US3245019A

US3245019A - Voltage dependent resistor

Info

Publication number: US3245019A
Application number: US406141A
Authority: US
Inventors: Heywang Walter; Schofer Rudolf
Original assignee: Siemens AG
Current assignee: Siemens and Halske AG; Siemens AG
Priority date: 1958-06-04
Filing date: 1964-10-23
Publication date: 1966-04-05
Anticipated expiration: 1983-04-05
Also published as: NL6605449A; CH380807A; FR1226309A

Description

April 5, 1966 w HEYWANG ETAL 3,245,019

VOLTAGE DEPENDENT RESISTOR 3 Sheets-Sheet 1 Filed Oct. 23, 1964 Fig.1a

April 5, 1966 w. HEYWANG ETAL, 3,245,019

VOLTAGE DEPENDENT RESISTOR VOLTAGE DEFENDENT RESISTOR 3 Sheets-Sheet 5 Filed Oct. 25, 1964 Fig.4 5-10 judo/f 5024 6 fa United States Patent many Filed Oct. 23, 1964, Ser. No. 406,141 Claims priority, application Germany, June 4, 1958,

S 58 485 9 Claims. (in. ass-20 This application is a continuation in part of copending application Serial No. 816,994, filed May 29, 1959.

The invention relates to resistors with high positive temperature coetficient of the resistance value over a range of temperatures, having the following features, namely: (a) the resistor is made of a ceramic ferroelectrical material whose Curie temperature, at which the material loses its permanent polarization, lies at least below the upper limit of the operating temperature range, especially at or below the lower limit of the range at which the resistor should have a positive temperature coefiicient of the resistance value, and preferably below 20 C.; (b) the resistor material is made conductive by the inclusion of impurity centers by donor or acceptor (preferably n-conductive whereby the respective spacing E between the energy level of the donor atoms and the conduction band, or between the energy level of the acceptor atoms and the valence band, is smaller, and especially appreciably smaller, than half of the width E of the prohibited zone between valence band and conduction band; and (c) the intrinsic conductivity or another impurity center conductivity of the resistor material not imparted by said donor or acceptor atoms is slight, at least in part of the operating temperature range in which the resistor has a positive temperature coefiicient, and especially negligibly slight as compared with that of the impurity center conductivity.

It has further been proposed, according to copending application Serial No. 809,478, filed April 28, 1959, now Patent No. 3,027,529, also owned by the same assignee, to dispose the contacts substantially free of barrier or blocking layers, so as to obtain in such a resistor negligibly slight voltage dependence of the total resistance. It was found, however, that a voltage dependence is despite these measures frequently present at sufiiciently high field strength. In order to permit operation of the resistor with high voltage without practically producing a voltage dependence of the resistance value, this copending application therefore suggests to provide for predetermined grain or particle sizes of the resistor material, which have the effect that the resistance remains up to praticularly high field strength practically voltage independent.

The present invention is directed to a voltage-dependent ceramic resistor of the type disclosed in said Patent No. 3,027,529, wherein the concentration of the donor or acceptor atoms (n in the interior of the particle and the particle size d are such that the magnitude of the product 61,71 is so much larger or smaller than that magnitude of the product dJ'I at which a maximum value (E for the critical field strength (E would be produced with any surface state density (11 that the actual value of the critical field strength is less than half of said maximum value (see FIG. 1a).

In order to obtain a resistor which is voltage dependent While being as temperature independent as possible, the present invention proposes to place the Curie temperature of the resistor material with respect to the operating temperature for which the resistor is designed, so that the value of the resistance is at the corresponding operating voltages to a high degree temperature independent while being strongly voltage dependent. This invention 3,245,019 Patented Apr. 5, 1966 is based on the concept, explained in the above referred to patent that potential walls are formed at the particle borders (see FIG. 3 of such patent) due to surface impurity terms, which are in the presence of sufficiently high field strength, resulting in the resistor material upon connection of the operating voltage, overcome thermally by the electrons flowing in the conducitivity band of the resistor material.

The invention and the various features and objects thereof will now be explained with reference to the accompanying drawings, wherein: 7 FIG. 1, which corresponds to FIG. 3 in said Patent No. 3,027,529, shows the effect of the particle size on the field strength, producing a critical field strength 13,, at which resistance value begins to break down;

FIG. 1a shows the critical field strength E plotted against the factors of the particle size;

FIG.2 shows an example of a resistor in which the invention may be incorporated; and

FIGS. 3 and 4 are performance curves.

