Depletion Zone in P-N Junction
The power of the depletion zone electric field gets increases as the reverse-bias voltage increases. One time the electric field intensity increases further than a critical level, the p-n junction depletion zone breaks-down and current starts to flow, generally by either Zener or avalanche breakdown processes. Both of these types of breakdown processes are non-destructive and are reversible, so long as the amount of current flowing does not reach levels which cause the semiconductor material to overheat and cause thermal damage.
This effect is employed to one's advantage in zener diode regulator circuits. Zener diodes have a specific - low - breakdown voltage. A standard value for breakdown voltage is for example 5.6V. The meaning of this is that the voltage at the cathode can never be more than 5.6V higher than the voltage at the anode, since the diode will break down - and therefore conduct - if the voltage gets any higher. This efficiently regulates the voltage over the diode.