THE UNIVERSITY OF MICHIGAN RESEARCH INSTITUTE ANN ARBOR A TABULATION OF VOLTAGE-VARIABLE CAPACITORS Technic'al Memorandum 'No.-O0..... Electronic De f'ense Group Department of, Ele~cri:-!rcal -Engineering -. '... T. W. Butler, Jr. Approved by: G. A. Roberts H. W. Farris Project 2262 TASK ORDER NO. EDG-4 CONTRACT NO. DA-36-039 sc-63203 SIGNAL CORPS, DEPARTMENT OF THE ARMY DEPARTMENT OF ARMY PROJECT NO. 3-99-04-042 SIGNAL CORPS PROJECT 194B April 1959

a"-'m t't. TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS iii ABSTRACT iv 1. INTRODUCTION 1 2. OPERATING CHARACTERISTICS - FERROELECTRICS AND BACK-BIASED DIODES 1 3. TABLE AND RELATED COMMENTS 5 4. COMMENTS CONCERNING MANUFACTURERS' CURVES 11 5. CONCLUSIONS 12 REFERENCES 14 DISTRIBUTION LIST 15 ii

LIST OF ILLUSTRATIONS Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Typical Capacity vs Voltage Curves at 100 Me (Normalized) Typical Q vs Voltage Curves at 100 Mc Typical Capacitance vs Temperature Curves Tabulation of Voltage-Variable Capacitors C-T-E Surface for Aerovox "HQ"-91 Page 4 6 7 8 13 iii

ABSTRACT A tabulation of voltage-variable capacitors is presented. The various types of capacitors available, the frequency ranges in which they will operate, types of circuits in which they can be used, and other pertinent operating characteristics are presented. In addition, a clarification of the distinction between the ferroelectric capacitor and the back-biased diode with regard to the principle of operation is presented. It should be noted that the list of manufacturers is not exhaustive but can be considered as representative. iv

A TABULATION OF VOLTAGE-VARIABLE CAPACITORS 1. INTRODUCTION Increasing interest in the use of voltage-variable capacitors for electronic frequency control has resulted in a number of queries as to what types are available, frequency ranges in which they will operate, types of circuits in which they can be used, and other pertinent operating characteristics. In many cases several different types could be used, each having somewhat different performance characteristics. The type of voltage-variable capacitor which is most suitable, whether it be ferroelectric or back-biased diode, can be chosen on the basis of a logical evaluation. The choice will involve simultaneous consideration of several factors for each available type of capacitor in terms of the basic requirements of the circuit in which it is to be used. 2. OPERATING CHARACTERISTICS - FERROELECTRICS AND BACK-BIASED DIODES A clarification of the distinction between the ferroelectric capacitor and the back-biased diode with regard to the principle of operation appears desirable. The following brief discussion is for this purpose.

In the case of the silicon junction diode, the density of charge carriers at a P-N junction (electrons in the N-region and holes in the P-region) is reduced almost to zero as a voltage is applied in the reverse direction across the junction. This region of zero charge-density, known as the depletion region, is not only swept clear of charge carriers but actually widens as the reverse bias is increased. In effect, the two conducting areas appear to act as two metal plates, which tend to move farther apart as the reverse bias is increased. The plate area and the dielectric constant remain the same, but the distance between the plates varies according to the applied voltage. When these back-biased diodes are used in a resonant tankcircuit where high RF voltages may be developed across them, the junction must be back-biased far enough so that no part of the signal-voltage swing causes the net voltage applied to the junction to go positive, or clipping will result. This effect can be avoided at the expense of reducing the circuit Q somewhat by placing two back-biased diodes in series opposition across the tank coil. In the case of ferroelectrics, the method of operation is completely different. Ferroelectricity can be described as a spontaneous polarization. Polarization in dielectric materials may be due to: (1) Alignment of permanent electric dipoles, (2) Displacement of the + and - ions relative to one another (e.g., Na+ and C1 in NaCl), or (3) Displacement, relative to the positive nucleus, of the center of gravity of the negative charge of the electrons. 2

