11.4 : Zener Diodes

Zener diodes are specialized semiconductor devices designed to operate in the reverse breakdown region, where they allow current to flow into the cathode, making it positive relative to the anode. This reverse operation distinguishes Zener diodes from conventional diodes and enables their use in various applications, most notably as voltage regulators. One of the defining characteristics of Zener diodes is their nearly vertical I-V (current-voltage) characteristic curve above a certain threshold current, known as the knee current which allows the Zener diode to maintain a relatively stable voltage over a wide range of currents.

Manufacturers specify a Zener diode's maximum power dissipation and its Zener voltage at a specific test current. The Zener voltage, which can range from a few volts to several hundred volts, varies slightly with changes in current. This variance is due to the dynamic resistance of the diode, defined as the inverse of the slope of its I-V curve in the operational region. A low dynamic resistance is crucial for maintaining voltage stability across varying currents, enhancing the Zener diode's effectiveness in voltage regulation applications.

However, it's important to avoid operating Zener diodes in low-current regions where their dynamic resistance increases significantly, as this can lead to instability in the regulated voltage. The temperature dependence of the Zener voltage is another critical factor, with the temperature coefficient (expressed in millivolts per degree Celsius) indicating how the voltage changes with temperature. Lower-voltage Zener diodes generally exhibit negative temperature coefficients, whereas higher-voltage diodes have positive coefficients. For applications requiring a stable reference voltage with minimal temperature influence, a Zener diode with a positive temperature coefficient can be paired in series with a forward-conducting diode, effectively compensating for temperature variations.