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In 1958 Leo Esaki, a Japanese scientist and Nobel Prize winner, discovered the tunnel diode phenomenon. If a semiconductor junction diode is heavily doped with impurities, its I-V curve will have a region of negative resistance (the slope is negative, or downward). Such diodes are called "tunnel diodes", and have broad applications in microwaves. This region has been exploited to create oscillators, but it also makes a very efficient detector. Why the word "tunnel"? We'd have to resort to quantum physics to explain that, but we won't, because no real microwave engineer cares!