Ferroelectricity will occur if any of these mechanisms, either singly or in combination, occurs spontaneously, i.e., without the application of an external electric field. A number of materials exhibit ferroelectricity, but on the whole the phenomenon is rather rare. Barium titanate (BaTiO3) is, from a practical point of view, the most important ferroelectric and, when mixed with a nonferroelectric buffer material such as strontium titanate, becomes a suitable material for many practical applications. Although the mechanism of spontaneous polarization which describes ferroelectricity is somewhat complicated, the basic properties may be stated simply. In the ceramic of which practical capacitors are formed, the barium titanate is made up of a multitude of tiny particles whose spontaneous dipoles are randomly oriented. Therefore, the statistical average of the elemental orientations is zero, and there is no net polarization before application of an electric field. When a dc bias is applied to the material, some of the dipoles originally randomly oriented will align themselves with the field; consequently, the dielectric constant decreases. As the biasing field is increased, more and more dipoles are reoriented, and the dielectric constant continues to decrease until saturation is reached, i.e., until a further increase in bias fails to produce a proportionate increase in polarization. This presumably occurs as a result of the exhaustion of the supply of randomly-oriented dipoles. Unlike the back-biased diodes, the ferroelectric capacitors can be biased in either the positive or negative directions, and clipping will not result when they are used in a resonant circuit. Figure 1 compares the capacity-vs-voltage curves at 100 me of an inexpensive commercially-available back-biased diode and the Michigan Ferroelectric Capacitor. These two curves have been normalized to 3

ACTUAL VALUES >I Q. N rj 0 Z 90 80 70 60 ____OO/100% CAPACITY 100% BIAS TEMP DIODE 25 pf 130 VOLTS 27~ C FERROELECTRIC 125 170 VOLTS 27~ C UNIVERSITY OF MICHIGAN FERROELECTRIC CAPACITOR 4: 40 HUGHES (HC 7001) BACK- BIASED- DIODE CAPACITOR 30 20 101 20 60 100% 140 180 220 260 NORMALIZED BIAS FIG. I TYPICAL CAPACITY VS VOLTAGE CURVES AT IOOMG (NORMALIZED)

compare their characteristics. The small table at the top right gives the actual values for the components. The two curves were normalized for the maximum capacitance range of the diode. Other comparisons can readily be made. Figure 2 compares the Q-vs-voltage curves at 100 me of the same back-biased diode and the Michigan Ferroelectric Capacitor, while Fig. 3 compares the capacity-vs-temperature curves of the same two voltage-variable capacitors. Note that the last two curves are not normalized. 3. TABLE AND RELATED COMMENTS The list of manufacturers is not exhaustive, but it can be considered as representative (see Fig. 4). Some of the units listed are experimental and not commercially available. In the ferroelectric field almost any manufacturer of high-dielectric-constant ceramic capacitors should be able to provide capacitors with some degree of voltage sensitivity. Unless otherwise stated, initial capacitance is the smallsignal capacitance that exists when the device has zero bias. When bias is the independent variable and other parameters are held fixed, the small-signal capacitance appears to be a maximum at approximately zero bias for both the diode and ferroelectric. The frequency range of the device is the typical range in which the device may find application. Minimum capacitance is the small-signal capacitance that occurs when the device has maximum bias applied. 5

60 Q Q 0 100 200 VOLTAGE (VOLTS) - 300 FIG. 2 TYPICAL Q VS VOLTAGE CURVES AT 100 Mc. 6

I.4 -z w 0 z a0.; 5 25 30 35 40 45 50 55 TEMPERATURE IN DEGREES C - 65 FIG. 3 TYPICAL CAPACITANCE VS TEMPERATURE CURVES ACA128

