the miniature wonders of transistors

THE MINIATURE WONDERS OF TRANSISTORSAuthor(s): L. J. DAVIESSource: Journal of the Royal Society of Arts, Vol. 110, No. 5073 (AUGUST 1962), pp. 638-655Published by: Royal Society for the Encouragement of Arts, Manufactures and CommerceStable URL: http://www.jstor.org/stable/41367189 .

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THE MINIATURE WONDERS

OF TRANSISTORS

A paper by

L. J. DAVIES , C.B.E. , M.A. , B.Sc ., M.I.E.E.,

Director of Research y Associated Electrical Industries

Ltd., read to the Society on Wednesday , 14^ March ,

1962, with G. S. C. Lucas , O.B.E. , President , Institution

of Electrical Engineers , in the Chair

the chairman : I have known Mr. Davies for many years but I have never been quite sure whether he takes a greater pride in being a scientist or an engineer. Like a skilled circus rider with two horses, he is delicately poised between them, in full command while they orbit in the ring. As Director of Research for Associated Electrical Industries his interests range widely over the whole field of light and heavy engineering from materials for steam turbines to semi-conductors, from lamps to electronics.

He is always keen to defend the research worker's right to explore a new field unfettered by preconceived ideas of its ultimate usefulness, but at the same time he is very well aware that a successful industrial research organization is one which can see the results of its past researches earning profits by the products of the factories. Nothing gives him greater pleasure than to see a piece of research at work in the factory and later in the service of the customer. This he has certainly seen with semi-conductor devices.

The following paper y which was illustrated with lantern slides, models and experiments, was then read.

THE PAPER

INTRODUCTION

This lecture attempts to give a general viewpoint of a technology which is very young but which is already having an important effect on our lives. The basis of it is an understanding of the phenomena due to minute modifications to lattice structures and the ability to achieve and control these under manufacturing conditions: not the 'macroscopic' ones to bring about strength or ductility or new chemical mole- cules, but sub-microscopic modifications to secure special electrical behaviour.

The transistor is thus a piece of electrical apparatus, but in comparison with orthodox items it is minute, silent, extremely efficient, solid and robust, with no moving parts and requiring only low voltages for its operation.

The development of the electronic valve in the 1920s and 1930s provided a device in which one could produce and control free electric charges. The ability to control electric current without moving parts with electrodes that do not absorb power, so positioned that they can amplify an incoming signal or produce oscillations, established a new industry, that of radio communication, with the valve as the unchallengeable deus ex machina . This was followed by 'electronics', the application

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AUGUST 1962 THE MINIATURE WONDERS OF TRANSISTORS

to industrial processes of electronic valve control circuits. But in industry the work of the valve could sometimes be done in other ways, and certain of its features such as the requirement of power to heat cathodes, its bulk and apparent fragility, and the short lives compared with those of orthodox repairable equipment, caused the designers of industrial apparatus and machinery not to be enamoured of them.

The development of the transistor in the '50s has revolutionized this position and has made of electronics a new industry capable of doing very well many things in which previously it had faltered. A range of uses theoretically possible with valves becomes practicable with transistors, because of a set of properties that fits them almost perfectly into the place in electronics that valves created but could not always elegantly fill. A computer requiring 50,000 switching devices (and in some cases up to 200,000) is a good example of an application in which the very small size and low internal losses - particularly the absence of cathode heating watts - of the transistor make it exactly right for the job. In addition, it provides the necessary robustness, reliability and long life to a very high standard.

It is at this stage that one should refer to a classic demonstration impossible without the transistor. In the original form,1 the transistor, in an oscillatory circuit, operates from the minute power available from a primary cell made from a copper and a silver coin separated by blotting paper moistened with saliva. A more powerful but equally striking demonstration is made by plunging electrodes of copper and magnesium alloy respectively into a lemon. This high impedance cell with an open circuit voltage of about 1.4 volts is sufficient to cause current to flow through the transistor and to produce oscillations of current at audio-frequency sufficiently powerful to operate a 'miniature loudspeaker' (a deaf-aid earpiece in fact). It is an easy step from this to the little radio sets that are already becoming known as 'transistors' and are so efficient and so plentiful that one notices them with both irritation and respect.

THE PARTS OF A SIMPLE TRANSISTOR

What is this device, discovered2 only 14 years ago, which is changing our environment by its adaptability to our personal requirements as well as by being a new and powerful adjunct to many of the things that the electrical engineer strives to do? If one is to be dissected, it is well to choose a simple form, and to use a microscope. {In the lecture a model 100 times full size was used.)

