disclosed in Canadian Pat. No. 831,691, issued Jan. 6, 1970, for example. While the alphas of these metal oxide varistors are identified by the current density range of 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter, which characteristically exhibits substantially constant alphas, it is appreciated that their alphas remain high also at higher and lower currents, although some decline from maximum alpha values have been observed.

The construction of a conventional metal oxide varistor having an alpha in excess of 10 is shown in FIG. 1. The metal oxide varistor 1 is formed of a sintered ceramic metal oxide body 3. The body includes a first major surface 5 and a second, opposed major surface 7. The major surfaces are separated by a thickness X. First and second electrodes 9 and 11 are associated with the first and second major surfaces respectively, so that they lie in ohmic contact therewith.

In placing the metal oxide varistor in use, when a potential difference is placed across the electrodes 9 and 11, a current is conducted through the bulk of the metal oxide body 3. Since the distance between the electrodes along the surface of the metal oxide varistor body is greater than through the bulk of the body, little, if any, current is conducted along the surface of the body. For various voltage levels across the electrodes the current follows equation 1. For a given cross-sectional area of the metal oxide body measured normal to the direction of current flow therethrough and for a given current level it has been observed that the voltage across the electrodes is a function of the thickness X.

For many circuit applications where relatively high voltage levels are desired at a given current conduction level this relationship is convenient, as it is quite simple to choose a thickness value X to yield the desired voltage characteristic for the varistor. In circuit applications where a relatively low voltage value is desired, however, for a given current conduction by the varistor, the value of X may become so small that it is quite difficult either to form or to handle the metal oxide varistor body without damage. For example, for comparatively low voltage applications a thickness for the metal oxide varistor body of only 2 or 3 microns may be indicated. Further, when comparatively low voltage characteristics are desired, the correct dimensions of the metal oxide body 3 can become quite important, as an error in thickness of only a few microns might double or halve the desired voltage characteristic.

It is an object of my invention to provide a metal oxide varistor construction in which the voltage characteristic is independent of the thickness of the metal oxide body. It is a more specific object of my invention to provide a metal oxide varistor in which a rugged and easily formed metal oxide body having no critical dimensions can be employed for even the lowest voltage applications. It is still another object to provide a metal oxide varistor according to my invention which is self-protected from overloading. It is an additional object to provide a metal oxide varistor capable of clamping at multiple voltage levels.

In one aspect, my invention is directed to the combination comprised of a substrate having first and second opposed major surfaces comprised of a sintered ceramic metal oxide varistor body lying along at least the first major surface and having an alpha in excess of 10 in the current density range of 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter. First and second electrodes lie in ohmic contact with the first major surface and are laterally spaced to form a conduction gap therebetween along the first major surface having a minimum width less than the thickness of the substrate between the major surfaces.

My invention may be better understood by reference to the following detailed description considered in conjunction with the drawings, in which

FIG. 1 is a schematic sectional view of the conventional metal oxide varistor discussed above;

FIG. 2, 3, and 4 are schematic sectional views of separate embodiments according to my invention;

FIG. 5 is a schematic circuit diagram;

FIGS. 6 and 7 are schematic sectional views of additional embodiments according to my invention;

FIG. 8 is a schematic circuit diagram utilizing the embodiment of FIG. 7;

FIG. 9 through 12 inclusive are plan views of additional embodiments according to my invention; and

FIG. 13 is a schematic sectional view of a packaged unit incorporating the varistor of FIG. 3.

In FIG. 2 a varistor 20 is shown formed according to my invention. The varistor includes a metal oxide varistor body 21 having an alpha as defined by equation 1 in excess of 10. The metal oxide varistor body may be formed according to the teaching of the Canadian patent cited above or in any other known manner. The body is provided with a first major surface 22 and a second, opposed major surface 23. The second major surface is shown to be parallel to the first major surface, but may take any geometrical form convenient for the specific application to which the varistor is to be placed. The thickness of the varistor body measured normal to the major surfaces is not critical and may vary widely. The varistor body thickness is in most instances chosen so that the varistor body is rugged enough to avoid damage both in fabrication and handling. For example, the varistor body will normally exhibit a thickness of at least 25 microns. In theory there is no limit to the maximum thickness of the varistor body, except that excessive thicknesses may unnecessarily add to the bulk and cost of the varistor as well as lengthening the thermal impedance path through the varistor body.