BACK BIASED DIODE CAPACITORS MANUFAC- INITIAL MINIMUM MAX. CUTOFF R TURER TYPE CAPACITANCE CAR VOLTAGE FREQ. sees SENSITIVITYOMMENTS BELL LABS SI 43-7 2.15pf,OV 1.2pf,6V IOV 77KMC 2.3 ESSENTIALLY T INSENSITIVE EXPERIMENTAL UNIT MA-460A 4pf 1I.2pf 2 20KMC MICROWAVE MA- 460 B 2pf Ipf 3 30KMC JUNCTION CAPACITANCE ONLY SHF AOCIA MA-460C 1.8pf OV 0.4pf 6V 4 AT O KMC 40KMC.. MA-460D 1.4pf 0.2pf 5 50KMC CASE CAPACITANCE -.4pf a MA-460E I.Opf 0.2pf 6 60 KMC WESTERN 427 -A 15. AT.. " f JUNCTION CAPACITANCE ONLY, UH ELECTRIC SERIAL 7-1026.p 3 KMC CASE CAP.SUBTRACTED -SEE REF. 9 HPA- 2800. 6pf 7V C7 ASE CAPACITANCE I pf JUNC HUGHES 2.5pf, OV 70 KMC CAP. HPA-2810. 6pf 7V.2pf ONLY SCH-51 2pf 0.35pf IOV 100 AT 50MC 850 UHF TRANSITRON S 4.V50 AT I0MC AT 4V 5KMC 430 SCH-52 4pf O.B 7V 50 MC HC 7001 88pf 6pf 130V 360 39 HC 7002 120pf 12pf 80V 330 36 HC 7004 170pf 20pf 60V 270 30 HUGHES HC-7005 240pf O. V 46pf 25V 200 M 23 MA MAX..V MAX5V HC 7006 88pf 14pf 25V 175 20 HC 7007 120pf 22pf 25V 175 20 HC 7008 170pt 32pf 25V 175 20 INTERNATION- 6.8 SC20 35pf } V 2.5pf 200V AL RECT 100 SC2 470pf 80pf 20V V- 7 8T'f 3pf 25V 18 43 V-10 26pf 4.3pf 25V 18 43 V-12 31pf 5.2pf 25V 18 43 V- 15 39pf 6.5pf 25V 18 43 V-20 50pf lOpf 20V 18.7 40.2 V-27 70pf 14pf 20V 15.7 33.8 V-33 85pf 17pf 20V 14.6 31.4 V-39 lOOpf 20pf 20V 15.1 32.4 V-47 120pf 24pf 20V 15.4 32.4 PACIFIC V-56 145pf 32pf 15V [3.5 24.8 VHF V-68 175pf 39pf 15V 14.0 50MC 25.8 50MC SEMICONDUC- V-82 210pf O.IV 47pf 15V 13.0 23.9 V-100 260pf 57pf 15V 11.0 4V 20.2 MAX.V TOR V-7E 18pf 1.5pf IOOV 4.5 22.5 V-IOE 26pf 2.2pf IOOV 5.5 27.5 V-12E 31pf 2.7pf IOOV 6.5 32.5 V-15E 39pf 3.3pf 1OOV 7.5 37.5 V-20E 50Pf 5.0pf 70V 18.7 78.5 V-27E 70pf 7.0pf 65V 15.7 63.5 V-33E 85pf 9.0pf 60V 14.7 56.5 V-39E lOOpf II.Opf 55V 15.1 55.8 V-47E 120Pf 14.0Pf 50V 15.4 53.8 V-56E 145pf 20pf 40V 13.5 41.8 PHILCO T-1606 35,0.5V 8 30V 20 AT 50MC a 0.5V SYLVANIA D-1156 4pf, O.IV 0.5 pf 20 V. SC-I 24pf 4.4pf 22V 9s SC-2 48pf 8pf 22V 4.50 SC- 3 90pf 15pf 18V 3.0 TRANSITRON SC-5 120pf O.IV 25pf IIV 350 AT 5MC, 33AT 50MC 1.8. SC-7 165pf 55pf 9V a 4V B 4V1.5 SC-II 245pf 85pf 6V 0.95 SC- 15 360pf 120pf 6V 0.6 FERROELECTRIC CAPACITORS UNIVERSITY EDH 4100 AT OV. 500 200V/MIL 205 AT 50MC a OV,ABOUT TEMPERATURE SENSITIVE EDG-HS24FI max mm EXPERIMENTAL UNIT UHF OF MICHIGAN 8 30~C 30C 8 X GREATER AT 200V/MIL CURIE 30~C TO CAPACITY VARIATION 80 EXPERIMENTAL UNIT AUDIO UNIVERSITY 125pf AT OV ~400 V AT OV 25 CAPACITORS CAN B F ABRICATEMPERAUDIuO OF MICHIGANB 25C pf 25C 87 AT ~400 V I0 2 ATURE RANGE OF - H CAPACITORS CAN BE FABRICATED HAVING VALUES 0.6pf TO O. IjLf TO +50~C,CURIE 12~C AT OV VHF MU VSR } 400pf TO 88pf TO 300 V CURIE 22.5~C 5%AC VHF MUCON TO LVSR O.I\Lf 0.022/f ~ 300V FOR +120C TO + 30~C TO AUDIO MUCON VEVSE 60pf TO 36pf TO ~200V CURIE - 700C 5% AC LVSE 300pf 1pf f ~200V FOR +67~C TO +75~C GLENCO MATERIAL 393 STORAGE APPLICATIONS FERROELECTRIC MATERIALS MAY BE AVAILABLE FROM THESE MANUFACTURERS: AEROVOX,CENTRALAB,MULLENBACH,SPRAGUE,AND OTHERS FIG.4 TABULATION OF VOLTAGE-VARIABLE CAPACITORS 8