Removal of the can discloses three leads going to pins or terminals in the base. Removal of these and a sub-frame of nickel leaves the essential part wherein occurs the transistor action. This consists of a thin slice of the semi-conductor material, germanium. (Although transistors have been made of other semi-conductor materials, in practice germanium or silicon is always used.) On to either side has been melted a small blob of indium. It is these two blobs and the slice of semi-conductor that are connected into circuit. Transistor action thus takes place in a solid. An enlarged cross-section3 (Figure 1) taken through the blobs of indium and the germanium slice, reveals, when suitably polished and stained, a structure with discontinuities. The indium has melted and alloyed with the germanium, dissolving and pene-

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962

[J. E. Hughes Figure i. Enlarged cross-section taken through the blobs of indium and the germanium slice

trating it. This process stops after a certain penetration and recrystallization takes place, the germanium throwing out most of the indium as it solidifies, but retaining a minute amount. A well defined and regular line of demarcation can be seen. The uniformity that results from such an ordered process is highly desirable for efficient manufacture. It arises from the fact that ease of penetration is different for different crystal planes. The germanium slice is cut, from a single crystal, parallel to a (hi) crystallographic plane. The lattice structure is such that in this plane each atom is joined to others in the plane by three double bonds, and the planes are joined by single double bonds from atoms of the respective planes. It is these plane-connecting bonds that are most easily broken, and the penetration progresses by the peeling off of layers. Evidence for this can be seen from an experiment in which the alloying was done with a crystal cut 9 degrees off the ( 1 1 1 ) plane. The junction is flat, but at the corresponding 9 degree angle with the surface.4 (See Figure 2.)

This is then the essential mechanical construction of the simplest form of transistor - a thin slice of germanium, one or two thousandths of an inch thick, bonded on either side by thin layers of germanium into which some indium has penetrated. The device must act by the manner in which electric charges move through it. It is therefore necessary to examine the nature of the conductivity through the semi-conductor material and what happens at junctions between portions of the material of very slightly different chemical purity.

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Figure 2. Diagram shozving an experiment in which the alloying of the indium and germanium zvas done with a crystal cut 9 degrees off the (hi) plane {From Mueller and Ditricky ' Uniform Planar Alloy Junctions for Germanium Transistors' in R.C.A. Review , Vol. XVII , March 1956, No. 1)

CONDUCTION OF ELECTRICITY IN SEMI-CONDUCTORS

If these parts conducted as metals do, the transistor would have no action other than that of a conductor. If one or more of the parts acted as an insulator it would have a purely negative action. How then does the conductivity of a semi-conductor differ from that of a metal?

Metals conduct electricity freely, without chemical change and with a decrease in conductivity as the temperature increases. When they show conduction, semi- conductors at high temperatures conduct almost as well as metals, and at low temperatures behave as insulators. They conduct without undergoing chemical

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962 cl langes and in general have the same electrical characteristics as do metallic conductors, but have a special characteristic, which is the considerable increase of conductivity caused by the introduction of the merest traces of impurities. There are a great number of semi-conducting materials, one of historical note being silver sulphide, whose properties were studied by Faraday in 1834. In industry, germanium and silicon have been developed physically and chemically with great finesse and expertise to provide the semi-conductors of commerce from which are manufactured transistors, rectifiers, controlled rectifiers, pn-pn switches, Zener diodes and other semi-conducting devices.

For an electrical current to flow, charges or carriers of electricity are needed, and the basic fact of electricity is that the carriers are negative charges or electrons. These are inherent in matter and form part of the lattice of ions and electrons which is the structure of solid conductors. In metals, the outer electrons at ordinary temperatures are mobile. They are not bound to the ions but are thermally agitated, suffering certain forms of collisions and moving about the lattice much as if they were a gas. Under an applied electrical field the cloud of electrons drifts through the conductor, which remains electrically neutral, the density of the mobile electrons being balanced by an equal density of immobile positive ions. The movement of electrons is constrained by the lattice and one cannot influence their flow, in practical terms, except by physically breaking the conductor as in a switch, or by varying the power consumed in the circuit by altering the main circuit voltage. The drifting electron cloud moves throughout the circuit freely crossing boundaries or junctions between different conductors.

The semi-conductors, silicon and germanium, do not conduct current very well at ordinary temperatures. In their lattice structure there is no great cloud of electrons in thermal agitation. Each atom is surrounded by four neighbours and each is connected to the others by four electron pair bonds, the valence electrons of the material. The electrons are tied to the lattice and are not available for conduction, unless disturbed by the vibrations of a sufficiently hot lattice. Then a valence electron may be freed, leaving behind an empty place or hole which also takes part in the conduction process. Electrons move through the lattice, as it were, from hole to hole, each valence electron in moving to fill up one vacancy leaving another. Electrons thus move one way through the lattice and the holes the other.