Mounted on the first major surface is a first electrode 24 and a second electrode 25. The electrodes may be ohmically conductively associated with the major surface in any convenient conventional manner. The electrodes are laterally separated by a width Y, referred to as the conduction gap width. In the varistor 20 the conduction gap extends linearly across the first major surface and is of uniform width throughout. The conduction gap width determines the voltage level to be observed across the electrodes for a given current conduction level. Accordingly, it is desirable in most instances to precisely control this width. This can be accomplished by positioning the electrodes using known masking techniques to assure that they are accurately spaced or by initially forming a single electrode and thereafter relieving an intermediate portion of the electrode in a controlled manner to leave the first and second electrodes in spaced relation.

The conduction gap width Y may be of any desired value, depending upon the voltage desired for a given level of current conduction. The lateral spacing of the electrodes of the varistor 20 is, however, particularly advantageous when the conduction gap width Y is less than the thickness of the varistor body, as would be the case in comparatively low voltage applications. To illustrate this, it is merely necessary to observe that if an electrode spacing of 2 microns between electrodes is indicated to yield the desired current and voltage characteristic for a varistor, it would be necessary to form the varistor body 3 with the thickness X being a value of only 2 microns. However, in my varistor 20 the varistor body 21 can be formed of any convenient thickness. It is only the conduction gap width Y that must be controlled at 2 microns. By comparison to forming the varistor body itself of this small thickness, like spacing of the electrodes is quite simple to accomplish employing techniques well known to the art.

The operation of the varistor 20 differs from that of a conventional varistor as shown in FIG. 1. When a potential is impressed across electrodes 24 and 25, the current that is conducted between the electrodes is along or immediately beneath the surface of the varistor body within the conduction gap Y. This is in direct contrast to the conventional varistor in which the current is more or less uniformly distributed within the bulk of the varistor body. Of course, in the varistor 20 there will be some fraction of the current that will be carried through the bulk of the varistor body beneath the surface of the body, particularly as higher voltages are reached, but this should still be only a small proportion of the total current and may under most circumstances be considered negligible. Hence, while the varistor 2 will follow equation 1 similarly as varistor 1, its internal conduction mode is quite dissimilar.

In addition to the varistor 20 I have also invented various alternative embodiments differing in one or more functional and structural aspects. Except for the specific differing features noted and discussed, the remaining embodiments of my invention should be understood to employ structural characteristics identical to those of the varistor 20.

In FIG. 3 a varistor 30 is shown, which is a modified form of my invention. A sintered ceramic metal oxide varistor body 31 is provided having a first major surface 32 and a second major surface 33. Electrodes 34 and 35, identical to electrodes 24 and 25, are associated with the first major surface and are separated by conduction gap width Y. A dielectric support 36 is associated with the second major surface. The dielectric support may be chosen from any one of a variety of electrically insulative, comparatively inert materials, such as, but not limited to, known glass, ceramic, and polymeric insulators. The advantage of using the support 36 is that the thickness X3 of the varistor body can now conveniently be reduced, since the ruggedness of the varistor body itself is supplemented to a considerable extent by the support. It is still a uniquely advantageous feature of my invention that the combined thickness of the varistor body and support, which together form a common substrate, can be greater than the conduction gap Y, although this is not absolutely essential to all applications of my invention. It is recognized that in some circumstances, particularly when the support is a ceramic, it may be advantageous to form the varistor body as a coating on the upper surface of the support. The varistor body and dielectric support can be bonded together to form a unitary substrate by conventional bonding techniques.

