The avalanche breakdown is something that occurs when a reverse biased diode (positive voltage on the cathode, negative voltage on the anode) has such a strong reverse voltage on it that the electromagnetic field starts accelerating electrons toward the depletion region. As the electrons accelerate, they hit other electrons, breaking them free from the valence band, creating an “avalanche” of free electrons. This usually occurs at a higher reverse voltage than a zener breakdown and, other than in a zener diode, is typically not desired.
The higher voltage breakdown in a zener diode.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
In silicon diodes, as the width of the depletion layer increases under reverse bias, the electric field increases. Free carriers in the depletion region are accelerated by this electric field, and as the carriers move through the depletion region, they collide with the fixed atoms. At some point, the electric field and the width of the space charge region become large enough that some carriers gain energy sufficient to break covalent bonds upon impact, thereby creating electron-hole pairs.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock
Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when carriers in the transition region are accelerated by the electric field to energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.