But conduction can be obtained in such lattices not only by the application of heat, but by the inclusion of imperfections, and the most effective and controllable of these are the substantial ones, or impurities. Atoms of Group III and IV will substitute in place of a germanium atom in a lattice of the latter. When, for example, an arsenic atom is substituted, four of the five outer electrons are used up to form four pair-bonds for binding to the four germanium neighbours, leaving a fifth valence electron. This is weakly held in the lattice and can become detached and available for conduction and thus cause an increase in conductivity. If the substituted atom be a trivalent one, such as aluminium, there will also be an increase of conductivity. However, as can be established by experiment, the charge carriers are now of apparently different sign, behaving as if they are positive. A trivalent element in a tetravalent element lattice produces 'an absence of an electron'.

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AUGUST 1962 THE MINIATURE W.ONDERS OF TRANSISTORS

One such experiment is to heat bars of very pure silicon which contain respectively minute traces of a pentavalent and a trivalent element. The bars are each connected in identical circuits with current indicating instruments. If one warms, say, the left-hand end of the 'pentavalent bar', the current moves the needle, say, to the left. If now we heat the corresponding end of the 'trivalent bar', the needle moves to the right. In this case a hot left-hand moves the needle to the right and vice versa. If bars are prepared with other trivalent and other pentavalent impurities, the experimental results are the same. The differences in the direction of current as shown by the needle movements are due to the differences in the valencies of the impurities.

The fact that a current is produced by heating one end of the bars can be explained in an elementary way by considering that the energy of the charges that are free to move in the lattice is greater at the hot end than at the cold, and a field is thus established. This will cause a current to flow as long as the difference in temperature is maintained. It would be expected that the direction of the field would reverse when the hot and cold ends are reversed. The reversal of the direction of the current by changing the valency of the impurity is not so obvious. It is as if charges of one sign are released when the semi-conductor contains a pentavalent impurity, and charges of opposite sign when the impurity is trivalent.

Although we may refer to these charges as negative and positive, there is, nevertheless, a oneness or sameness about them, for in both cases it is the movement of electrons in the same lattice that causes the effect. In the one case the substitutional impurity has an excess valency electron. Four of its valency electrons pair with those of the tetravalent main lattice member. The fifth is not associated, so to speak, with the structure of the lattice and it can more easily absorb energy and become detached, leaving a positively charged ion fixed in the lattice. In the other case, the substitutional impurity has a deficit when it goes in place of the tetravalent main lattice member; it has only three valence electrons to pair with four. This means that now there is vacancy at a main lattice point into which an electron from one of the electron pair-bonds of a main lattice point can move if it receives sufficient energy. The particles that move in each case are electrons, but they differ from each other as regards the amount of energy they possess. It is this difference that makes them behave - if we wish to describe the behaviour of semi- conductors and the action of transistors in ordinary circuit language - as if they were positive and negative charges.

To return to the dissected transistor, the germanium slice contains a carefully controlled amount of impurity to give it negative carriers. It is called 'n-type' The indium, a trivalent element, has alloyed with it on either side, and penetrating to a certain depth, converted layers of it into germanium with positive carriers, called 'p-type'. We thus have n-type layer sandwiched between layers of p-type, giving a p-n junction in series with a second junction mirroring the first. Consider one only of these junctions with its p-type part joined to its n-type part. Each

piece is electrically neutral but has charges within it moving in thermal agitation, in one the charges being negative and in the other behaving as if they were positive. At the junction the thermal movements in effect bring the negative and positive

64З




charges close together and they neutralize each other. This means that they are no longer available to neutralize the fixed impurity ions in the lattice, and the ions that face each other at the junction thus form an electric double layer, one side positive and the other negative - a potential barrier. It is the establishment at will of positive and negative charge availability, and the establishment of junctions, that is the basis of the semi-conductor industry.

THE P-N JUNCTION If connected in circuit, the field of the potential barrier will not assist the

thermally moving clouds of charges to drift across it, but will rather oppose such a drift. If an additional potential is set up by placing a battery in circuit so that the p-type side is made negative and the n-type positive, the field of the barrier is strengthened and no current flows, but if the polarity of the battery is reversed, then the field of the junction is opposed and current will flow. The junction thus acts as a gate which opens or closes to the passage of current according to its direction.

A short length of animated diagrammatic film was shown at this point.

Figures 3 and 4 may be noted at this stage. This action is of great importance to electrical engineering technology, for some

operations such as electric traction are better done by using direct current, and for some, for example, the electrolytic production of materials, it is essential.

It may be of interest to note how easy it is, given the right materials , to produce a primitive form of rectifying junction and to note the profound electrical change that can be produced by a simple physical process. A thin slice of silicon is taken and first put between contacts in a circuit with a current indicating meter, a battery and a reversing switch to show that it will conduct current in either direction. A thin slice of aluminium is also put through the same test and also conducts current in both directions. The two slices are now held together and placed in a furnace at about 6oo°C. for about three minutes, taken out, allowed to cool, and then placed between the contacts in the circuit. It will now be found, on operating the reversing switch, that the current will pass in one direction only. (Although an argon atmosphere is maintained in the furnace, the silicon may oxidize to an extent that may make cleaning necessary, or in order to avoid this a gold contact may be melted on to one side of the silicon.)