In FIG. 4 a varistor 40 is shown provided with a varistor body 41, which may be identical to 21, having a first major surface 42 and a second, opposed major surface 43. A first electrode 44 is ohmically conductively associated with a portion of the first major surface. A second electrode 45 is provided with a portion 45A ohmically conductively associated with the first major surface and laterally spaced from the first electrode by conduction gap width Y. A remaining portion 45B of the second electrode is associated with the second major surface, and an intermediate portion 45C ohmically conductively connects the portions 45A and 45B of the second electrode. It is to be noted that the first and second major surfaces of the varistor body and, hence, the first electrode and the portion 45B of the second electrode are separated by a thickness X2, which exceeds the conduction gap width Y.

When the varistor 40 is called upon to conduct low current levels, its operation is identical to that of varistor 20. That is, current is conducted almost exclusively across conduction gap Y, and a relative stable low level voltage range (compared to that obtainable using a resistor) is maintained across the electrodes. Should, however, the voltage level continue to rise across the first and second electrodes, as might occur in the case of a high power surge requiring current conduction beyond the capacity of the conduction gap at the first major surface, the voltage across the electrodes can be stabilized again at a somewhat higher voltage level determined by the spacing X2 between the first electrode and the portion 45B of the second electrode. This will become more apparent when it is recognized that the conduction gap width Y, though lower in value than the thickness X2, relies for current conduction upon a relatively restricted area of the varistor body lying adjacent or immediately below the surface of the conduction gap, and for this reason its current conducting capabilities are limited. By contrast the somewhat more widely spaced first electrode and portion 45B of the second electrode are capable of conducting current therebetween through the bulk of the varistor body over a relatively extended area. In this instance it can be seen that the varistor 40 combines the very low voltage characteristics of the varistor 20 while also incorporating as an added feature the larger power handling capability of a conventional varistor, such as shown in FIG. 1, which also offers a second range of voltage stabilization.

Each of the varistors 20, 30, and 40 can be placed in an electrical circuit to provide a shunt path around a high voltage degradable circuit unit, as is illustrated in FIG. 5. The varistor is connected in the circuit to selectively shunt current around the degradable unit in proportion to the voltage across the terminals 50 and 51. The current through the varistor rises exponentially with any increase in voltage and hence serves to stabilize the voltage across the terminals.

In FIG. 6 a varistor 60 is illustrated. The varistor includes a varistor body 61 having a first major surface 62 and a second major surface 63 opposed thereto. Associated with the first major surface are first, second, and third electrodes 64, 65, and 66, respectively. The electrodes are each laterally spaced with the second electrode being interposed between the first and third electrodes. The first and second electrodes are separated by a conduction gap width Y1 and the second and third electrodes are separated by a conduction gap width Y2, which exceeds conduction gap width Y1 in value. The varistor 60 possesses all the advantages of the varistor 20 plus the added advantage that the first and third electrodes can be simultaneously and independently referenced to the second electrode. Further, by controlling the placement of the second electrode 62 with respect to the first and third electrodes, the resistance to current flow between the first and second electrodes can be related to the resistance to current flow between the second and third electrodes to provide any desired ratio of these resistances. For certain applications the gap width Y1 and Y2 may be equal in value.

In FIG. 7 a varistor 70 is illustrated which is provided with a varistor body 71 that may be identical to varistor body 41. Adjacent first major surface 72 first and second electrodes 74 and 75 are located separated by conduction gap width Y. A third electrode 76 is associated with the second major surface 73. The third electrode is separated from the first and second electrodes by a thickness X2 of the varistor body. The thickness X2 exceeds the gap width Y. Both the first and second electrodes can be referenced to the third electrode while at the same time being referenced at a lower voltage range with respect to each other.