Maximum voltage is the highest bias voltage at which the device can be operated. In the case of the diode it is the maximum peak inverse-voltage. The maximum voltage for the ferroelectric is determined by its dielectric strength. Q is the energy-storage figure of merit. For a capacitor Q may be expressed as 1/WR C, where Rs is the equivalent series resistance. s s 1 Cutoff frequency, f, is defined as 2[ RCm. Specifications of are based on a measurement of and C at some frequency below of f are based on a measurement of R and C. at some frequency below c s mm f, then fc is calculated. Recent investigations indicate that Rs may not be a constant; in this event, fc would not be given by the above equation. Rs is the equivalent series-resistance of the device. For diodes, this is assumed to be a constant. For ferroelectric capacitors, however, no simple relationship for Rs as a function of the frequency is known, although it can be assumed constant at a given frequency. Temperature sensitivity is used here as a measure of the response of the component to incremental temperature changes. The performance of diode capacitors shows only a slight dependence upon temperature, whereas that of the ferroelectrics is quite sensitive to temperature. Storage-temperature or wiring-temperature of the ferroelectric capacitor is limited by the method of attachment of the leads to the ceramic, which is usually done with ordinary solder. In general, all of the devices may be used as tuning elements in oscillators and filters. The maximum power-level is a function of the device, the frequency, and the tuning range. Ferroelectrics have been used in oscillators for tuning, with a power output of 0.4 watt at 9

400 me with a 20 me tuning range, showing a negligible shift in capacitor characteristics due to heating of the capacitor by dielectric losses. Other general applications so described in the references are in parametric amplifiers, modulators, adjustable delay-lines, tunable antennas, thermometers, frequency converters, and distributed-variable devices. Some specific applications are in FM modulation, automatic frequency-control, electric receiver-tuning, panoramic-receiver tuning, tunable audio-oscillators, and tunable filters. The frequency range of the device is a function of its Q and the range of capacitance values that can be fabricated. That diodes can be made which have high Q's at high frequencies is illustrated by the Bell diode. Ferroelectrics can not now compete in this field. The Q of a good ferroelectric under the best conditions may be 4 at 3000 mc. In the future better ferroelectric materials may be developed. The maximum capacity of a ferroelectric can be large. For instance, an initial capacitance of 0.1 dfd can be attained with a slab approximately 1.2 x 1.2 x 0.1 cm in size. Fabrication of a diode of this capacitance value would be difficult. Both diodes and ferroelectrics are physically small components. For instance, a diode with an initial capacitance of 88 prtfd can be packaged in a case 0.25" long x 0.1" in diameter. A ferroelectric capacitor of 100 qtfad can be packaged in a plastic sphere about 0.1" in diameter. The cost of diodes in small quantities ranges from about one dollar to several hundred dollars. Because of the complexity and nonhomogeniety of diodes it is reasonable to expect that their minimum 10