FIG. 1, corresponding to FIG. 3 of said patent, shows in schematic manner an n-doped titanate and the formation of the previously mentioned potential wall. The vertical line 1 indicates the particle border, for example, of two intersintered barium titanate bodies.

Numerals

2 and 2" represent donor terms of two particles I and II lying closely to the lower border 3' and 3" of the conductivity band of the barium titanate. The upper limit of the valence band of those titanates is indicated at 4' and 4". At the particle border 1 there is at A an acceptor term which, due to its energetically low position leads to a discharge of the neighboring donors 21', 21, such donors accordingly forming a space charge zone and tending to bend the conductivity curve upwardly. The height of this bend is indicated by (p thus designating the height of the potential wall which is now within the range of the Curie temperature due to the very strong temperature dependence of the dielectric constant (e), and is to a high degree temperature dependent. It rises, for example, in the case of barium titanate at an assumed acceptor density at the particle border, of l0 l0 /cm. and at a donor density of about 10 -10 /cm. from about 0.1 volt at 20 C. to about 1 volt at- 200 C.

A low voltage dependence of the resistivity value occurs when the potential wall is by the electrons flowing in the conduction band substantially not overcome by the wavemechanical tunnel effect but only thermally; however, in order to achieve this, the maximum field strength (E g) at which the resistor is operated, must not be appreciably greater, at the particle borders, even at the lowest temperatures, than the quotient formed by the height to of the potential wall divided by the particles size d. I The value E /d is the mean critical field strength 13,; in the volume of the resistance material at which the resistance value caused by the potential wall at the particles boundaries starts to collapse height of the potential wall, see FIG. 1, d length of particle measured in the direction of the working field strength in the volume of the resistance material hereinafter referred to as particle size. The mean critical field strength 15;, is given by the formula:

depending on whether the particle size a is larger or smaller than n /n In these formulae 11 is the surface state density (number of acceptor of donor atoms for unit area of the particle boundaries), m the impurity center concentration (total number of donor or acceptor atoms for unit volume of the particles), s the dielectric constant of a vacuum, and e is the dielectric constant of the resistance material. In any particular ceramic the surface state density n is a constant and, if (see FIG. 1a of the present application) the magnitude of the mean critical field strength E of the potential wall is plotted against the product n -d for a particular ceramic the curve shown in FIG. 1a is obtained. So that the collapse of the resistance occurs at critical field strengths E; which are less than half of the maximum value, the particle size d must either be kept below the magnitude which corresponds to half the maximum field strength for a given density of the impurities (11 embedded in the resistance material, or else must be made so large that the product n -d is so much greater than that corresponding to the broken line in FIG. 1a that the critical field strength falls to less than half its maximum value. In one particular ceramic with a convenient impurity density it has been found advisable to make the particle diameter of the major portion of this ceramic voltage-dependent resistor either less than microns and preferably less than 2 microns, or else greater than 15 microns and preferably between 20 and and 30 microns.

FIG. 3 hereof is a graph illustrating the dependence of the resistance of the resistor at very low voltages on the temperature. It will be seen that well below the Curie temperature which is indicated by an arrow 11 the resist ance is at a low value which is even below approximately 100 ohm-cm. At higher temperatures, the resistance rises to nearly ohm-cm. and has therefore, increased by several powers of ten. In order to obtain in such a resistor according to the invention a resistance value which varies with temperature as little as possible, the resistance value of the resistor body measured at a low voltage of the Working voltage range and at low temperatures of the working temperature range should be large compared with the resistance value of the same resistor measured at the same voltage but at the Curie temperature or at lower temperatures. In FIG. 3 arrow b indicates the lower limit of the working temperature range at which a resistor according to the invention in which the ferroelectric substance is barium titanate operates as a voltage-dependent resistor and has only a small or no temperature dependence. This temperature b is between 50 and 100 above the Curie temperature indicated by the arrow a. If, therefore, the lowest working temperature at which the resistor is required to serve as a varistor independent of temperature lies near room temperature (approximately 20 C.) it is advisable to use as starting material a material the Curie temperature of which lies below 20 C., and preferably below 0.

