How does a high voltage condition break down a semiconductor?
There's no heat until there's current. Of course, when device features are very small, it doesn't take very much current; localized heating is proportional to localized current density. When there's enough current density and enough heating, the dopant atoms begin to diffuse through the substrate. This destroys the integrity of the structure of the affected PN junction region, so that you no longer have the desired electrical properties.