The experiment described was then carried out.

The physical reality of the junction can be demonstrated by sectioning a rectifier disc radially to expose the layers and placing a fine powder on them. If a battery is connected to the opposite sides of the junction, the field by electrostatic attraction will cause the powder to concentrate at the junction or disperse, according to the direction of the field.

Л short film of this was shown here.b

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There are many other types of rectifier - high vacuum, mercury arc, contact - some of which have been completely replaced by the semi-conductor type for certain operations. This is because of its very high efficiency. It is of interest to look at it this way. The circuit conveying power will consist of a cable made of copper. Through the lattice of this will drift under the applied field the cloud of electrons that is the current. One can put into this a one-way gate consisting of a thin slice of germanium within which is the junction. The diameter of the slice is not very much larger than the cable. It is very thin because it need not be otherwise ; and since it is of higher resistance than copper, the thinner the better. Flow of current in one direction is completely halted, provided that the voltage attempting to drive it through does not rise too high - not more than a thousand volts or so. In the other direction the only energy required to move the electrons through the junction corresponds to less than one volt. The semi-conductor rectifier is therefore the best for electrolytic operations, and for many other uses such as traction it is likely to supersede other forms.

The junction on which this rectifying action depends, which also appears in the transistor, is brought about by technical control of lattice structure, but only by accepting fantastic standards of purity and delicate manipulation.

PURITY REQUIREMENTS AND ZONE REFINING The amount of donor or acceptor additive will vary according to the electrical

characteristics required from the finished device. In many cases the amount is as small as one part in a million, and this means that the initial purity, if the additive situation is to be under control, should be as good as one part in a million million.

It is a characteristic of some molten materials containing impurities that, as solidification takes place, the lattice structure as it forms out of the liquid phase may squeeze out the impurities so that the concentration becomes less in the frozen solid and greater in the supernatant liquid. The technologist takes advantage of this phenomenon of segregation by a process known as zone refining.6 The material, for example, silicon, is prepared with very considerable chemical care, to a purity of about one in io8, in bar form. A narrow transverse molten zone is produced and caused to traverse the length of the bar. The material that solidifies in the wake of the zone contains less impurity than that in the molten zone which takes up more and more impurity out of the bar as it traverses its length. The portion at the end, rich in impurities, is abandoned. Repeated traverses can be given to obtain improved purity, but the result will fall short of what is desired for the simple reason that molten silicon is a very reactive chemical (its melting point is i425°C.), and it is not possible to avoid impurities from the container con-

taminating the molten zone and thus preventing the continual decrease in concentration which is the aim of the multiple traversing. The technologist, therefore, makes use of another atomistic property - the netlike structure of atoms that forms the surface of a liquid, and which will not break below a certain applied force. If one melts only a narrow zone, a container for the rod can thus be dispensed with, for the surface skin of atoms is now the container for the molten zone. The

energy to melt the silicon is suitably obtained from the coil of a high frequency

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962 furnace or from an electron bombardment furnace. The coil, for example, traverses the bar of silicon for as many times as the degree of purity requires. Furthermore, if the bar was formed as a single crystal in the first place, it will remain as one, for it forms its own seed on which the molten zone can recrystallize.

At this point a film of the floating zone process was shown.1

The adding of the donor or acceptor atoms is carried out in various ways, but the first stage is usually to make the slice of material either p-type or n-type according to the requirements of the designer. This can be done by inserting pellets of the required additive into the bar along its length. The floating zone refining process can then be caused by segregation to distribute the additive along the length of the bar, leaving it at the required concentration. Then by alloying or diffusion, a junction is produced in a slice off the bar by the introduction of the opposite impurity. If a transistor is required, two junctions are necessary, and on the type and spacing of these will depend the operating characteristics of the transistor. The manner in which these junctions are, in fact, produced in the slice of semi-conductor forms the basic art and science of the transistor manufacturing industry.

Figure 3. Transistor action I

JbiGURE 4. lransistor action II

TRANSISTOR ACTION A transistor will amplify, and

from this springs other operations in which it will act as a switch or as an oscillator. The amplifying action of a transistor can be appreciated by considering diagrammatically the behaviour of carriers of electricity in an assembly of semi-conducting materials, such as a p-n-p or an n-p-n sandwich placed in a circuit with a source of low voltage direct current.

Consider a p-n-p transistor, taking first (Figure 3) only the p-n part of the sandwich, a piece of semi-conductor material having on the left hand p-type characteristics and on the right n-type, forming a p-n junction between them. Metal contacts are alloyed to the two parts so that current can flow into them from an external circuit. This (including a battery, not shown)

646




is arranged so that the p-type part is positive with respect to the n-type. Under this field there will be a drift of positive charges through the potential barrier into the n-type. Electrons will flow from the external circuit into the n-type part to neutralize the positive space charge. There is thus, by a movement of negative charges to the left and positive charges to the right, a current flow around the circuit. (Figure 4.)

