The Four Layer Diode
The four-layer diode was the key to William Shockley's plan to revolutionize AT&T's phone system. It was a great device in theory, but not in practice -- at least not at the time when Shockley wanted to build it. The four-layer diode, also known as a Shockley Diode, is a crystal made of alternating layers of N- and P- type semiconductors. By putting in four layers, instead of the three used in transistors, the Shockley Diode could do more than a transistor. For one, it acted like a rectifier, able to turn alternating current into direct current. Two, it switched on and off when a specific amount of voltage -- known as the breakover voltage -- was applied. The four-layer diode, therefore, could be used to replace both the rectifiers and transistors necessary to connect long distance phone calls.
In essence, the four-layer diode was the first integrated circuit since it did the work of two transistors, two resistors, and a diode -- all in a single crystal. Unfortunately, they were so tricky to make that Shockley's company, Shockley Semiconductor, never managed to build any that were truly commercially viable. When the integrated circuit was invented in 1958, it eclipsed the four-layer diode's capabilities and any market for the diode quickly dried up.
Four-layered semiconductors are, however, used today. They're known as "thyristors" and a variety of types, including Shockley Diodes, exist. Thyristors are chiefly used as switches to control power supplies -- often in electrical utility systems.
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