Read more: “The nature of nothingness“
THE sound of New Year’s Eve celebrations drifting up from the Palace Theater did not distract William Shockley. Nor did the few scattered revellers straying through Chicago’s snow-covered streets below. Rarely a mingler, Shockley had more important things on his mind. Barricaded in his room in the art-deco opulence of the Bismarck Hotel, he was thinking, and writing.
Eight days earlier, on 23 December 1947, John Bardeen and Walter Brattain, two of Shockley’s colleagues at Bell Laboratories in Murray Hill, New Jersey, had unveiled a device that would change the world: the first transistor. Today, shrunk to just nanometres across and carved into beds of silicon, these electrical on-off switches mass in their billions on every single computer chip. Without them, there would be no processing of the words, sounds and images that guide our electronic lives. There would be no smartphone, router, printer, home computer, server or internet. There would be no information age.
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Bardeen and Brattain’s device, a rather agricultural construction of semiconductor, gold-enwrapped polystyrene and a spaghetti twist of connecting wires, did not look revolutionary, and it would have taken a seer to foretell the full changes it would bring. Even so, those present that December at Bell Labs knew they had uncovered something big. In Shockley’s words, the transistor was a “magnificent Christmas present”. Magnificent, but for one thing: no one knew quite how it worked.
Holed up in his Chicago hotel, Shockley needed to change that. As head of Bell Labs’ solid-state physics group, he had been the intellectual driving force behind the transistor, yet Bardeen and Brattain had made the crucial breakthrough largely without him. To reclaim the idea as his own, he needed to go one better.
That meant getting to grips with a curious entity that seemed to control the transistor’s inner workings. Its existence had been recognised two decades earlier, but its true nature had eluded everyone. For good reason: it was not there.
Transistors – both Bardeen and Brattain’s original and those that hum away in computer processors today – depend on the qualities of that odd half-breed of material known as a semiconductor. Sitting on the cusp of electrical conduction and insulation, semiconductors sometimes let currents pass and sometimes resolutely block their passage.
By the early 20th century, some aspects of this dual personality were well documented. For example, the semiconductor galena, or lead sulphide, was known under certain circumstances to form a junction with a metal through which current travelled in only one direction. That had made it briefly popular in early wireless receivers, where a filigree metal probe – a “cat’s whisker” – was tickled across a crystal of galena to find the contact that would transform oscillating radio signals into steady direct current.
This process had to be repeated afresh each time a radio receiver was switched on, which made tuning a time-consuming and sometimes infuriating business. This was symptomatic of all semiconductors’ failings. There seemed little rhyme or reason in their properties; a slight change in temperature or their material make-up could tip them from conduction to insulation and back again. It was tempting to think their caprices might be tamed to make reliable, reproducible electrical switches, but no one could see how.
And so in the radio receivers and telephone and telegraph systems of the 1920s and 30s – such as those operated by Bell Labs’ parent company, AT&T – vacuum tubes came to reign supreme. They worked by heating an electrode in a vacuum and applying electric fields of varying strength to the stream of electrons emitted, thus controlling the size of the current reaching a second electrode at the far side. Bulky, failure-prone and power-hungry though they were, vacuum tubes were used as switches and amplifying “repeaters” to hoist fading signals out of a sea of static on their long transcontinental journeys.
Even as they did, however, the seeds of their demise and semiconductors’ eventual triumph were being sown. In 1928 Rudolph Peierls, a young Berlin-born Jew, was working as a student of the great pioneer of quantum physics, Werner Heisenberg, in Leipzig, Germany. The convolutions of history would later make Peierls one of the UK’s most respected physicists, and pit him against his mentor in the race to develop the first atomic bomb. At the time, though, he was absorbed by a more niggling problem: why were electrical currents in some metals deflected the wrong way when they hit a magnetic field?
To Peierls, the answer was obvious. “The point [was] you couldn’t understand solids without using the quantum theory,” . Just as quantum theory dictates that electrons orbiting an atom couldn’t have just any old energy, but are confined to a series of separate energy states, Peierls showed that within a solid crystal, electrons are shoe-horned into “bands” of allowed energy states. If one of these bands had only a few occupied states, electrons had great freedom to move, and the result was a familiar electron current. But if a band had only a few vacant states, electron movement would be restricted to the occasional hop into a neighbouring empty slot. With most electrons at a standstill, these vacancies would themselves seem to be on the move: mobile “absences of electron” acting for all the world like positive charges – and moving the wrong way in a magnetic field.
Nonentities named
Peierls never gave these odd non-entities a name. It was Heisenberg who gave them their slightly off-hand moniker: ö – or “holes”. And there things rested. The holes were, after all, just a convenient fiction. Electrons were still doing the actual conducting – weren’t they?
Although Peierls’s band calculations were the germ of a consistent, quantum-mechanical way of looking at how electrical conduction happened, no one quite joined up the dots at the time. It was 10 years before the rumblings of war would begin to change that.
“1970 1.3×1012 Transistors in use globally 1.3×1010 Microchips in use globally”
Radar technology, which involves bouncing radio waves off objects to determine their distance and speed, would become crucial to Allied successes in the latter stages of the second world war. But radar presented a problem. If the equipment were to fly on bombing missions, it needed to be as compact and lightweight as possible. Vacuum tubes no longer cut the mustard. Might the long-neglected semiconductors, for all their failings, be a way forward?
