What is the principal characteristic of a tunnel diode?

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The key idea is that a tunnel diode shows a region of negative differential resistance in its current–voltage curve, caused by quantum tunneling in a heavily doped p-n junction. The extreme doping makes the depletion region very thin, so electrons can tunnel through as the junction is biased just enough in the forward direction. This creates a peak current at a low forward voltage. As the voltage increases further, the energy bands misalign and the tunneling probability drops, so the current actually falls even though the voltage rises. That produces a portion of the I–V curve where increasing voltage reduces current, i.e., negative resistance. At still higher forward bias, normal forward conduction resumes and current rises again. This negative resistance region is what makes tunnel diodes useful for very fast switching and microwave oscillators, unlike devices that are defined by high peak inverse voltage, high forward current rating, or high forward resistance.