Zener Diode

A high reverse-biased voltage applied to a diode may create a high reverse cur­rent, which can generate excessive heat and cause a diode lo break down. The applied reverse volt­age at which the breakdown occurs is called the breakdown voltage or Peak Reverse voltage. A spe­cial diode called a zener diode is connected lo operate in the reverse-biased mode. It is designed to operate at those voltages that exceed the break­down voltage. This breakdown region is called the zener region.

When a reverse-biased voltage is applied that is high enough to cause breakdown in a zener diode, a high reverse current (1z,) flows. The re­verse current is low until breakdown occurs. Alter breakdown, the reverse current increases rapidly. This occurs because the resistance of the zener diode decreases as the reverse voltage increases. The breakdown voltage of a zener diode (Ez) is determined by the resistivity of the diode. This, in turn, is controlled by the doping technique used during manufacturing. The rated breakdown voltage represents the reverse voltage at the zener test current (1zt). The zener test Current is some­what less than the maximum reverse current the diode can handle. The breakdown voltage is typ­ically rated with 1 to 20% tolerance.

The ability of a zener diode to dissipate power decreases as the temperature increases. Therefore power dissipation ratings are given for specific temperatures. Power ratings are also based on lead lengths: shorter leads dissipate more power. A de­rating factor is given by the diode manufacturer to determine the power rating at different tempera­tures from the ones specified in their tables. For example, a derating factor of 6 milliwatts per de­gree Celsius means that the diode power rating de­creases 6 milliwatts for each degree of change in temperature. Zener diodes are packaged like FN junction diodes .Low-power zener diodes are mounted in either glass or epoxy. High-power zener diodes are stud mounted with a metal case. The schematic symbol for the zener diode is similar to the FN function diode except for the diago­nal lines on the cathode bar.

Questionnaire:

• What is Zener Diode?