A specific application for the varistor 70 is shown in FIG. 8. Circuit terminals 80 and 81 are shown. These terminals may be connected to a series related electrical load and power source. The anode terminal 82 and the cathode terminal 83 of an SCR 84 are shown connected to the terminals 80 and 81, respectively. Gate terminal 85 of the SCR is connected to the cathode of a diode 86 and the anode of the diode is connected to other conventional trigger circuitry 87 which is in turn electrically connected to the terminals 80 and 82. The first electrode 74 of the varistor is connected to the gate terminal 85. The second electrode 75 of the varistor is connected to the SCR anode terminal 82, and the third electrode 76 of the varistor is connected to the cathode terminal 83 of the SCR.

It can be seen that in circuit operation the varistor 70 acts as a shunt across the SCR 84. Should a voltage surge develop across the SCR it would be shunted through the varistor body between the second and third electrodes 75 and 76. At the same time the varistor 70 is also capable of shunting a lower voltage that might develop across the diode 86 and coventional trigger circuitry 87. This could occur, for example, if a reverse voltage were applied to the SCR well within its voltage blocking capability, but approaching the voltage blocking capability of the diode 86. In this instance the diode is protected by the varistor's voltage clamping ability through conduction gap width Y between the first and second electrodes. It is to be further noted that the portion of the varistor having the highest power handling capability is used to protect the power handling portion of the circuit, namely the SCR, while the portion of the varistor having a lower power handling capability, the first major surface associated conduction gap, protects the signal portion of the circuit. It is to be still further noted that excessive gate voltages are prevented by the varistor, since in this instance conduction can occur through the varistor body between electrodes 74 and 76. It is recognized that the varistor 60 could be substituted for the varistor 70 in the circuit shown iwth first electrode 62 being connected to gate terminal 85, second electrode 65 connected to anode terminal 82, and third electrode 66 connected to cathode terminal 83.

While I recognize that the limited current carrying capacity of my varistors may be a disadvantage in certain applications requiring substantial power handling capabilities, their current handling capabilities may be enhanced by increasing the distance traversed by the conduction gap over the major surface so that it exceeds the maximum dimension of the major surface. In other words, the conduction gap need not extend linearly across the major surface as described for simplicity in the foregoing embodiments.

A simple approach for increasing the distance traversed by the conduction gap on a major surface of a varistor according to my invention is best appreciated by reference to FIG. 9. In this figure is shown a varistor 90 having a circular first eleectrode 91 and an annular second electrode 92 which is concentric with the circular electrode and which is uniformly spaced from the circular electrode by a conduction gap width Y. It may be readily observed that the distance traversed by the conduction gap exceeds the outer diameter of the annular electrode 92. In this way the current carrying area is increased over what would be present if two semicircular electrodes were employed in association with the same underlying varistor body.

In FIG. 10 an approach for further increasing the area available for current conduction is illustrated. A varistor 100 is provided with a central first electrode 101 having a plurality of regularly spaced fingers 102 extending radially outwardly. An outer electrode 103 is provided with a plurality of radially inwardly spaced fingers 105 interdigitated with the fingers 103. In this arrangement a variable spacing between the inner and outer electrodes is required if an equal amount of current is to be conducted throughout the conduction gap, as the different curvatures presented by the different portions of the fingers will produce differing electrical fields if a uniform spacing is employed. Where unequal stresses can be tolerated on the fingers, it may be most convenient to provide a uniform spacing between the fingers or an approximately uniform spacing.

In FIG. 11 a varistor 110 is illustrated which is provided with a first electrode 111 and a second electrode 112 associated in laterally spaced relation to an underlying varistor body. The electrodes are formed so that they are laterally separated by a minimum conduction gap width Y3 and progressively diverge to a maximum conduction gap width Y4. The effect of varying the conduction gap width in this manner is to cause the varistor to present a somewhat lower alpha than should be present based upon the characteristics of the varistor body, per se. This approach is particularly useful in using metal oxide varistors incorporating varistor bodies having an alpha in excess of 10 in the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter to replace previously utilized varistors, such as selenium and silicon carbide varistors having alphas appreciably below 10.

