Avalanche:

When a diode is reverse-biased and the voltage becomes high adequate, the p-n junction will conduct. This is termed as avalanche effect. The reverse current that is near to zero at lower voltages increases dramatically. The avalanche voltage differs among various kinds of diodes. Figure shown below is a graph of the characteristic current-versus-voltage curve for a typical semiconductor diode viewing the avalanche point. The avalanche voltage is significantly greater than and is of the reverse polarity from the forward breakover voltage. The Zener diode makes use of avalanche effect. The Zener diodes are specially produced to have exact avalanche voltages. They build the basis for voltage regulation in the dc power supplies.

Figure: Characteristic curve for a semiconductor diode.