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The Aerovox Research Worker is edited and published by the AerovoxCorporation to bring to the Radio Experimenter and Engineer, authori-tative, first hand information on capacitors and resistors for electricaland electronic application.

VOL. 34 NOS. 10 - 11 - 12 OCTOBER - NOVEMBER - DECEMBER, 1964 Subscription ByApplication Only

Semiconductor Devices as Nonlinear Resistors

By the Engineering Department, Aerovox Corporation

Modern sophisticated electron designoften calls for a nonohmic resistance ele-ment; i. e., one in which R varies non -linearly with current or voltage (andsometimes which may vary approximatelylinearly but is dc -sensitive) . This re-quirement is not recent in origin, how-ever, as is attested by the article "Non-linear Resistors" in the October -Novem-ber 1953 issue of the Research Worker.

That article was devoted only to the non -ohmic resistance of thyrite, thermistors,tungsten -filament lamps, forward -biasedgermanium diodes, and miniature fuses.

But during the ensuing decade, othercomponents have become available, whichextend the usefulness of nonlinear resis-tance.

Other components in which nonlinearresistance is available as a supplementaryeffect, and which merit present discus-sion, include silicon diodes and rectifiers,zener diodes, transistors, a n d tunneldiodes. The peculiar resistance characte-ristic of these components opens newvistas of application in control, computa-tion, instrumentation, and communica-tions. The performance of these compo-

nents is discussed separately below.

SILICON DIODE

Forward -biased germanium small -signaldiodes have long been used as nonlinearresistors to provide an average resistancevariation of 100:1 (resistance decreasesfrom 10,000 ohms to 100 ohms as thediode voltage drop is increased from 0.1 vto 1 v) . Forward -biased small -signal sili-con diodes provide similar nonlinear re-sistance change, with the added advan-tage of improved temperature characte-ristics.

AEROVOX quie,st 9#i eleektanic CainizaaeottsPrinted by Aerovox Corporation, New Bedford, Mass., U. S. A. Copyright 1965 by Aerovox Corporation

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NFIGURE 1

RESISTANCE CHARACTERISTIC OFPLANAR PASSIVATED EPITAXIAL

SILICON DIODE

Figure 1 shows the resistance -vs -dc volt-age characteristic of a forward -biased(i. e., anode positive, cathode negative)planar epitaxial passivated silicon diode(General Electric Type 1N3606) at 25°C.Note that the resistance decreases from40K at 0.4 vdc to 4 ohms at 1 vdc, aresistance change of 1000:1 for a voltagechange of 2.5:1. Corresponding currentlevels are 0.01 ma at 40,000 ohms, and25 ma at 4 ohms. At +150°C, the re-sistance change, for the same voltagechange, becomes 266 ohms to 3.3 ohms,and at -55°C, it becomes 4 megohmsto 33 ohms.

SILICON RECTIFIERS

When a high value of current and/orvoltage must be used in a nonlinear cir-cuit, a silicon rectifier having suitablecontinuous dc ratings must be substitutedfor the small -signal diode.

Rectifiers (sometimes called powerdiodes) are supplied in a wide variety ofcurrent and voltage ratings - from lessthan 100 v to several kv, and from afew milliamperes to several hundred am-peres. Correspondingly varied resistancevalues (both from a standpoint of ab-solute levels and range of variation) areprovided by these rectifiers. Figure 3,

for example, shows the resistance charac-teristic of an 850 -ma rectifier (RCAType CR101) . Here, the resistance de-creases from 1.8 ohm to 0.23 ohm (a re-sistance change of 7.84:1) as the forwardd -c voltage is increased from 1.8v to 3.4v.(This corresponds to a current variationfrom 1 A to 15A. A rectifier may beselected to operate at the current and/orvoltage available for- the application, andthe obtainable resistance range then maybe determined from calculations of staticd -c E/I values, the slope of the resultingcurve resembling that given in Figure 2.

Like the small -signal silicon diode, thesilicon rectifier has improved tempera-ture ratings, compared with those of itsgermanium counterpart.

ZENER DIODE

Operation of the zener diode is charac-terized by its sharp reverse breakdown(rapid increase of current at a discrete re-verse voltage). This peculiarity, which hasbeen much exploited in voltage regula-

tion, gives rise to a distinctive nonlinearresistance feature. The zener diode isreverse -biased; i. e., anode negative andcathode positive.

Figure 3 shows the resistance charac-teristic of a reverse -biased 3.9 -volt zenerdiode (International Rectifier Type1N1518) . Here, the resistance decreasesfrom 140K to 11 ohms as the voltagedrop is increased from 0.6 v to 3.9 v (aresistance change of 12,700:1 for a voltagechange of 6.5:1). The corresponding cur-rent increases from 4 microamperes to250 milliamperes.

Zener diodes are available in a widevariety of current, voltage, and powerratings. As is true with small -signaldiodes and rectifiers, a unit may bechosen to withstand current and/or volt-age available in the circuit. The obtain-able resistance range then may be de-termined from static d -c E/I calcula-tions, the slope of the resulting curveresembling that given in Figure 3.

