Voltage regulator diodes utilize the reverse characteristics of a pn junction. When raising reverse voltage of pn junction diodes, high current starts flowing at a certain voltage, and constant voltage can be obtained. (This phenomenon is called breakdown and this voltage is called breakdown voltage.)
Voltage regulator diodes actively use this characteristic. Since this breakdown voltage is also called Zener voltage, voltage regulator diodes are also called Zener diodes. This voltage may be used as a constant voltage power supply or a reference voltage for electronic circuits.
(Note: Generally, the Zener phenomenon is observed when the voltage is 6 V or less. If it exceeds 6 V, the avalanche phenomenon becomes dominant over the Zener phenomenon. Zener voltage and avalanche voltage have different temperature characteristics; the former has a negative temperature coefficient and the latter has a positive one.)