The Silicon P-N Junction

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.

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.

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.