Next (Figure 5) form between the n-type and its metal contact a further p-type piece. No current will now flow because there is no source from which negative charges can flow into the n-type part to neutralize the space charge created if positive charges

Figure 5. Transistor action III

Figure 6. Transistor action IV

try to enter trom the leit hand p-type part. JNegative charges cannot now trom the external circuit into the right hand p-type piece because they would set up a negative space charge, and no positive charges can flow in to neutralize this. Movement of current carriers is blocked in both directions.

But if (Figure 6) a metal contact is again made to the n-type part and it is connected into circuit so that it is negative with respect to the left hand p-type part, positive charges will now drift to the right and negative charges will flow from the external circuit into the n-type piece to neutralize their space charge, and recombinations will take place between the positive and negative charges. If the recombination is not immediate - if, that is, the charges have a lifetime before they recombine - then some of the positive charges will drift into the right hand p-type piece, because the n-type part is positive with respect to it, and assisting the field of the junction. Negative charges can flow in from the outer circuit into the right hand p-type piece to neutralize the positive charges drifting into it, and a current flows round the circuit once again. Note that it can only do so because of the current that flows to the centre n-type piece. If these two currents are equal nothing has been gained - we have no viable device. But if the initiating current that flows into the centre n-type piece can be made small compared with that which flows into the right hand p-type piece - if, say, for every two electrons that flow into

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Figure 7. Transistor action V

centre n-type, 98 will flow into right hand p-type - then amplification has been achieved and a very important device results.

This desirable state of affairs can be produced by making the centre n-type piece very thin and with characteristics that allow a sufficiently long lifetime for the carriers. Then negative charges that flow into it serve to neutralize the space charge

that would otherwise prevent the stream of positive charges flowing through and suffer but few recombinations. (Figure 7.) This is the amplifying transistor, a device that depends upon 'miniature wonders' that have to be understood, controlled and reproduced by the design engineer and the factory personnel. The purity and the mechanical perfection of the material have to be to hitherto unknown standards to secure the desired lifetime and other characteristics. The dimensions formed, not by previously normal factory processes, but by diffusion and recrystallization, are thus by atomistic ones, but nevertheless have to be appreciated in thousandths of an inch and less and reproduced in vast numbers, just as much as if they were marked on normal engineering drawings. They cannot, however, be measured in the normal way by micrometers, etc., as the job proceeds, but by test results on the finished device : a procedure which, as every production man knows, is fraught with the hazards of shrinkage and wastage until firm understanding and meticulous controls are established.

A considerable range of transistors has been developed, the majority more complex, sophisticated or 'second generation' than the simple 'cooking' type so far described.

The problems that confront design and production engineers when they come to translate research laboratory practice into production for these more difficult types can be appreciated by a film sequence. This shows the processes used to develop in a research laboratory a prototype high frequency mesa type transistor. Processes carried out under these conditions are easier to follow than when viewed under the rhythm of mechanization, the paraphernalia of quantity handling, and the special arrangements for safety and cleanliness necessitated by the use of non-scientific labour in the production area.

An enlarged model showing the general assembly and the active part of the transistor were exhibited . A film of the laboratory construction was then shown*

USES OF SEMI-CONDUCTOR DEVICES

Semi-conductor devices in general have not made possible new circuit operations but have made such operations more efficient and attractive, both industrially and commercially. Where high power is required in single units at high voltages or

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Figure 8. Part of the 18,400 kW semi-conductor rectifier installation at the Ellesmere Port plant of Associated Ethyl Co. Ltd.

at very high frequencies, the electronic valve has so far not found a competitor in semi-conductor devices. But in every other direction, and especially where the saving of space and high efficiency are important, with ruggedness, long life and ease of servicing equally so, the semi-conductor device is proving of great interest and value. Its small size and exceptionally low power requirements enable applications that a few years ago would have been considered extraordinary to be carried out with ease.

On the power side it should not be overlooked that power semi-conductor devices, though small in output compared with many orthodox items of electrical equipment, can be made with surprisingly large outputs compared with those of transistors. For example, rectifiers are being made that will deliver a D.C. current of 150 amperes at 300-400 volts. (There is much experience required in assessing the output possibilities of power semi-conductor devices. The inverse peak test voltage for such service is required to be some 1200 volts.) Each unit is quite small, and it is practicable to connect them in series and parallel and to build up in cubicles of considerable power-handling capacity. The illustration (Figure 8)

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962 shows an 18 megawatt set for the electrolytic production of chlorine. The electrolytic smelting of aluminium is an important use for D.C. rectification by means of semi-conductor rectifiers, for the amount of power consumed is so considerable that the superior efficiency of the semi-conductor rectifier may represent annual savings of many thousands of pounds. The largest set so far installed is one of 108 megawatts in Norway. It is of interest to reflect that such an equipment is possible because we know how correctly to dispose in pure germanium of an array of impurity atoms.