In 1940, a team at Bell Labs led by engineer Russell Ohl was exploring that possibility by attempting to tame the properties of the semiconductor silicon. At the time, silicon’s grouchy and intermittent conduction was thought to be the result of impurities in its crystal structure, so Ohl and his team set about purifying it. One day, a glitch in the purification process produced a silicon rod with a truly bizarre conducting character. One half acted as if dominated by negatively charged carriers: electrons. The other half, though, seemed to contain moving positive charges.
That was odd, but not half as odd as what happened when you lit up the rod. Left to its own devices, the imbalanced silicon did nothing at all. Shine a bright light on it, however, and it flipped into a conducting state, with current flowing from the negative to the positive region.
A little more probing revealed what was going on. Usually, a silicon atom’s four outer electrons are all tied up in bonds to other atoms in the crystal. But on one side of Ohl’s rod, a tiny impurity of phosphorus with its five outer electrons was creating an excess of unattached electrons. On the other, a small amount of boron with just three electrons was causing an electron deficit (see diagram).
Peierls’s holes had suddenly found a role. When kicked into action by the light, electrons were spilling over from the region of their excess to fill the holes in the electron structure introduced by the boron. However passively, it was the presence of an absence of electrons that was causing the silicon rod’s unique behaviour. Ohl named his discovery the positive-negative or “” junction, owing to its two distinct areas of positive and negative charge carriers. Its property of converting light energy into electric current made it, incidentally, the world’s first photovoltaic cell.
It was a few years before Shockley got wind of Ohl’s breakthrough. Already a senior member of Bell Labs’ physics team before the war, the hostilities had taken him in a very different direction, as head of the US navy’s anti-submarine warfare operations research unit. Resurfacing in 1945 leading Bell’s solid-state physics division, it did not take Shockley long to spot the p-n junction’s potential.
He was fascinated by the thought that, by pressing a metal contact to the junction’s midriff, you might use an external electric field instead of light to control the current across it. In a sufficiently thin layer of n or p-type silicon, he reasoned, the right sort of voltage would make electrons or holes swarm towards the contact, providing extra carriers of charge that would boost the current flow along the surface layer. The result would be an easily controllable, low-power, small-scale amplifier that would smash the vacuum tube out of sight. That was truly a prospect to pique the interest of Shockley’s paymasters.
“2010 1.4×1020 Transistors in use globally 2.7×1012 Microchips in use globally”
His first attempts to realise the dream, though, were unsuccessful. “Nothing measurable, no measurable results,” he noted of an early failure. “Quite mysterious.” And with his mind now on the broad sweep of Bell Labs’ solid-state research, Shockley was obliged to leave further investigations to two highly qualified subordinates: Bardeen, a thoughtful theorist, and Brattain, an inveterate tinkerer.
It proved a frustrating chase, and it was a classic combination of experimental nous and luck that led the pair to success – plus Bardeen’s spur-of-the-moment decision to abandon silicon for its slightly more predictable semiconducting sister germanium. This finally produced the right sort of amplification effect, boosting the power of input signals, sometimes by a factor of hundreds. The magnificent Christmas present was unwrapped.
Just one thing didn’t add up: the current was moving through the device in the wrong direction. Although the germanium slab had n-type material at the top, it appeared to be positive charges making the running. The puzzlement is almost palpable in Brattain’s lab-book entry for 8 December 1947. “Bardeen suggests that the surface field is so strong that one is actually getting p-type conduction near the surface,” he wrote. It was a mental block that stopped Bardeen and Brattain understanding the fruits of their labours.
No doubt they would have done, given time. But in his Chicago hotel room that New Year’s Eve, Shockley stole a march on his colleagues. There was a way out of the impasse, he realised, and he did the first hurried calculations to firm up his case.
If a hole were merely the absence of an electron, then electrons and holes could hardly co-exist: whenever an electron met a hole, its presence would by definition negate the absence of itself that was the hole. By that measure, the existence of positive charges in a negative region, as Bardeen and Brattain had seemingly observed, was a nonsense.
But what if a hole were real, Shockley asked: not just an absence of something, but a true nothing-that-is? What if it were a particle all on its own, with an independent existence just as real as the electron’s? If this were true, holes would not need to fear encountering an electron. They could happily co-exist with electrons in areas dominated by them – and that would explain what was going on in the transistor.
It was a daring intellectual leap. In the weeks that followed, Shockley used the idea to develop a transistor that exploited the independence of electrons and holes. This was the “p-n-p” transistor, in which a region of electron excess was sandwiched between two hole-dominated areas. Apply the right voltage, and the resistance of the middle section could be broken down, allowing holes to pass through hostile electron-populated territory without being swallowed up. It also worked in reverse: electrons could be made to flow through a central region given over to holes. This was the principle that came to underpin the workings of commercial transistors in the decades that followed.
The rest, as they say, is history. For Shockley, it was not a happy one. He did not at first tell Bardeen and Brattain of his new course, and even attempted to claim over the first transistor. The relationship between the three men never recovered. By the time they shared the Nobel prize in physics for their discovery in 1956, Shockley had left Bell Labs to form the Shockley Semiconductor Laboratory to capitalise on his transistor alone. But his high-handed and increasingly paranoid behaviour soon led to a mass mutiny from the bright young talents he had hired, such as Gordon Moore and Robert Noyce, who went on to found Intel, which remains the world’s largest manufacturer of microchips.
The hole, meanwhile, went from strength to strength. Today you will find it at the heart of not just every computer chip, but every energy-saving LED lightbulb, every laser that reads our CDs and DVDs, and every touchscreen. Modern life has become unimaginable without this curiosity whose nature took two decades to reveal: the nothing that became a something and changed the world.
