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Transistor

Fall 2010 | Volume 25 |  Issue 3
Era:
1940s

On December 23, 1947, in the Bell Telephone Laboratories at Murray Hill, New Jersey, physicists John Bardeen and Walter Brattain spoke over the world’s first transistor-amplified telephone circuit, a quarter-inch-tall device composed of a thin strip of gold foil sliced in two in order to create two metal contacts over a crystal of germanium. Their success was the culmination of eight years of research conducted alongside their team leader, 37-year old William B. Shockley, and triggered a wave of new electronics. Within a decade, transistors had found applications in hundreds of devices: telephones, radios, hearing aids, guided missiles, and pocket calculators, the predecessors of modern computers.

Up until the end of World War II, vacuum tubes were considered the future of electronics, making everything from radar to radio possible. The vacuum tube served two purposes: amplifying weak signals to make them stronger, and serving as a switch to stop the flow of electricity instantly. Mervin Kelly, Bell Labs’ research director, recognized their serious limitations, especially when long-term reliability was necessary, as in telephone networks, because they relied on generating heat to work. And when thousands of tubes were used together in new digital computers it was extremely difficult to keep so many hot devices operating without burning out. Alternatively, a solid-state switching device generally needed no power at all and would be capable of responding immediately to an incoming signal, a necessary requirement if a telephone system was not to drown in its own complexity and size.

Kelly’s solution was to hire William Shockley, a recent Ph.D. graduate from MIT, to focus on semiconductors, new solid-state devices falling somewhere between the spectrum of a conducting material such as copper or silver, through which current always passes, and an insulating material such as glass, through which current never passes. Physicists theorized that, by learning how to pass and amplify a current through a semiconductor, they might produce electrical output comparable in value to that of a vacuum tube.

During his first three years at Bell Labs, Shockley produced nothing of value. While fiddling with a strip of copper oxide in his home office during the 1939 holiday season, however, he stumbled upon the field effect. He had discovered that the thin metal film of a semiconductor would substantially increase in conductivity if turned into one of a pair of strongly charged condenser plates. His experiments with copper oxide ultimately failed, but the field effect became the basis for the successful point-contact transistor developed under Shockley’s guidance.

At the end of World War II, Shockley gathered a team at Bells Labs including a former colleague, Walter Brattain, and John Bardeen, a theoretical physicist and friend of Brattain’s. From the summer of 1945 until the end of 1947, they performed experiments on silicon and germanium crystals, semiconductors with a crystalline atomic structure. Shockley’s experiments still failed, but after several more tries, Bardeen and Brittain produced a low amplifying effect by placing a gold foil sliced in two on the surface of a slice of germanium. When an electric signal generated through one part of the foil traveled into the germanium, some of the semiconductor’s excess electrons became attracted to the gold directly on its surface, leaving positively charged holes. These holes were in turn attracted to the second, negatively charged piece of foil, creating a flow of current across the semiconductor. The result was the point-contact transistor.

At first only 20 percent of the transistors they made worked. To complicate matters further, the transistor might do the same sorts of things a tube could do, but it didn’t do them in the same way. This meant that circuit designers—the engineers responsible for turning components into workable tools and instruments—would have to reconfigure their often highly complex circuits to accommodate the transistor’s special characteristics. For many, it hardly seemed worth it, but others made enormous efforts to get the transistor working and out into the world of applications. Finally, in 1951, Shockley surprised his team by presenting work on the junction transistor, which used the internal properties of semiconductors rather than the surface effects on which the point-contact device depended.

Shockley took a silicon semiconductor and “doped” it, introducing impurities such as arsenic or boron that, respectively, created free electrons to produce a negative charge (n-type) or stole electrons from the original semiconductor’s lattice to create positively charged holes (p-type). When joined together, the electrons and holes of the doped n-type and p-type semiconductors flowed forward toward the junction where they met. As electrons filled the holes, the holes and free electrons ceased to exist, but others took their place, creating a current that flowed in one direction. This concept provided the fundamental basis for all subsequent transistor development.

In 1954 the introduction of the first all-transistor radio spearheaded the commercial success of the transistor. That year 1.3 million transistors were sold, and by 1966 the number had spiked to 850 million. The appearance of the integrated circuit in 1971 marked the next stage in the evolution of electronics, one that has still not run its course: in 2009 semiconductor manufacturer Intel released its 32 nanometer (nm) chip, containing nearly 2 billion transistors; by the end of 2011 Intel will have released the 22 nm, containing 2.9 billion transistors packed into an area as small as a fingernail.

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