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No P-N Intended

A cracked crystal launched the silicon revolution.
February 1, 2003

When Russell Ohl began working at Bell Laboratories in 1927, vacuum tubes were seen as the future of electronics. It was his chance discovery, however, that led to the creation of both the transistor and the solar cell and helped spark the “silicon revolution.”

In the late 1930s Ohl was a radio researcher trying to create a receiver that would be more effective than vacuum tubes. The tubes easily picked up low-frequency radio signals, but had trouble with higher frequencies such as those being tested in radar-a technology that was gaining importance as war brewed overseas. Ohl thought an alternative might lie in the crystal receiver, an antiquated radio device from the 1920s. He devoted himself completely to his research: when his workweek was shortened during the Depression, Ohl used his extra time to study crystal structure.

Crystal receivers were tricky, poorly understood devices. To get a signal, an operator would search the surface of a crystal with a metal strand for the “hot spot,” which caused current flow in only one direction. After exhaustive experimentation, Ohl concluded that the best receivers were the elements now known as semiconductors. He theorized that purer materials would make better receivers and had special samples prepared for his tests.

Early in 1940 Ohl examined a silicon sample that had a crack down its middle. Something was strange about that crystal: when it was exposed to light, the current flowing between the two sides of the crack jumped significantly. Baffled, Ohl showed the bizarre sample to his Bell colleagues, who were equally amazed. No one had ever seen a photovoltaic reaction like it.

The researchers discovered that the crack was a dividing line between two  impurities in the silicon. One type of silicon had an excess of electrons, the other a deficit. They named them p-type for positive and n-type for negative, and the barrier between the two was dubbed the p-n junction. Gradually, the group realized that photons give the excess electrons in the n-type material enough of an energy boost to cross the junction and produce a current.

Although Ohl’s original crystals didn’t produce nearly enough power for commercial use, his research into p- and n-type silicon led to Bell Labs’ creation of the first modern solar cell in 1954. The first transistors also were based on the p-n junction. When Ohl held his unusual crystal to the light in 1940, he unwittingly began the transition from vacuum tubes to integrated circuits.

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