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ransistors versus Vacuum Tubes*D. G. FINKt, FELLOW, IRE

IT IS NOT the intention of this review to empha-size competition between transistors and vacuumtubes, as the title might suggest, although it must

certainly be admitted that there is a contest betweenthese devices in the important matters of technicalmanpower and funds supporting their development.Rather, the purposes in contrasting these devices are:first, to compare the status of their development and,second, to define their respective spheres of utility in thehope that such a survey may assist in guiding theirfuture development along constructive lines.The triode vacuum tube is forty-eight years old, the

transistor seven, going on eight. Based on their relativematurity, one would expect sharply different scales ofactivity, and this is indeed the case. For example, oneindex of activity is the type numbers which have beenassigned to transistors and vacuum tubes. The Inter-national Radio Tube Encyclopedia, the Vade Mecumpublished in Europe, lists all the type numbers of allthe vacuum tubes ever produced anywhere in the world.The present total is 18,500 type numbers, many as-signed to identical structures to be sure, but a very im-pressive figure nonetheless. Simnilar figures for transis-tors are harder to come by, but the latest compilation oftype numbers produced in the United States containsapproximately 235 types; and if we add foreign numbers,the list probably does not exceed 350.

Another index of activity is production. Total tran-sistor production in seven years to date certainly doesnot exceed 4,000,000 units. We make that many re-ceiving-type vacuum tubes in two working days in thiscountry alone, as many as 42 million of them in onemonth, 450,000,000 in the last 12 months. Moreover,at a conservative estimate, the all-time worldwideproduction of vacuum tubes exceeds seven billion-three tubes produced in a generation for every man,woman and child on the globe.A third, most significant, index of technical accom-

plishment is the variety of practical applications to whichthe two devices have been put. It is feasible only toindicate the range of application in vacuum tubes bycomparing two types which illustrate extremes of power-handling capacity. The grandfather of all vacuum tubesin this respect is the super-power beam triode, type5831, a water-cooled, three-foot-high electronic enginewhich will absorb the astonishing plate input power of650,000 watts. A pair of these tubes, in broadcast-bandor medium-shortwave service, will deliver an averagecw power of one million watts.

* Original manuscript received by the IRE, October 31, 1955;revised edition received December 27, 1955. Luncheon addresspresented on October 24, 1955 before the Professional Group onElectron Devices, Washington, D. C.

t Philco Corp., Philadelphia, Pa.

At the other extreme of power-handling capacity intubes, consider the miniature rf amplifier tube used inthe tuners of a majority of the 20,000,000 televisionsets produced since 1952, that is, since the adoption ofthe gold-plated grid in such tubes. This tube will ampli-fy usefully at 200 megacycles a fringe-area televisionsignal of 10 microvolts across 300 ohms, representing apower available from the antenna of a mere one-sixthof a micromicrowatt. We thus find that the type 5831super-power transmitting triode customarily handles asignal power which exceeds that handled by the 6BQ7miniature rf pentrode by nineteen orders of magnitude.This is, perhaps, a fair indication of the power rangecovered by vacuum tubes. Transistors cover a muchsmaller range of power, about thirteen orders of magni-tude, reaching into the watts, rather than megawatts.

In one field, the transistor has completely displacedthe vacuum tube. In 1954 the hearing aid industry pro-duced some 360,000 instruments, only 25,000 of whichused vacuum tubes. In 1956, no hearing aid in produc-tion uses any vacuum tubes at all.The technical reason for this complete victory is the

fact that the transistor is the most efficient amplifierof low-level audio-frequency energy known to science.It needs no A battery and it makes extraordinarilygood use of the current provided by a low-voltage Bbattery. The net result is that the transistor hearingaid pays for itself, in lower cost of battery power, in ayear of regular use. No wonder the transistor has takenover in this department. A similar trend, having lesseconomic urgency behind it, is showing up in portableradio sets. And in battery-operated and airborne mili-tary electronic equipment the transistor definitely hasits foot in the door, for similar reasons.The growing strength of the transistor is even more

impressive in another field, the electronic digital com-puter. Here economy of power consumption is impor-tant, but not so important as utter reliability. Thetransistor designers have been arguing for many monthsthat transistors are more reliable than vacuum tubes incomputing machinery, but those in charge of the fundswhich support computer developments have had un-derstandable doubts. Now it appears that the doubtsare being resolved and that there will be general agree-ment, before this year is out, that the transistor is in-deed the preferred active element in computing ma-chines.The emphasis on reliability here comes from the fact

that digital computers capable of taking on the problemsof modern warfare and modern industry must containthousands, or tens of thousands, of tubes or transistors,and the computers must operate without error or inter-ruption for many days or weeks at a time. Since the