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FIGURE 2. RESISTANCE CHARACTERISTIC OF SILICON POWER RECTIFIER(RCA CR101)

CFigure 6 shows the circuit for a field

-effect transistor. Here, R is a 50,000 -ohm1/2 -watt resistor, B a 11/2 -volt bias battery,and X the field effect transistor (AmelcoType FE200). The resistance availableat terminals A and B (with A positive)increases from 10K to 41K as the appliedvoltage is increased from 5 v to 45 v(corresponding to a current change from

0.5 ma, to 1.1 ma).

Selection of Transistor Resistance Range.Changing the emitter -base input biaswill change the resistance of the transis-tor at any discrete value of collector volt-age. Thus, the resistance range of anytransistor, together with its absoluteresistance limits, may be selected simplyby changing the value of fixed bias -current bias in conventional small -signaland power types; voltage bias in thefield effect type.

TUNNEL DIODE

The forward -biased tunnel diode pro-vides not only nonlinear resistance, butalso negative resistance over a portionof its ELI characteristic. Figure 7 showshow this resistance varies with low valuesof forward d -c voltage drop in a GeneralElectric Type 1N2940 tunnel diode.

The diode resistance is positive andstable between its 10-mv and 55-mvvalues, negative and unstable from 55my to 350 my, and positive and stableagain from 350 my to 400 my andbeyond. The current peak (1 ma) occursat 55 my, and the current valley (0.1

ma) at 350 my, for the particular diodetested by the editors.

WHERE USED

Some of the automatic actions providedin a simple manner by nonlinear resistorsinclude voltage regulation, frequencymultiplication (harmonic generation),curve shaping, curve correction, ampli-tude stabilization, signal compression,signal expansion, modulation, bias con-trol, signal clamping, instrument protec-tion, a n d transient suppression. Thesemiconductor devices discussed in thisarticle have extended the range of use-fulness of nonlinearity and are valuableadditions to the stock of early nonlinearresistors.

Equipment in which nonlinear resistoraction may be exploited include test in-struments, control devices, communica-tions apparatus, electromedical gear, anddomestic and industrial electrical andelectronic machinery.

NONLINEAR RESISTANCEAVAILABLE HERE

FIGURE 6. FIELD EFFECT TRANSISTOR AS NONLINEAR RESISTOR

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FIGURE 7. RESISTANCE CHARACTERISTIC OF TUNNEL DIODE

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NONLINEAR RESISTANCEAVAILABLE HERE

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FIGURE 4. TRANSISTOR AS NONLINEAR RESISTOR

TRANSISTOR

The common -emitter - connected tran-sistor will provide a nonlinear resistancein its collector -emitter (output) circuitwhen a small fixed bias current is sup-plied to its base -emitter (input) circuit.Figure 4 shows the circuit: X is thetransistor, B a 11/2 -volt d -c bias source,and R a 20,000 -ohm wirewound rheostatfor setting the bias current to 0.1 milli-ampere. The internal collector -emitterresistance varies nonlinearly as a voltageapplied to terminals A and B (with Anegative) is varied linearly.

Conventional Small - Signal Transistor.

For low -voltage applications (i. e., wherethe cl-c voltage drop across the transistorwill not exceed 221/2 v) , a conventional,small - signal transistor is satisfactory.Common - emitter response curves forvarious transistors may be inspected forthe absolute resistance values and resis-tance variation desired. (Resistance maybe calculated from the collector voltageand collector current values obtainedfrom the curves: R=Vc/Ic.)

Figure 5 shows the collector resistancevariation for the circuit given in Figure4. When rheostat R is set for a biascurrent, I, of 0.1 ma, the resistance variesfrom 118 ohms at a collector -to -emittervoltage drop of 1 v to 1160 ohms at 12volts. (This resistance change of 9.84:1corresponds to a current change from8.5 ma to 10.32 ma.) The transistor is

RCA Type 2N109. Unlike the previous

examples, here the resistance varies di-rectly as the applied voltage; and al-though it is very nearly linear, it isvoltage -dependent.

Power Transistor. When higher voltageand/or current must be used, a suitablepower transistor must be chosen. Thecircuit is the same as Figure 4, exceptthat B and R must be chosen for higherbias current. With a Delco Type 2N2827transistor, biased at 1 ma, the resistanceincreases from 40 ohms to 156 ohms asthe voltage is increased from 5 v to 25 v(corresponding to a current increase from125 ma to 160 ma) . When this transistoris biased at 4 ma, the resistance increasesfrom 17.7 ohms at 5 v to 61.6 ohms at20 v (corresponding to a current increasefrom 280 ma to 325 ma) .

Field Effect Transistor. Operation ofthe unipolar field effect transistor (FET)approximates that of a vacuum tubemore closely than is possible with a con-ventional transistor. In particular, theinput impedance (resistance) of t h eFET is very high (similar to the tubegrid input) , so this device is essentiallyvoltage operated as far as an input -biassignal is concerned.

1400

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VOLTAGE DROP ACROSS TRANSISTOR (-DCV)

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FIGURE 5. COLLECTOR RESISTANCE CHARACTERISTIC OFSMALL -SIGNAL TRANSISTOR

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FIGURE 3. RESISTANCE CHARACTERISTIC OF ZENER DIODE

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