A development of the diode rectifier is the controlled rectifier, a three-electrode device that resembles in its operation the thyratron, a gas-filled electronic valve. In such devices current does not necessarily flow over the whole of the period of the conducting half cycle, but over a part only, the duration being under control by means of a third electrode and its auxiliary circuit (in which the power consumption is small or negligible). The variable member in such a circuit may be, for example, a very lightweight resistance. (In the experiment demonstrated one is being used to control a load of 3 kilowatts.) It is not unreasonable to suppose that controlled rectifiers will, with further research, be capable of controlling 100 to 200 h.p. or more, and when this stage is reached and a demand established that will result in an economical price, very considerable usage should result.

An experiment was carried out , showing the controlled rectifier regulating the speed of a motor.

Of the many uses for transistors, the dominant ones will be those which become for the first time practicable because of the unique characteristics of the device and which have widespread applications. Pocket and portable radio sets provide an example of one class in this group, and computers another, and in general for the same reasons, viz., small size and low power consumption and heat dissipation.

Such characteristics make possible a contactless circuitry for future telephone exchanges. Industrial control circuits are already using small trains of semi- conductor devices encapsulated in blocks of resin to form sets to operate con- tactors, for the control of electrically operated equipment. This is doing what has previously been done another way, a process for which one hears the general des- cription 'transistorization'. This process may mean either the replacing of valves by transistors in apparatus that has previously used electronic control circuits, or the adaption of electronic control, possible under the circumstances only by the use of transistors, to a process that has previously been done mechanically or manually. It is, of course, the latter process that is the most dramatic. In order to demonstrate, a facetious example has been devised purely for illustrative purposes. It is the 'transistorized' bugle. Normally a bugle is sounded by the vibration of the lips modulating air forced out of the lungs through the throat, mouth and lips into the resonant horn of the bugle. A metal diaphragm, by moving at sufficient ampli- tude and at the desired frequencies, takes the place of the lungs and lips. The audio-frequency oscillations are obtained by four (this is the number of notes required to sound infantry bugle calls) separate transistor circuits, each brought into operation by a key.

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The transistorized bugle was then sounded .

This particular operation is possible with valves. However, the valves and the batteries to operate them are considerably larger than the alternative equipment when using transistors. Batteries would have to be carried in a separately slung case. Under the conditions of bugle-blowing usage there is little doubt that transistorization would be acceptable, reliable and convenient, whereas circuits using valves would not find favour.

Medical research men may wish to follow reactions of the body while it is actually under violent stress, such as, for example, when a soldier trains on a combat course. The use of transistor radio transmitting sets means that, carrying virtually no extra weight to vitiate the experiment, the soldier can, for example, transmit over a distance the sound of his heart, which can be heard in a listening post several hundreds of yards away, much as if a stethoscope was applied in the normal way.

The most striking example of this type of application is the radio pill. In this a capsule that can easily be swallowed contains a transistor radio transmitter and a transducer which according to design can signal, if the pill be swallowed, either internal pressure, temperature or acidity.

A radio pill was demonstrated and swallowed at this point .

CONCLUSION

Design, manufacture and use of transistors is still in the early stages of development, and many new forms and processes are under examination. So far in this paper no mention has been made of what is generally referred to as microminiaturization. Studies of this are an inevitable consequence of the characteristics of transistors. These are already so small in mechanical size and power dissipation that circuits involving tens of thousands can be envisaged. It is then a natural step to reflect that, just as when the motor car was first introduced it was exactly like the horse-drawn carriage without the horse, so the transistor - to a large extent - is like the electronic valve without the vacuum. It has a stem or mount or header and a can or bulb, and it has lead wires or pins. It need not be like this, however, for the active part, the junction, and the semi-conductor portions immediately adjacent, need to be only a thousandth of an inch or so in thickness. It follows that if conventional lines of thought are abandoned, entirely different methods of construction of devices and of placing them in circuits can be devised. For example, the semi-conductor devices, along with associated circuit components such as resisters, capacitors and batteries, can be made as thin discs or washers stacked together to make up the required circuits.

Alternatively, conventional methods of assembly can be abandoned, and each component - for example, the 'bulb' or 'can' - made extremely small. Made by such methods, the overall size of a diode rectifier may be no more than a cylinder 30,000ths of an inch high and 6o,oooths of an inch in diameter. An even more advanced method of microminiaturization is that known as solid state circuitry. In this method not only is the transistor or diode formed from junctions within a

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962 block of silicon, but so are connecting leads and associated circuit components. Using any of these methods of miniaturization it is possible to contemplate quite new designs of computers and such-like devices so far as overall size and servicing is concerned.