When a high voltage dependence of the resistance is desirable, a starting material is advisable whose Curie temperature lies more than 100 below the lowest limit of the working temperature range; some temperature dependence of the resistance value must then be tolerated, see for instance FIG. 3 in which above approximately 300 C. the resistance begins to drop again in dependence on the temperature. If, however, as high a temperature in dependence as possible is of importance, it is advisable to operate the resistor on which FIG. 3 is based bebetween the temperatures b and 0 (approximately between 180 and 340 0.). It is, therefore, advisable to choose the material of the resistor for a given working temperature range to be such that the maximum resistance value measured at low voltages of the working voltage range lies within the working temperature range. In the case illustrated in FIG. 3, the maximum resistance value lies at point M, that is at a temperature at approximately 250 C. which is between the limits b and c of the working temperature range.

FIG. 4 is a graph illustrating the dependence of the resistance of the resistor, the R-T characteristic of which is illustrated in FIG. 3 against the field strength in the resistor for various temperatures. As may be seen the resistance is strongly voltage-dependent at field strengths above approximately 10 v./cm. In particular it is advisable to dimension the resistor in such a way that the working voltage range has a lower limit corresponding to a field strength of approximately v./cm. In this way the resistor is made strongly dependent on the applied voltage (see the larger slope of the curves in FIG. 4).

The measurements of FIGS. 3 and 4 are based on a resistor of the type specified, the starting material of which is pure barium titanate. Consequently the Curie temperature lies at approximately C. and the working temperature at which this resistor is to be operated as a varistor, that is with a strong voltage dependence but with a small temperature dependence, lies above approximately C. It is possible to shift the Curie temperature and, therefore, the steeply rising branch of the resistance curve to corresponding low temperatures in those cases in which other working temperature ranges are desirable such as, for instance, a temperature range between 0 and 100 C. For this purpose a different ferro-electric material with a considerably lower Curie point may be used as a starting material for a resistor according to the invention. Preferably, a barium strontium titanate may be used in which the proportion of the strontium in the titanate amounts to 30 to 50 mol percent or more of the titanium in the titanate. In particular the Curie temperature of the starting material may lie at or below 0 C., for instance at approximately -20 C., for a working temperature range extending from approximately 0 C. or room temperature up to approximately 100 C. As mentioned above, it is, however, advisable to locate the Curie temperature for this working temperature range still lower, in particular at between 50 and -l00 C.

It was already explained with respect to FIG. la it is in general advisable, in order to initiate the collapse of the resistance in dependence on the applied voltage at low field strengths, to work in the left hand portion of the linear branch of FIG. 1a, that is at as small a grain diameter d as possible. The resistance material should, therefore, be sintered from particles which are in general smaller than 5 microns and preferably smaller than 2 microns. However, the critical field strength E is inversely proportional to the abscissa d-n in the right hand portion of FIG. 1a. The curve extends therefore hyperbolically to the right of the broken line in FIG. 1a. Since the magnitude n frequently cannot be made as small as desired the particle diameter d must often be made undesirably small in order to arrive at sufliciently low value B In these cases it is advantageous to make the particle diameter particularly large, that is to make it so large that the product n -d is greater than that corresponding to the broken line in FIG. 1a. The critical field strength E; at which the resistance of the ceramic body starts to collapse and its resistance value becomes, therefore, strongly voltage-dependent is then achieved at relatively low values of the voltage applied to the resistor. In particular, it is then advisable to make the particle diameter d greater than approximately 15 microns, preferably approximately equal to 20 to 30 microns or more. In these cases too, however, as stated'in said Patent No. 3,027,529 the particles preferably are to be used which are as uniform as possible so that the particle sizes of the major portion of the ceramic resistance material deviate relatively little, that is by only approximately :L-20 to 25% from their mean value in this body.

The invention may be applied in the construction of resistors of the type as shown, for example, in FIGS. 1 and 2 of said patent, one such embodiment being shown herein in FIG. 2.

As explained in said patent, the contacts are placed upon the resistor body substantially free of barrier or blocking layer, especially vaporized thereon, so as to reduce the voltage dependence of the total resistance value of the resistor or to make it negligibly low at room temperature. The material for the contacts is for this purpose selected so that it does not form a barrier or blocking layer with the resistor material; more particularly, in the case of aresistor sintered of ferroelectric crystallite particles made n-conductive, the current supply contacts will consist of a base metal, preferably aluminum or zinc or of an alloy containing at least a high propor tion of one of these metals. The surface parts of the ceramic material serving for the contacting are more- :over prior to or if desired incident to the contacting preferably particularly treated as compared with the interior particles of the resistor body, especially mechanically treated, for example, they are made to be 'well conductive by solder rubbing or by sanding or by electrical or chemical treatment applied prior to vaporizing metal thereon. The purpose of this pretreatment is to provide a clean surface free of troublesome terms. This may be obtained, for example, by glow treatment or by chemical reduction. The treatment may be such as to affect not only the surface but penetrating to some depth the marginal layer of the resistor material.

