The Evolution of the Transistor

As the transistor has been around for most of my lifetime, an account of its early days may not come amiss. Indeed in imperfect form they may have been produced by accident in the common Lead ore Galena, ages ago.

In my early days I , like many others, had a my ‘Crystal Set’. As the number of users increased, so did the claims that some crystals could produce oscillations. As this seemed unlikely, the story was at first disbelieved, but in the end there were so many reports that a number of qualified scientists got involved and confirmed that some Crystals did indeed oscillate. This was about 1922, and if they had bothered to look up the records, they would have found that this effect had been reported as far back as 1910. My memory is that nothing much was published at the time.

I have often thought about the oscillating crystals since. The general transistor-like properties could well have been due to accidental ‘doping’ impurities, but where was the power coming from? Possibly from the junction of dissimilar metals, possibly from rectified RF, or even as a very long shot, from the use of Baker’s Fluid, or the action of over heated soldering irons on resin. Like others I used to fiddle with the ‘Cat’s whisker’ , but I did discover than any clapped out old crystal was capable of excellent rectification over its entire surface if you put the voltage up a volt or so. This is probably the reason why the Channel Islanders were able to receive a good signal on crystal sets which used tiny pieces of Galena, for long periods during the German occupation. They had a relatively enormous signal obtained form the universal and extensive overhead telephone wires.

A friend of mine used to make and sell quite nice Crystal Sets. For himself he kept one with two ‘Cats whiskers’ arranged just like the ones shown in the Bell Labs sketch of later years. He certainly had no batteries connected, but it always gave a signal a bit louder than his production models. A pity he did not patent it!

The local investigations petered out, but a considerable amount continued on in Germany resulting in many patents. In the UK when something is patented, all may inspect the patent after a short interval. In Germany it was the custom to register a good patent, and at the same time to register a lot of similar ones to confuse the issue. However the Germans did manage to register at least one device which was effectively a FET which caused future generations a lot of trouble. It would seem they must have gone quite a long way in their developments. However nobody was interested, and the valve was doing very nicely thank you. Solid state effects were a scientific curiosity.

In the above account I have not mentioned the biased Steel-Carborundum combination. Although technical journals and other sources often described them, I myself never saw one in operation. The bias would of course be available as power for a would be oscillating crystal, but this source was never quoted as being more likely to oscillate than any other.

During the 1939-45 war the only notable work carried out using solid state material was on mixer diodes. With the much higher frequencies produced by Magnetrons for Radar, valve mixers were very inefficient.  The favourite combination was Carborundum with a Tungsten point. After the war, the Bell Laboratories set a team of Theoretical and Practical Scientists to investigate Solid State. They were believed to have sorted out the German patents and to have carried out much original work of their own. They seemed to have concentrated on the Carbon Group Semiconductors viz.: Carbon, Silicon and Germanium. In particular they studied Germanium. They soon found that Germanium when pure, was in fact a nonconductor and needed certain impurities to enable it to conduct.

The power of any Chemical element to combine with another, is known as its Valency. The metals and non metals of the Carbon Group are all Tetravalent, that is they each could be described as having four arms on every atom seeking another one to grab. In a Crystalline state these bonds connect to neighbouring atoms of the same material forming a Matrix of atoms in orderly rows. The Diamond is a very stable example of this bonding of Carbon. The Germanium Matrix is far less stable, and will accept other substances in small quantities which have three or five bonds. The modified crystal acquires new electrical properties from this ‘Doping’. Germanium will accept Aluminium or Indium ( Trivalent ), and Arsenic or Antimony ( Pentavalent ). The first produces ‘p’ type and the second ‘n’ type semiconductors.

Conduction in these devices is not only carried out by the electrons, but also by , we are assured a very rum object named a ‘hole’, which appears to be a vacancy in the valency structure acting as a positive charge, a sort of positron. These are not so mobile as the electrons. This positive charge hole dodging about takes a lot of believing, but it does give something to work with.

The Bell Labs rather concentrated on Germanium as most of its properties are superior to those of Silicon when it comes to making a Transistor. However the same wattage applied to both Silicon and Germanium samples of the same mass, will give a rise in temperature of the second twice that of the first owing to its lower Specific Heat.  Germanium also melts at a much lower temperature than Silicon. In practice Germanium Transistors are very easily destroyed by overload.

The first Bell labs Transistor was launched with great publicity and Ballyhoo in December 1947. The Press and the Technical Press really went to town. No credit was given to earlier work. Sketches of the assembly were published, and also the mandatory Lab notes signed up as ‘read and understood by me’ by an observer as required by American Patent law. These two sheets still appear from time to time. It is very difficult to see what the first one is about.  The Emitter and Collector are connected together by a ghostly battery, and what is possibly the Base connection hangs down at the back like a mouse’s tail. You can do a bit better with the Patent sheet. This is just about recognisable as a Point Contact Transistor. From the battery connections it would appear to be a p-type. This is rather surprising as these were mainly produced as n-type. The Contact Point Type became quite quickly a favourite with the manufacturers of Computers. They were much better switchers than amplifiers, being noisy and having a far from linear response.

