Zener diodes are two-terminal electronic devices which act as conventional diodes when forward-biased, i.e., with unidirectional conduction, but when reverse-biased above a certain threshold voltage, conduct in the reverse direction. The term “Zener diode” is traditionally applied to devices comprised of p-n junctions formed in conventional semiconductor materials, e.g., Si, which junctions undergo avalanche breakdown at reverse bias potentials above about 5 volts, for example, and such devices may be utilized in voltage regulating and circuit protection circuitry.
A current (I) vs. voltage (V) plot of an idealized Zener diode is illustrated in FIG. 1, from which it is evident that, when reverse-biased above a certain voltage, i.e., the Zener threshold voltage, generally above 5 V, for example, for Si-based devices, a sudden rise in the reverse current occurs. Thus, when forward-biased, a Zener diode functions as an ordinary rectifier, but, when reverse-biased, exhibits a knee, or sharp break, in its I-V plot. A characteristic of Zener avalanche or breakdown is that once conduction occurs under reverse-bias, the voltage across the device remains essentially constant upon further increase of reverse current, up to a maximum allowable dissipation rating. As a consequence of this characteristic behavior, Zener diodes find utility, inter alia, as voltage regulators, voltage references, and overvoltage protectors.
During a surge event, it is desired to limit the voltage drop across the device to a minimum value. Accordingly, an important characteristic of a Zener diode is its reverse surge capability.