In order to keep as small as possible the varistorlike volume effect caused by the transition resistances between the crystallites of the sintered resistor body, it is furthermore proposed that the resistor material be sintered at suitable high temperature for a sufiicient time so as to avoid these transition resistances as far as possible. In the event that an impermissible voltage dependence of the specific resistance cannot be avoided in the production of the resistor body, by suitable selection of the sintering conditions, the volume effect may be subsequently reduced by forming operations.

For example, as explained in said patent, a sufliciently voltage-independent resistance may be produced by first sintering the resistor body without regard to the voltage dependence of its resistance, thereupon contacting the resistor body in the manner explained above, and thereafter passing through the resistor a strong current surge which reduces the voltage dependence of the transition resistances between the crystallites and therewith the conductivity of the resistor body to a tolerable point, by effecting in a manner weldingtogether of the individual crystallites.

Since a reduction of the volume resistance could in connection with the treatment he observed, if at all, only at high field strength, while the resistance remained at lower field strength practically independent of the voltage applied, it is possible by providing suitable dimensions of the resistor body to produce for any given voltage a thermistor with any desired resistance value, which is practically free of varistor effects, by making the spacing between the two resistor contacts relatively great, therewith the field strength low, and making the cross-sectional area fo the resistor body correspond to the desired capacitance and the length of the current path in the resistor body.

The surface treatment is in a particularly advantageous embodiment carried out by subjecting the surface to a glow effect. For this purpose, the semi-conductor is suitably disposed in a vacuum vessel, at relatively low gas pressure, opposite an electrode and the surface parts of the semi-conductor body which are subsequently to be contacted are brought to a glowing condition by the connection of an alternating voltage or, preferably, a direct voltage, which becomes effective between the resistor body and the electrode. When using a direct voltage, it is advisable to place the semi-conductor on the positive pole of the voltage source. The resistor material may be subjected to a strong glow effect, for example, at a current density from about to 30 milliamperes per square centimeter, for example, at roughly 3000 to 5000 volt, so as to liberate the surface to be metallized also from adhering residual gas or other contaminations such as deposited hydrogen. The contact metal which does not form a blocking layer with the semi-conductor is thereupon applied, advantageously by vaporization in the same vacuum vessel at further reduced gas pressure The con- 6 tact metals to be used in the case of n-doped ferroelec'tric resistor materials are preferably base metals with a normal potential below that of silver, especially below that of copper, for example Al or Zn, which are particularly suitable. The use of noble metals as contact materials is, however, not inherently excluded.

The current connections may be mechanically secured on the semi-conductor resistor, for example, in the case of rod-shaped resistors (not shown herein), in the form of caps attached thereto by press-fit or in the form of clasps embracing the resistor body.

FIG. 2 shows a disc or wafer-shaped resistor comprising a resistor body 1 with a width L. The resistor body 1' is made of sintered ferroelectric crystallites. Its

opposite sides

11 and 12 are pro-treated as descnibed before, for example, chemically or electrically to remove surface terms. Instead of providing current connections in the form of caps or the like, as in the case of a rod-shaped body, base metal, for example,

aluminum

13, 14 is placed upon the resistor body 1, preferably by vaporizing. These vaporized layers or

coatings

13, 14 form with the

surfaces

11, 12 of the ceramic resistor body 1 very good and substantially barrier-free contact. The base metal coatings are thereupon provided, chemically or electrochemically, With solderable material, for example, silver, forming thin reinforcing

layers

15, 16, to which are soldered disc shaped

portions

171, 181 carried by the respective current terminals 17 and 18,

numerals

19 and 20 indicating the resulting solder pearls.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

We claim:

1. Ceramic resistor exhibiting strong voltage dependence of its resistance value, said resistor being made of ferroelectric-al material having a Curie temperature, above which the material loses its permanent polarization, lying below the lower limit of the range at which the resistor shall have said positive temperature coefl'icient of the resistance value, said resistor having conductive impurity content with a spacing of the donors from the conduction band and of the acceptors from the valence band which is smaller than one-half of the width of the prohibited zone between the valence band and the conduction band, the intrinsic conductivity of the resistor material being, at least within the range of the positive temperature coeificient, small as compared with the conductivity of said impurity content centers, the resistance value of said resistor, measured at a low voltage of the voltage range and at low temperatures of the operating temperature range, being high as compared with the resistance value measured at identical voltage but at a temperature not exceeding the Curie temperature of the basic resistor material, the concentration of the impurity atoms (n in the interior of the particles and the particle size (0.) being such that the magnitude of the product (d 1%) difiers so greatly from that magnitude of the product d -n at which a maximum value (E for the critical field strength (B would be produced with any surface state density (n that the actual value of the critical strength is less than half of said maximum value.