With so much that is new to be developed and engineered, one may give a thought to the education and training of those students who one day will find themselves in industry doing work of this kind. This is not the place to contribute an essay on education, but much of the work that has to be done is design without experience, and inevitably it will fall to the lot of physicists to carry it out. It is to be hoped that these designers-to-be will receive, in addition to an education in physics, an appreciation of industry and its objectives.

REFERENCES i. First demonstrated by Bell Telephone Laboratory, 1048. 2. J. Bardeen and W. H. Brattain, ' The Transistor, a Semiconductor Triode ' : Phys.

Review, 74, 1948. 3. This photograph was kindly supplied by Dr. J. E. Hughes, A.E.I. Research Laboratory, Harlow. 4. Taken from the paper ' Uniform Planar Alloy Junctions for Germanium Transistors '

by C. W. Mueller and N. H. Ditrick, published in the R.C.A. Review, Volume XVII, March 1956, No. i.

5. The film of the electrostatic attraction of powder on a p-n junction was taken by Mr. C. Gilson of the A.E.I. Research Laboratory, Rugby. 6. Zone Melting, by W. G. Pfann (Tohn Wiley, N.Y., 1958). 7. & 8. The film of the floating zone process and of the laboratory construction of a

transistor was taken by Mr. C. Gilson of the A.E.I. Research Laboratory, Rugby. 9. I am indebted to Dr. H. S. Wolff of the Medical Research Council for help and advice

on ' radio pills '. I should like to thank Mr. R. V. Mills and Dr. J. Shields and many other members of the A.E.I. Research Laboratories for help and advice in connection with this lecture.

DISCUSSION

the chairman : May I say that the part of the lecture that I found most fascinating and most bewildering was the idea of positive and negative current flow from pentavalent and trivalent bars.

Being an ordinary electrical engineering student, I was taught that an electric current was a movement of electrical charges, and nothing that Mr. Davies has said contradicts this, but I always had the idea that there could not be a flow of current without free electrons to carry the charges. Now, as I understand Mr. Davies, we can get a flow of positive charges without an equal flow of negative charges or free electrons if we are short of the number of electrons to fill the orbital pattern. Would Mr. Davies like to say whether I am right or wrong?

the lecturer: The current is electronic. It is important to realize, however, that the behaviour of the electrons in solids differs considerably from their behaviour in vacuum; the electrons in solids are not really free electrons. Their behaviour is governed by the properties of the crystalline lattice in which they are moving.

It is not unreasonable to expect that the change in energy of electrons produced by the application of electric fields will result in effects which are not easily explained if one imagines the electrons as free electrons. One of these effects is the apparent movement of electrons against an accelerating electric field in certain semi-conducting crystals; this effect can be considered as due to the reflection of electrons in atomic planes of the crystal. These reflected electrons are effectively moving in the direction, under the applied field, which would, for free particles, be typical of positive charges. It is for this reason that we call them positive charges.

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MR. G. R. E. Cleveland, A.M.i. E.E. : What is the minimum (supply) voltage at which a junction transistor will operate?

the lecturer: Mr. Chandler, will you be]so kind as to answer this? MR. j. A. chandler, (Research Laboratory, A.E.I . (Rugby) Ltd.) : With a germanium

transistor you can certainly get amplification with a d.c. voltage of about 0.2 to 0.3 volts from collector to emitter. For a silicon transistor somewhat higher voltages are required.

MR. G. vivían davies: Transistors being so very small in size, it seems to me that we have left the field of electrical engineering and entered that of the watchmaker. I have a new transistor radio set in my car, and I wonder whether, if anything goes wrong with it, it is possible to repair it, or whether the whole of the inside need be taken out and replaced?

the lecturer: Transistors are on the whole much more reliable than valves, and it is possible to repair a transistor set just as you would repair a valve set. You make the normal tests and find out the faulty component and replace it.

You have raised a very big point by your remark about the way electrical engineering is going. I should like to take this opportunity of quoting Professor Cullwick (who quoted Sir George Thomson) that an engineer is someone who designs things for factories to make, and an engineer calls not only upon his knowledge but upon his experience. Nowadays we are having to design things of which we have no experience. You realize that the transistor is a most important development the moment you see the first demonstration of it ; what must be done in order to get it going is to design it ; you have, however, no background of experience. This is where I get my own back on Mr. Lucas : who should design it, the electrical engineer or the physicist?

the chairman: Before very long I do not think there will be much difference between the two.

MR. E. F. s. Clarke (G.P.O. Research Station, Dollis Hill): Mr. Davies touched on the subject of longevity of a transistor. I wonder if he could say what are the factors affecting its life, and whether there is any prospect of making that life indefinite?

the lecturer: The transistors are encapsulated so that their surfaces cannot be contaminated, and if you have made them properly then they should go on for ever. In practice, however, I concede that things do go wrong with them, but development is making their life longer and longer, and as you probably know, they are subjected to 'reliability testing', so that a class of device known as a reliable device can be specified - a shocking piece of terminology because it suggests that all the others aren't reliable. What it really means is that this type of transistor is very reliable indeed ; the prob- ability of failure is so low that, even if you have two hundred thousand of them in a set, only once a year will you have to replace one of them.