At this time it was thought that the free electrons or holes in the Matrix followed the rules of Space Charge, such as that which is produced by a hot valve filament. Later it was found there were differences.

The Bell Labs licensed a numbler of people to manufacture Transistors. The range was greatly improved when npn and pnp sandwich types were evolved, and the local manufacturers began their own publicity. Mullards had a scheme whereby senior engineers in electrical companies had a sort of seminar. I was working for Rediffusion in the fifties, and all the chief engineers of the regions and selected bodies from Head Office were drafted to attend one such party. We were lectured freely and were shown the factory processes.  I remember being greatly intrigued by the sight of a single crystal some 3 inches across being drawn very slowly out of the melt to the height of a foot or more.  As Germanium was at that time believed to be obtained from old flue deposits, I fell to wondering how many sacks of soot that little lot represented! We were also given a set of bits and pieces to make up a very low powered audio amplifier. This amplifier was basically a valve design with transistors substituted for valves. The input was a transformer at the front driving 2 transistors in push-pull, all class A-B with another transformer to drive the loudspeaker. Due to the high impedances of the 2 transistor outputs the transformer had to be relative enormous in order to cope. Even then a trial run sounded as if there was at least 10% of odd harmonics present. We were not impressed and at the state of the art the valve looked the best bet for Quality products.

Soon things began to change. There was a lot of work put into the production of the more robust Silicon Transistor, which enabled it to be sold at a more reasonable price. Also the Computer manufacturers decided that their future was bound up with the Transistor. Enormous sums of money were expended on Research. The computers of that day needed greater stability and reduction of size. The work extended also to a review of fundamental circuit elements.

From that period appeared increased use of negative feedback, the Op Amp, the Integrated Circuit, and of course the chip in more or less its present form. Transistors of dot size were deposited, and the reduction of size is still continuing at the present time. Developments such as the Op Amp have been also a Godsend to the Hi Fi industry. Another very useful arrangement appeared about then, namely the use of a npn and complimentary pnp transistor together with a large capacitor to feed the loudspeaker. This saved a transformer and produced an output about the same quality as could be obtained from 2 valves in push-pull.

The miniaturisation of components and the corresponding drop in current far from making things unreliable, seems to make for greater reliability.  When I was working for one of the larger Television companies in the days of valves, we used to have around 20 engineers to make and mend. In these days they have only 1 or 2, and they do not even have the circuits of the apparatus which they use. On the rare occasions of a breakdown, the faulty unit goes back to the manufacturer. Small receivers appeared about 1953, at first hybrid valve and transistor, but soon all transistor.  These later sold on the novelty of their small size. as this included the loudspeaker, the sound was rather dreadful.

To end on a personal note slightly off the main theme of this article, I had two experiences with computers which at that period were stuffed with quite large transistors and in total consumed Kilowatts of power. I worked for one of the large Television Companies at the time.  We in Engineering happened to find out that our Sales Department had a tender out for a computer which would list not only all Commercials in detail, but would also arrange them so that they did not clash, e.g. send out like commercials one after the other. This they proposed to place on the floor above the Technical area. Accordingly I had a go at Management prophesying that this would cause our product to come out in stripes. After a considerable wrangle I was allowed to take a look at the 6 manufacturers and make an assessment.  Taking a transistor radio with me to get some idea of the virulence of the internal ‘Clock’ oscillator, I duly went the rounds and set up a sort of league table. They all radiated ‘Clock’ about the same amount, they were all about half a ton in weight, but they differed in the power consumed over a range of 5 to 20 Kilowatts.

They chose to buy the greediest one. I had to go up to the distant end of TV House next to a room which had to provide cooling for it, which in turn consumed another 20 Kilowatts, to say nothing of the trunking. When it all settled down I removed the Earth lead, and measured the RF current in it. It was half an amp of mainly ‘Clock’ frequency. It must have been radiating quite a few watts.

Later with another company this management decided to hand over the entire Presentation to a computer – much to the indignation of the Unions. Presentation has to keep a continuous output going despite in house changes of commercials and those down the line if networking. Usually not the sort of thing that would alter the start of a program a lot, but niggly 15 and 30 second adjustments right up to the last moment.

A pleasant young lady appeared and made notes in Master Control, and then went away to think about it. After a suitable interval the management paid over what would be half a million in today’s money, and the device was delivered. Soon it was installed and the Gen fed in.  A dummy run in parallel with the normal presentation produced a program running about 3 hours late on the computer. The party in charge of went away to lick their wounds. After an interval a further attempt was made. This time the computer was only an hour and a half late. The next we heard was that the size of the computer would have to be doubled. At this stage the management gave up. Perhaps there is some truth in the old saying that the best computers are human beings turned out in millions a year by people who love the work!

Eric Vast.

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