2. A resistor according to claim 1, wherein the Curie temperature of the resistor material lies about 50 C. to over C. below the lower limit of the operating temperature range.

3. A resistor according to claim 1, wherein the Curie temperature lies at about 20 C. to 0 C.

4. A resistor according to claim 1, wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material, is smaller than 5 microns.

5. A resistor according to claim 1, wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material, is smaller than 2 microns.

6. A resistor according to claim 1, wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material exceeds 15 microns.

7. A resistor according to claim 1, wherein the diameter of the particles of the resistor material, measured in the direction of the operating field strength and the main part of the resistance material is from about 20 microns to about 30 microns.

8. A resistor according to claim 1, wherein the particle 10 sizes in the material of the preponderant part of said resistor deviate by less than 25% from the average particle size therein.

9. A resistor according to claim 1, wherein theresistor body is disk shaped, contacts of base metal disposed barrier-free on said body, said contacts reinforced by layers of solderable metal disposed thereon.

References Cited by the Examiner UNITED STATES PATENTS 2,864,713 12/1958 Lewis 106-39 2,911,370 11/1959 Kulcsar 25262.9

RICHARD M. WOOD, Primary Examiner. W. D. BROOKS, Assistant Examiner.

Claims

1. CERAMIC RESISTOR EXHIBITING STRONG VOLTAGE DEPENDENCE OF ITS RESISTANCE VALUE, SAID RESISTOR BEING MADE OF FERROELECTRICAL MATERIAL HAVING A CURIE TEMPERATURE, ABOVE WHICH THE MATERIAL LOSES ITS PERMANENT POLARIZATION, LYING BELOW THE LOWR LIMIT OF THE RANGE AT WHICH THE RESISTOR SHALL HAVE SAID POSITIVE TEMPERATURE COEFFICIENT OF THE RESISTANCE VALUE, SAID RESISTOR HAVING CONDUCTIVE IMPURITY CONTENT WITH A SPACING OF THE DONORS FROM THE CONDUCTION BAND AND OF THE ACCEPTORS FROM THE VALENCE BAND WHICH IS SMALLER THAN ONE-HALF OF THE WIDTH OF THE PROHIBITED ZONE BETWEEN THE VALENCE BAND AND THE CONDUCTION BAND, THE INTRINSIC CONDUCTIVITY OF THE RESISTOR MATERIAL BEING, AT LEAST WITHIN THE RANGE OF THE POSITIVE TEMPERATURE COEFFICIENT, SMALL AS COMPARED WITH THE CONDUCIVITY OF SAID IMPURITY CONTENT CENTERS, THE RESISTANCE VALUE OF SAID RESISTOR, MEASURED AT A LOW VOLTAGE OF THE VOLTAGE RANGE AND AT LOW TEMPERATURES OF THE OPERATING TEMPERATURE RANGE, BEING HIGH AS COMPARED WITH THE RESISTANCE VALUE MEASURED AT IDENTICAL VOLTAGE BUT AT A TEMPERATURE NOT EXCEEDING THE CURIE TEMPERATURE OF THE BASIC RESISTOR MATERIAL, THE CONCENTRATION OF THE IMPURITY ATOMS (ND) IN THE INTERIOR OF THE PARTICLES AND THE PARTICLE SIZE (D) BEING SUCH THAT THE MAGNITUDE OF THE PRODUCT (D.ND) DIFFERS SO GREATLY FROM THAT MAGNITUDE OF THE PRODUCT D.ND AT WHICH A MAXIMUM VALUE (EK MAX) FOR THE CRITICAL FIELD STRENGTH (EK) WOULD BE PRODUCED WITH ANY SURFACE STATE DENSITY (NA) THAT THE ACTUAL VALUE OF THE CRITICAL STRENGTH IS LESS THAN HALF OF SAID MAXIMUM VALUE.