A Zener diode, also known as a voltage regulator, is a junction-type silicon semiconductor diode manufactured using a special process. Its symbol is shown in Figure (a). As mentioned earlier, this diode has a relatively high impurity concentration, resulting in a high charge density within the space charge region. This narrow region easily forms a strong electric field. When the reverse voltage reaches a certain value, the reverse current increases sharply, causing reverse breakdown. Its characteristics are shown in Figure (b). VZ in the figure represents the reverse breakdown voltage, or the voltage regulator's stable voltage. The voltage regulation principle of a Zener diode is that a large increase in current causes only a small voltage change. The steeper the reverse breakdown curve, the lower the dynamic resistance, and the better the voltage regulation performance of the Zener diode.

A current-limiting resistor, R, is generally added to the voltage regulation circuit of a Zener diode to keep the diode current within the voltage regulation range of IZmax and IZmix. Furthermore, appropriate measures should be taken to limit the current through the diode to prevent it from overheating and burning out.