Russell Ohl, working at Bell Labs
in 1940, stumbled on the
semiconductor pn junction. Ohl
was working with a silicon sample
that had an accidental crack down
its middle. He was using an
ohmmeter to test the electrical
resistance of the sample when he
noted that when the sample was
exposed to light, the current that
flowed between the two sides of
the crack made a significant jump.
This discovery was fundamental to
the work of the team that invented
the transistor in 1947.