Zener breakdown is a quantum mechanical effect, which, in itself, shouldn’t scare you. But it’s where, in a reverse biased diode (a positive voltage on the cathode, negative voltage on the anode), electrons will *tunnel* through the depletion region and give rise to a reverse current. From a high-level, this may seem like an avalanche breakdown but they operate under two completely separate mechanics and avalanche typically requires more energy.
The lower voltage breakdown in a zener diode.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
In highly doped Zener diodes, the high doping results in a very narrow depletion-region width, and application of a reverse bias causes carriers to tunnel directly between the conduction and valence bands, resulting in a rapidly increasing reverse current in the diode.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock
In electronics, the Zener effect (employed most notably in the appropriately named Zener diode) is a type of electrical breakdown, discovered by Clarence Melvin Zener. It occurs in a reverse biased p-n diode when the electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor, leading to numerous free minority carriers which suddenly increase the reverse current.