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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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