price will remain at about the one-dollar level. The ferroelectrics are inherently easier to fabricate and therefore should have a lower price. Some applications require tracking of several circuits. The characteristics of diodes will vary significantly from one unit to another and introduce tracking problems unless units are selected. With ferroelectric capacitors it is possible to make all units for a given circuit from the same ceramic wafer, which results in good tracking of the capacitors. 4. COMMENTS CONCERNING MANUFACTURERS' CURVES Both diodes and ferroelectrics have complex characteristiccurves. It appears that.manufacturers, in an attempt to simplify the data, have left out information that is important. The result is that persons unfamiliar with the device may be misled. In general both diodes and ferroelectrics have capacitance that is a function of bias and temperature, and Q as a function of bias, temperature, and frequency. Some diode manufacturers tabulate Q at a specific frequency at maximum bias and fail to indicate the Q at zero bias or describe the law for Q. Assuming that R is independent of bias and that the initial capacitance s is 10 times the minimum capacitance, a minimum Q of 1/10 the tabulated value is given. It is also necessary to consider signal level relative to the bias voltage for, if a diode is driven into conduction, the Q will drop and the effective capacitance will change. 11

In presenting data on ferroelectrics some manufacturers plot percent capacitance change vs bias, with temperature as a parameter and all curves starting at zero bias and 100% capacitance. This tends to give a misleading picture of the temperature characteristics. The c-T-E surface in Fig. 5 is a more descriptive type of presentation. 5. CONCLUSIONS In conclusion it should be reiterated that the tabulation is not exhaustive but merely representative. It would seem that a complete listing of all the available diodes which might possibly be used in reactance and other voltage-tunable devices would make the tabulation very unwieldy indeed. In general, recommendation of a particular diode for a specific purpose was avoided, since it was considered likely that the reader might then be biased against using that diode for other applications. It is perhaps more beneficial to require the reader to study the characteristics and make his own choice for a particular application. 12

EAC =.005KV CM-' RMS AT IKC * CYCLING FIELD ONE POLARITY *CYCLING FIELD RATE = IOOKV CM-' MIN-' *DATA PLOTTED ONLY FOR E DC INCREASING K-, 1 (1 C-T-E SURFACE FOR AEROVOX "HQ''91 FIG. 5 13

REFERENCES 1. L. J. Giacoletto and J. O'Connell, "A Variable-Capacitance Germanium Junction Diode for U.H.F.," RCA Review, Vol. XVII, No. 1, March, 1956. 2. Gene F. Straube, "A Voltage Variable Capacitor," Electronic Industries, May, 1958. 3. Ao Uhlir, Jr., "The Potential of Semiconductor Diodes in High Frequency Communications, Proc. IRE, Vol. 46, June, 1958. 4. H. Heffner and K. Kotzebue, "Experimental Characteristics of a Microwave Parametric Amplifier Using a Semiconductor Diode," Proc. IRE, Vol. 46, June, 1958. 5. T. W. Butler, Jr., W. J. Lindsay, and L. W. Orr, "The Application of Dielectric Tuning to Panoramic Receiver Design," Proc. IRE, Vol. 43, September, 1955. 6. George S. Shaw and James L. Jenkins, "Nonlinear Capacitors for Dielectric Amplifiers;" Electronics, October, 1953. 7. P. Popper, "An Electrical Engineering Review - Ferroelectric Materials," Journal I.E.E., August, 1956. 8. T. W. Butler, Jr., "Ferroelectrics Tune Electronic Circuits," Electronics, January 16, 1959. 9. K. L. Kotzebue, "A Semiconductor-Diode Parametric Amplifier at Microwave Frequencies," Stanford Electronics Laboratories, Technical Report No. 49, November 4, 1958. 14

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