When a pn junction is reverse-biased the current is carried solely by the minority carriers, and at a given temperature the number of minority carriers is fixed. Ideally, therefore, we would expect the reverse current for a pn junction to rise to a saturation value as the voltage is increased from zero and then to remain constant and independant of voltage, as shown below. In practice, when the reverse voltage reaches a particular value which can be 100V or more the reverse current increases very sharply, again shown below, an effect known as breakdown. The effect is reproducible, breakdown in a particular junction always occuring at the same value of reverse voltage. This is known as the Avalanche effect and reverse-biased diodes known as Avalanche diodes (sometimes called - perhaps incorrectly - Zener diodes) can be used as the basis of a voltage stabiliser circuit. The junction diodes used for this purpose are usually silicon types.

Zener Diode VI Characteristic

Zener Diode Symbol