Explanation: A tunnel diode's principal characteristic is its **negative resistance region**. Unlike a typical diode, as the forward voltage across a tunnel diode increases beyond a certain point, the current through it *decreases* for a range of increasing voltage before eventually rising again. This unique behavior, resulting from quantum mechanical tunneling due to heavy doping, allows them to be used in high-frequency oscillators and amplifiers. Options A (High forward resistance) and D (High forward current rating) are generally incorrect; tunnel diodes are typically low-power devices and have low initial forward resistance. Option B (Very high PIV - Peak Inverse Voltage) is also incorrect; their heavy doping actually leads to a relatively low reverse breakdown voltage.