MR. P. J. HARRIS (Dictograph Telephones Ltd.): I have come across the term 'thermal runaway', and am puzzled by it.

the lecturer: Let me ask one of my colleagues on the job to explain. What is thermal runaway, Mr. Castle?

MR. P. F. CASTLE, B.sc. (A.E.I. (Rugby) Ltd): Leakage current in a transistor increases with increasing temperature. If, therefore, the junction temperature rises due to an increase in internal power dissipation, or an increase in ambient temperature, this will cause an increase in leakage current, leading to higher power dissipation, and therefore higher internal temperature, with a further increase in leakage current, and greater dissipation, and so on. If the rate at which the internal dissipation increases due to this mechanism is greater than the rate at which the transistor can

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JOURNAL OF THE ROYAL SOCIETY OF ARTS AUGUST 1 962 lose heat to its surroundings, the current through the device will continue to increase until the device destroys itself. This effect is known as thermal runaway.

MR. Edward j. Humphreys: May I ask the lecturer whether the transistor will be able to contribute to the aerial transmission of energy?

the lecturer : I do not think that transistors can contribute to the transmission of appreciable amounts of energy. If ever it is realistic to beam energy, they would contribute to the control circuits involved. But the problem there would be to beam the energy and to find some method of charging people for it !

MR, i. L. hurst, A.M.Brit. i.R.E. (De Havilland Aircraft Co. Ltd., Ballistic Missile Division) : Can Mr. Davies give us any hints as to some methods of controlling the minute impurity concentration to one part in io10?

the lecturer : The main method is to find a chemical way of getting it as pure as possible, and then to use the zone refining method. You combine this technique with methods of measuring, such as solid mass spectrographs and radio-activation chemistry.

There is also the making of devices from the vapour phase so that you do not necessarily have to go through the production of a bulk rod, which may introduce impurities. You have to be meticulously clean and careful in everything. Does this answer your question, Sir?

MR. hurst: Yes, thank you, but there is the further query. If you have to introduce accurately measured quantities of 'donor' or 'acceptor' impurity atoms into the purified intrinsic germanium (e.g. arsenic, indium) this must surely require very intricate control in order to produce predetermined junction characteristics.

the lecturer : There are various ways of doing that : you can do it by diffusion : you can do it as you saw in the model : put a blob of indium on, let it melt, let it alloy, let it solidify. You can do it by taking your pure rod, drilling little holes along it and poking in little bits of impurity, zone-refining again and thus getting the kind of distribution you want.

MR. j. G. cocks, B.sc. : The Lecturer mentioned the fact that higher voltage silicon controlled rectifiers are now required by engineers. Bearing in mind that single junction rectifiers are readily available over 1 k.v., what is the difficulty with the silicon controlled rectifier?

the lecturer : The point which you raise is one of general significance in all semi- conducting devices. Silicon controlled rectifiers of high voltage rating can be produced on a laboratory scale; the problems are, of course, certainly more numerous than those in the single layer rectifier. During the past few years the single layer rectifiers have, in manufacture, been obtained with steadily increasing voltage ratings. So it will be with the controlled rectifier - a newer device than the single layer rectifier. As manufacturing techniques improve so the voltage ratings of the controlled rectifier will increase.

MR. v. H. GILBERT (Gilbert Photo-Electronics Ltd.) : The transistor is also a light- sensitive device, and I wonder whether any developments are taking place which could bring that type of light-sensitive device into the same part as the photo- multiplier, where secondary emission is the source of amplification. Is anything similar possible in the case of the photo-transistor?

the lecturer: I personally do not know whether the photo-transistor is likely to be developed, or can be developed, as a photo-multiplier. I think, however, that the straightforward photo-transistor will not be as sensitive as the photo-multiplier. There are, I think, some developments in hand to attempt to produce a small solid

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state photo-multiplier - working, however, on a principle different from the photo- transistor.

the chairman: We have listened, ladies and gentlemen, to a fascinating paper and an excellent discussion on a very topical subject. Mr. Davies in his talk has surveyed the progress that has been made since transistors first came on the scene in 1948 and he has given us a glimpse of the enormous possibilities of these tricky little devices in the future.

We have listened to a lecture that stirs our imagination and makes us think, and I am sure that I shall be speaking for all present here this evening when I ask that there should be recorded our best thanks to Mr. Davies.

If you agree, will you please show your appreciation in the usual manner.

The vote of thanks to the Lecturer was carried with acclamation and , another having been accorded to the Chairman upon the proposal of Sir Ronald Nesbitt-Hawes , a Member of Council of the Society , the meeting then ended.

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the miniature wonders of transistors

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