In FIG. 12 a varistor 120 is illustrated having a first electrode 121 and a second electrode 122. The two electrodes are laterally spaced on a varistor body in two discrete stepped increments. The left hand portion of each electrode is laterally spaced by a conduction gap width Y5 which is less than the conduction gap width Y6 of the right hand portion of each electrode. It has been observed that with a constant direct current bias placed across the electrodes of a metal oxide varistor a gradual increase in the voltage level across the electrodes can occur as an aging function, particularly where the device is biased at near its power handling capacity. In the varistor 120 the voltage between the electrodes will initially be determined by the conduction gap width Y5. As the varistor ages in use it is possible that the voltage across the gap width Y5 may approach the voltage level in which the right hand portion of the device becomes active. In this way an aging device is protected against runaway voltages developing for a period of time permitting replacement before uncontrolled voltage increase occurs.

As described above the varistors formed according to my teachings are free of any protective packaging or external lead connections. In the form shown the varistors may be utilized in protected environments without additional packaging. For example, the varistors could be incorporated in a hermetically sealed housing alone or in combination with other electrical components. For most applications it will be desirable to attach terminal leads to the electrodes and to encapsulate the varistors in a dielectric material to assure protection from environmentally encountered substances altering their electrical characteristics.

To illustrate the packaging of a varistor formed according to my invention, the varistor 30 shown in FIG. 3 is illustrated as the packaged varistor 130 shown in FIG. 13. Elements of the varistor 130 corresponding to those of the varistor 30 are assigned like reference characters and are not redescribed. Terminal leads 134 and 135 are soldered or otherwise suitably attached in low impedance relation to the electrodes 34 and 35, respectively. A substantially impervious dielectric body 136, preferably formed of a dielectric glass of a type conventionally employed in the passivation and/or packaging of semiconductor crystals, is shown overlying the conduction gap and the adjacent edges of the electrodes. Inasmuch as the conduction characteristics of the varistor are most appreciably influenced by the conduction properties present at or near its surface along the conduction gap, this is the area to which maximum protection should be given. Note should be taken of the fact that this relationship is directly in contrast to that for the varistor 1 in which conduction occurs through the bulk of the varistor body.

It is anticipated that for many applications the only protective packaging needed or desired for the varistor will be the dielectric body covering the conduction gap. For more general applications, however, it is normally desirable that an additional dielectric covering 137, which may take the form of any conventional plastic or glass semiconductor packaging composition, be used to cover the remaining exterior surfaces of the varistor body and, optionally, its electrodes. It is further anticipated that the packaging dielectric 137 may be used alone with the dielectric body 136 being omitted. As shown, the dielectric package cooperates with the dielectric substrate 36 to completely cover the exterior surfaces of the varistor body. Where the varistor is of a form lacking a dielectric substrate, it is appreciated that the package dielectric 137 may also completely envelop the varistor body and, optionally, its attached electrodes.

While I have described my invention with reference to certain preferred embodiments, it is appreciated that numerous variations in form will readily occur to those skilled in the art. For example, while I have disclosed the varistor bodies to be of limited and regular lateral extent, it is appreciated that the lateral extent of the varistor body beyond the conduction gap width is not critical to its current conduction capabilities. For this reason I contemplate that varistors according to my invention may be formed with the varistor body extending laterally well beyond (or short of) the electrode outer edges, if desired, and utilizing lateral outline geometries of any convenient regular or irregular configuration. In the forms of my varistors with electrodes attached to only one major surface it is appreciated that it is unnecessary to provide a second major surface of any regular geometrical form or to have the second major surface parallel to the first major surface. While in most instances it will be most convenient to form the major surfaces so that they lie in a single plane, it is anticipated that the major surfaces may, if desired, be curved or bent, so that they lie in more than one plane. While I have shown a varistor for purposes of illustration to be packaged as a lead mounted device, it is anticipated that the varistors formed according to my invention may be attached to electrical terminals of various configurations. It is specifically contemplated that the terminals attached may also serve to draw heat from the varistor body, as is well understood in the fabrication of electronic components. In this regard it is noted that the varistor body is itself a fairly good thermal conductor and will dissipate heat readily from the area immediately underlying the conduction gap. While a specific form of electrode interdigitation has been shown for purposes of illustration, it is appreciated that electrode interdigitation is per se well known in the electronic components arts and that many alternate forms of electrode interdigitation could be easily substituted.

