As you learned on the mechanics of the PN junction, when a negative
voltage from anode to cathode is connected, the electrons do not have
enough energy to cross the widened depletion zone & no current flows.
But what happens when enough voltage is applied, the electrons have
enough energy to cross the barrier & knock some other electrons free
along the way. This creates what is called an electron avalanche where
more & more electrons break free & conduct current. The voltage at
which this phenomenon starts remains across the diode constant even if
the outside source is increased.
In most semiconductor diodes this effect is destructive to the diode,
since they are not designed to handle the current produced by the
avalanche.
The zener diode works in what is called the zener region, a voltage
where a small & controlled electron avalanche can be used to generate a
constant voltage across the diode.
This property gives the zener diode many of its uses as a voltage
regulator, voltage monitor & many other fixed voltage applications