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failure of even one tube or transistor from among thethousands in use can produce large errors in the com-putation, and since failure occurs in a statistical distri-bution, it is necessary that the life expectancy of theindividual tube or transistor shall be many thousandsof times as long as the period of uninterrupted servicedemanded of the computer as a whole. Thus a week oftube life is not sufficient for a week of computer service;rather thousands of weeks of life are needed in the indi-vidual units. The life expectancy of the transistor, forreasons to be described later, is far greater than that of avacuum tube of similar properties and cost; hence, itscommanding position in this field.On another front, however, the transistor does not

have the commanding position. This is the field of ex-tremely high-frequency operation. To achieve high-frequency operation in any electronic device, the chargecarriers (electrons or holes) must move through theactive control region of the device in an extremely shortperiod of time. This means that the active region mustbe of small dimension, normal to the charge flow, andthat the motion of the charge carriers must be rapid.On the first score, small dimensions, the transistor

is already ahead. For example, the smallest active regionin a production-type transistor, the base thickness ofthe 2N128 surface-barrier transistor, is one ten-thou-sandth of an inch. The smallest corresponding region ofa triode vacuum tube, the grid-cathode spacing in themicrowave triode used in the transcontinental televisionnetwork, is six times as great.The high-frequency trouble with transistors is not

dimensions. Rather, it is the principle underlying themotion of the charges within the transistor. In vacuumtubes, the charge carriers (electrons) move through avacuum within which the applied electric field is fullyeffective in accelerating them to high speed. Moreover,since a good vacuum is an excellent insulator, highvoltages may be applied to achieve phenomenally rapidtransits of the electrons.

In present-day transistors, however, the useful chargecarriers are interspersed among a vast multitude of non-useful charges of the opposite sign, and the presence ofthe latter charges prevents the applied voltage fromproducing an electric field within the body of the tran-sistor. Consequently, the useful carriers, instead ofbeing whisked across the transistor by electric attrac-tion, move in the relatively aimless motion of diffusion,like people drifting away from an overcrowded room.

This relatively slow motion is a block on the road tohigher-frequency performance of transistors, and it faroutweighs the effect of small dimensions. For example,the surface-barrier transistor mentioned is a 50-mega-cycle unit; the microwave triode with a sixfold disadvan-tage in dimensions, achieves an 80-fold advantage infrequency. It works nicely as an rf amplifier at 4,000megacycles, higher than any transistor has been evenrumored to operate to date.

We know the general path around this slow-motionroadblock. This is to remove, partially or wholly, thenonuseful majority carriers which dissipate the ap-plied electric field, which can be done by changing thedistribution of impurities in the semiconductor in aknown manner. This is not to say that the work ontransistors using intrinsic materials or graded distribu-tions of impurities has gone very far in a productionsense. It merely points the way to be followed if thetransistor is to imitate the charge-motion patterns ofvacuum tubes.

It must be emphasized that the diffusion mode ofmotion, typical of transistors available today, is a realadvantage, when the frequency of application lies inthe range up to 100 megacycles which such transistorsnow cover. This advantage lies in the fact that lowvoltage sources suffice to operate these transistors. Andlow voltage, particularly in battery-operated equip-ment, is an important practical consideration.On still another front, tubes and transistors are differ-

ent breeds of cat. This is high-power operation. Highpower in an electronic device means high current, whichrequires a large cathode and a large plate; and it meanshigh voltage, which requires good insulation. Neitherrequirement is any particular hindrance to the vacuumtube designer. In fact, vacuum-tube cathodes can bedesigned larger and larger almost without limit; thefilament current of the 5831 super-power beam triode,previously mentioned, is 2,220 amperes; the filamentheating power is 13 kilowatts, and the typical platecurrent is 41 amperes.To get rid of the heat generated in a high-power de-

vice, it must be big enough to present a large surface tothe cooling medium. Large size is also dictated by theneed to get sufficient insulation to permit high voltagesto be applied. Large dimensions work against high-fre-quency operation. For many years the size of high-powertubes limited the amount of power that could be gene-rated at such frequencies as, say, 1,000 megacycles. Butthe technique of compartmentalizing the electrons intogroups, as in the klystron, cavity magnetron, and travel-ing wave tube, has changed all this. Today, advancedversions of the klystron will generate 10,000 wattscontinuously at 1,000 megacycles, and cavity magne-trons will generate brief peaks in the order of hundredsof kilowatts in the range of 10,000 megacycles and above.Where does the transistor stand in this situation? The

part of a transistor which corresponds to the vacuumtube cathode is its emitter. The semiconductor emitterhas the important advantage, relative to a cathode, ofbeing able to produce more carriers per square centi-meter.But as of the moment, the transistor designer is

having a very difficult time designing large emitters,even though his brother in the high-power semiconduc-tor diode department is making some startling break-throughs in this region. The semiconductor surface onwhich the transistor emitter is formed must be perfec-