Still other variations are contemplated. It is accordingly intended that the scope of my invention be determined by reference to the following claims.

Claims

What I claim and desire to secure by Letters Patent of the United States is:

1. The combination comprising

a substrate having first and second opposed major surfaces comprised of a metal oxide varistor body lying along at least said first major surface and having an alpha in excess of 10 in the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter and

first and second electrodes lying in ohmic contact with said first major surface and laterally spaced to form a conduction gap therebetween along said first major surface having a minimum width less than the thickness of said substrate between said major surfaces.

2. The combination comprising

a substrate having first and second opposed major surfaces;

said substrate consisting of a metal oxide varistor body;

said varistor body having an electrical resistance which varies as a function of applied voltage in accordance with the formula

I = (V/C).sup.alpha

where V is the voltage in volts applied to the body, I is the current in amperes through the body resulting from such voltage, and C and alpha are constants;

said body having an alpha in excess of 10 in the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter;

first and second electrodes lying in ohmic contact with said first major surface and laterally spaced on said first major surface to form a conduction gap therebetween along said first major surface;

said conduction gap having a minimum width less than the thickness of said varistor body measured normal to said first major surface.

3. The combination according to claim 2 wherein said varistor body comprises predominantly zinc oxide.

4. The combination according to claim 1 in which said substrate is additionally comprised of a dielectric support associated with said varistor body.

5. The combination according to claim 1 in which a third electrode lies in ohmic contact with said first major surface, said third electrode lying in laterally spaced relation with said second electrode and laterally separated from said first electrode by said second electrode, said second and third electrodes forming a conduction gap therebetween along said first major surface having a minimum width along said first major surface exceeding the maximum width along said first major surface of the conduction gap between said first and second electrodes.

6. The combination according to claim 1 in which said substrate is formed entirely by said varistor body and additionally including a third electrode lying in ohmic contact with said second major surface.

7. The combination according to claim 1 in which adjacent edges of said first and second electrodes are substantially parallel.

8. The combination according to claim 1 including

a metal oxide varistor body having an alpha in excess of 10 within the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter and presenting at least one major surface,

first and second electrodes lying in ohmic contact with said body along said major surface and laterally spaced relatively to form a conduction gap therebetween, and

dielectric means overlying said varistor body along the conduction gap and cooperating with adjacent edges of said electrodes to protect said varistor body against alteration of its electrical characteristics.

9. The combination according to claim 8 in which said dielectric means and said electrodes together completely envelop said varistor body.

10. The combination according to claim 8 additionally including packaging means cooperating with said dielectric means and said electrodes to envelop said varistor body.

11. A varistor comprising

a metal oxide varistor body having an electrical resistance which varies as a function of applied voltage in accordance with the formula

I = (V/C).sup.alpha

where V is the voltage in volts applied to the body, I is the current through the body in amperes resulting from such voltage, and C and alpha are constants;

said body having an alpha in excess of 10 in the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter;

said body having at least one major surface;

first and second electrodes lying in ohmic contact with said major surface and laterally spaced on said major surface to form a conduction gap therebetween along said major surface;

said conduction gap having a minimum width less than the thickness of said body measured normal to said major surface.

12. The combination comprising

a metal oxide varistor body having an alpha in excess of 10 in the current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter and presenting first and second opposed major surfaces,

first and second electrodes lying in ohmic contact with said first major surface and laterally spaced to form a conduction gap therebetween along said first major surface having a minimum width less than the thickness of said body between said major surfaces, and

a third electrode lying in ohmic contact with said second major surface. pg,24

13. A varistor according to claim 11 wherein said varistor body comprises predominantly zinc oxide.