Russell Ohl Accidentally Discovers the

Silicon P-N Junction

"He called these fellows up and said to drop their work and come on up here and take a look at something. Then all these men gathered in Kelly's office. He wouldn't take no for an answer. He said, 'Drop it, and come on up here!' And so they all came up here."
-- Russell Ohl, 20 August 1976, about a day in 1940 when he demonstrated the P-N junction to his boss

In 1939, vacuum tubes were state of the art in radio equipment. People had previously used crystals for radios, but the crystals were so maddeningly inconsistent and mysterious it was a wonder they worked at all. Vacuum tubes were simple, and they worked. Most scientists agreed tubes were the future for radio and telephones everywhere.

Russell Ohl didn't agree. He kept right on studying crystals, occasionally having to fight Bell Labs administration to let him do it. Ohl thought silicon crystals' erratic behavior was due to impurities in the crystal, not any problem in the silicon itself. He thought that if he could purify silicon enough, the crystals just might provide the improved radio broadcasting capabilities for which everyone was looking.

A Quirky Crystal

Much of his research in 1939 was devoted to producing ultra-pure crystals. As he expected, his purified silicon crystals-- now 99.8 percent pure -- were much more consistent. They worked the way a rectifier should, allowing current to flow in one direction and not the other. At least, most of them worked. On February 23, Ohl sat down to examine a particularly curious crystal that was as quirky as the cat's whisker crystals of old.

The crystal had a crack down the middle. Ohl was examining how much current flowed through one side of the crack versus the other, when he noticed something peculiar. The amount of current changed when the crystal was held over a bowl of water. And a hot soldering iron. And an incandescent lamp on the desk in the room.

By early afternoon, Ohl realized that it was in fact light shining on the crystal that caused this small current to begin trickling through it. On March 6, he showed his prize silicon rod to Mervin Kelly. Kelly quickly called Walter Brattain and Joseph Becker to the scene.

Ohl had his coal-black crystal attached to a voltmeter in front of him. He turned on a flashlight, aimed it at the silicon, and the voltage instantly jumped up to half a volt. This was ten times anything Brattain had ever seen before. He was stunned, but not too stunned to produce an off-the-cuff explanation. The electrical current must be due to some barrier being formed right at the crack in the crystal.

The Quirks Explained

With more research, what was going on became clear: the crystal had different levels of purity on either side of the crack. Due to the subtle traces of extra elements, one side had an excess of electrons, and the other side a deficit. Since opposites attract, the electrons from one side had rushed over to the other -- but they went only so far, creating a thin barrier of excess charges right at the central crack. That barrier created a one way street -- electrons could now only travel in one direction across it.

When Ohl shined light on the rod, energy from the light kicked sluggish electrons out of their resting places and gave them the boost they needed to travel around the crystal. But due to the barrier, there was only one way they could travel. All those electrons moving in a single direction became an electric current. Ohl's crystal was the ancestor of modern day solar cells, which take energy from the sun and convert it into electricity. But for Bell Labs on that day, it opened up the idea that crystals might be just the thing needed to replace vacuum tubes.

Resources:
-- Russell Ohl Interview with Lillian Hoddeson on 20 August 1976
-- Walter Brattain interview with King and Holden in January of 1964
-- Crystal Fire

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