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tion itself. If any crystal defect or other inhomogeneityis included within the emitter, the number of usefulminority carriers emitted falls away all out of propor-tion.The way out here is also clear in principle but fraught

with practical difficulties. We must seek a wholly neworder of control in the metallurgical preparation of semi-conducting materials, which is already the most skilledart that metallurgy has to offer. We must know, also,a lot more about the surface properties of the semicon-ductors under the emitter electrode and around itsedges to prevent a fatal loss of useful carriers, evenwhen the metal itself has been urged to perfection.Sometime we will, no doubt, know how to design and

produce an emitter of any required size. If, by then,we also know how to apply high voltages to the materialwithout burning it out at hot spots or fracturing it fromelectric stress, we will have the basis of transistor de-sign worth the name "high power" as it is used in thetube laboratories.Such structures will have to be large to get rid of heat

losses, and this means that the charge-carrier groupingprinciple will have to be introduced to the transistorif it is to achieve both high power and high frequency.At present there is no analog to electron grouping intransistors. But if we arrange that the useful carriersare in the majority, rather than in the minority as atpresent, there seems no fundamental bar to chargegrouping in transistors. At this stage, the really high-power, high-frequency transistor becomes a possibility.

All this will, of course, take a lot of effort and it maywell be asked, "Why bother?" After all, these high-power, high-frequency jobs are being very well handledby vacuum tubes right now, Why insist on a transis-torized version? Transistors in the kilowatt class won'tbe battery operated. Moreover, they are likely to con-tain a large amount of highly refined silicon, and, hence,are likely to be rather expensive.

Should we take on the massive labor of improvingtransistors until they compete on every front withtubes? From a long-range point of view, perhaps so;after all, we can't stop, and we don't want to stop, thesteady extension of the transistor principle, nor thesteady extension of the vacuum tube principle, to fron-tiers not now in sight. But there is a great deal to besaid for another shorter-range point of view, extendingperhaps only over the next five years. During that peri-od, it would appear prudent to emphasize that we have,in tubes and transistors, two quite different electroniccitizens, capable of holding down quite different jobs,and we might do very well to concentrate on puttingthem to the most effective uses, not in competition, butas members of a team.

It is appropriate, therefore, to recommend that wepay particular attention to the special abilities of tran-sistors in the areas of immediate application. The justi-fication for this goes back to the matter of life expec-tancy previously noted for its importance in large assem-

blies of electronic devices, such as digital computers'To this we must add the importance of long life insmaller assemblies, where safety of life and effectivenessin military operations depend on completely dependableoperation, unfettered by a statistical expectation offailure.On this question of life expectancy we have some

important new evidence. In at least two laboratorieslife tests of certain types of transistors over periods inexcess of 10,000 hours have shown such a phenomenallylow failure rate that the extrapolated life expectancy,using statistics applicable to vacuum tubes, exceedsone million hours, a period extending from now to 2069A.D.! It must be admitted that the exponential failurerates assumed in these statistics may not (in fact, prob-ably do not) apply to transistors, and that much morelife testing must be completed before the million-hourfigure can be accepted with the same assurance thatnow surrounds the life expectancy of the 40-year(350,000-hour) repeater tubes, now being laid in cablerepeaters at the bottom of the Atlantic. But the evi-dence is so startling that even conservative forecastersare willing to concede that transistor life is on a newplateau, far above that of vacuum tubes. In fact, withevery passing day, the conviction grows that transistors,properly made and properly applied, can outlast justabout any garden-variety component used in present-day electronic equipment, except possibly the chassisbase.We have become used to the fact that the vacuum

tube is the weak sister in just about every assemblageof electronic components. We accept the fact that tubeshabitually fail in service, at times which can be predictedonly statistically and which give no assurance, for ex-ample, that the autopilot definitely will be working dur-ing the next hour. We go along with the fact that tubesare the most delicate item on the chassis deck. And weforget that it's an unusual tube whose technical capa-bilities, in power or frequency, fully match up to thoseof the components with which it is connected. Weexcuse all this by saying, "The tube is the active ele-ment; it has the toughest job; to make it an equal part-ner with every resistor, condenser and coil is just noteconomical."

