ZENER DIODES

A zener or reference diode is a silicon PN junction made to operate only under reverse bias or voltage conditions. At a known reverse voltage an avalanche breakdown occurs, indicated by the knee in the curve shown on the left side of Fig. 1a. Beyond that point the reverse voltage remains constant enough to serve as a useful reference voltage. Zener diodes exhibit sharp reverse knees at less than about 6 V. Large quantities of electrons within the depletion region break the bonds with their atoms, causing a large reverse current to flow, as indicated by the vertical dropoff of the curve.



Figure 1 Characteristic curves for a PN diode: (a) forward bias (right) and reverse bias (left), and (b) symbol for zener diode.


Zener diodes are stable voltage references because the voltage across the diode remains essentially constant for wide variations of current. These diodes are used as generalpurpose voltage regulators and for clipping or bypassing voltages that exceed a specified level. Variations of the zener diode called transient voltage suppressors (TVSs) serve as circuit-protective devices because of their ability to bypass unwanted high-input voltage transients.

The schematic symbol for a zener diode is shown in Fig. 1b. It differs from the conventional diode schematic symbol because of its S-shaped anode representation. Zener diodes have nominal reference voltage values from 1.8 to 200 V and power ratings from 250 mW to as high as 50 W. They are packaged in a variety of glass, metal, and plastic cases, some for surface mounting. TVS diodes have ratings from 5 to 300 V, and can handle up to 5 W steady-state or 1,500 W peak power. Although both of these diodes can operate in the small-signal region, they are considered to be regulator and suppressor diodes rather than small-signal diodes.