What happens when an electron gathers enough energy to “slide down the hill” in impact ionization?

In the context of impact ionization within semiconductors, when an electron obtains sufficient energy, it collides with a lattice atom at high speed. This collision can provide enough energy to free another electron from its bond. The freed electron is then referred to as a "secondary electron." As a result of this process, additional electrons are released into the depletion region, which ultimately contributes to the generation of electron-hole pairs.

This phenomenon is critical in the operation of devices such as avalanche photodiodes, where the multiplication of carriers through impact ionization leads to an increase in the current. Thus, the generation of additional electrons in the depletion region is a fundamental aspect of how impact ionization operates and is integral to understanding the behavior of semiconductors under certain conditions.

The other options, while related to the broader context of semiconductor interactions, do not accurately characterize the direct consequence of impact ionization. Creating a chemical bond or altering the band structure are not direct effects of the sliding down process. Similarly, while heat may be produced in some processes, it is not the primary or defining outcome of impact ionization.