But, before the electronic industry will have reachedmaturity we must accept, as a categorical imperative,that electron devices, tubes and transistors, must beequal partners with other components, equal in life,equal in ruggedness, equal in sharing the load.

In this department, the transistor is taking the lead.Transistors are rugged, naturally so. And long life intransistors, given a certain amount of essential know-how, comes relatively easy. Tubes are not naturallyrugged; ruggedness must be developed. And they arenot naturally long-lived.The fundamental limit to vacuum tube life, assuming

it doesn't meet an accident or suffer a mechanical fail-ure, is exhaustion of electron emission at the cathode.

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Electron emission depends not only on a steady flowof electrons to the cathode from the external circuit,but also on the presence of very special conditions atthe cathode surface, including impurity elements whichlower the work function and allow the electrons to es-cape. The surface condition appears, by every evidencegathered over nearly fifty years, to be subject to in-evitable deterioration as the impurity elements boiloff or otherwise escape. And there is no sure mechanismfor putting them back. So, sooner or later, the emissionfalls off and the tube goes dead. Much can be done toarrest the deterioration, as in the case of the 40-yearsubmarine cable repeater tube. But the life expectancyof this tube (about 0.02 per cent failures at 1,000 hours)was achieved in spite of the fact that it uses cathodeemission, not because of it. As a result, the tube is usedso far under normal ratings, and is so painstakinglyput together that its use can be justified only in veryspecial circumstances.

Contrast this with the "million-hour transistor"postulated above. It achieves long life expectancy with-out overdesign or costly production methods. This ispossible because the charge emission process in a tran-sistor is fundamentally different from that in a vacuum

tube. The transistor emitter is self-replenishing, in-definitely. Transistors do burn out, of course, for alarge number of other reasons; and they do lose theiramplifying function if they are overheated. But theydo not fail due to exhaustion. Tubes do.At the moment this difference appears to be funda-

mentally rooted in the principle of operation of the twodevices. If further investigation confirms this view,we may then be sure that long life (which means un-varying ability to amplify) will always be easier to getin a transistor than in a vacuum tube. The conclusionthen is evident. We should use transistors, now, wherelong life and ruggedness is important. We should usetubes in the many areas where transistors, for the pres-ent anyway, can't handle the job.

Perhaps, then, we can conclude with the observationthat the two devices have a lot to learn, one from theother. In such a situation, fast and easy communicationfrom one group of technical workers to another is essen-tial to rapid progress. The Professional Group on Elec-tron Devices, a single group having cognizance of bothdevices, is in an ideal position to foster this communica-tion through its conferences, its TRANSACTIONS, andthrough the PROCEEDINGS OF THE I RE.

The Cryotron-A SuperconductiveComputer Component*

D. A. BUCKt

Summary-The study of nonlinearities in nature suitable forcomputer use has led to the cryotron, a device based on the destruc-tion of superconductivity by a magnetic field. The cryotron, in itssimplest form, consists of a straight piece of wire about one inch longwith a single-layer control winding wound over it. Current in the con-trol winding creates a magnetic field which causes the central wireto change from its superconducting state to its normal state. The de-vice has current gain, that is, a small current can control a largercurrent; it has power gain so that cryotrons can be interconnected inlogical networks as active elements. The device is also small, light,easily fabricated, and dissipates very little power.

THE CRYOTRON PRINCIPLE

BEFORE describing the cryotron as a circuit ele-ment and potential computer component, thebasic physical phenomena underlying its opera-

tion will be described.* Original manuscript received by the IRE, November 10, 1955.

The research in this document was supported jointly by the Army,Navy, and Air Force under contract with M.I.T.

t Division 6, Lincoln Lab., M.I.T., Lexington 73, Mass.

Superconductivity

Superconductivity was discovered in 1911 byH. Kammerlingh Onnes at Leiden, three years after hesucceeded in liquifying helium. While extending electri-cal resistance measurements to this new low-tempera-ture region he found that the resistance of mercury dropssuddenly to zero at 4.12°K. Soon many other materialswere shown to display this same unusual behavior.Niobium becomes a superconductor at 8°K, lead at7.20K, vanadium at 5.1°K, tantalum at 4.40K, tin at3.70K, aluminum at 1.20K, and titanium at 0.50K. Inaddition to 21 elements, many alloys and compoundsare superconductors with transition temperatures rang-ing between 0 and 170K.1 2

1 D. Schoenberg, 'Superconductivity," Cambridge UniversityPress, Cambridge, England; 1952.

2 F. London, "Superfluids," John Wiley & Sons, Inc., New York,N. Y., vol. 1; 1950.

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