Most of the diodes have avalanche breakdown voltages higher than the reverse bias ever gets. The value of avalanche voltage depends on the manufacture of diode. Zener diodes are to have well defined and constant avalanche voltages.
Assume that a certain Zener diode has an avalanche voltage, called as Zener voltage, of 50 V. If the reverse bias is applied to P-N junction, diode acts as an open circuit below 50 V. When voltage reaches 50 V, the diode begins conducting. The more the reverse bias tries to increase, more current flows through P-N junction. This prevents reverse voltage effectively from exceeding 50 V. The current through the Zener diode, as the function of voltage, is shown in the Figure given below. The Zener voltage is indicated by abrupt rise in the reverse current as reverse bias increases. A characteristic Zener diode voltage limiting circuit is shown in the figure given below.
There are other ways also to get voltage regulation besides use of Zener diodes, but Zener diodes provide the simplest and least expensive alternative. Zener diodes are available with a extinsive variety of voltage and power handling ratings. Power supplies for the solid state equipment commonly use Zener diode regulators.