A semiconductor diode normally conducts current in only one direction, like a spring-loaded door: it opens easily one way but pushes back the other. But if you dope the crystal heavily, the door starts behaving strangely: under reverse bias, the current doesn't just flow: it shoots up. Such diodes are called tunnel diodes because it was thought that electrons pass through the barrier like ghosts through a wall: that's quantum tunneling.
The authors of the new study propose a different, more down-to-earth mechanism. Any warm object emits invisible infrared radiation: that's how thermal imaging works. A semiconductor at room temperature also glows in the infrared. Heavy doping creates a host of defects in the crystal — atoms out of place, vacancies. These defects can absorb infrared photons and generate electron-hole pairs, and the photon energy is sufficient even if it is smaller than the band gap.
The electrons and holes generated this way end up near the p-n junction, where a built-in electric field exists. This field sweeps them apart, creating a reverse current. Under reverse bias, the junction region widens, capturing more defects, and the current grows. And with sufficiently heavy doping, avalanche multiplication of carriers also occurs, like a snowball rolling down a mountain, gathering more and more snow. As a result, the reverse current increases sharply with voltage, and an ordinary diode acquires the characteristics of a tunnel diode.
The authors emphasize that they do not completely deny quantum tunneling. In different devices, the contribution of one mechanism or the other may be larger or smaller. But the proposed explanation via infrared radiation and defects provides a simple and intuitive picture that links the thermal properties of the crystal to its electrical behavior.
The tunnel diode was invented in 1957 and was the first semiconductor device to practically use quantum tunneling. Now, perhaps, its operation can be explained without this effect.