Zener diodes are semiconductor diodes which have been manufactured to have their reverse breakdown occur at a specific, well-defined voltage (its “Zener voltage”), and that are designed such that they can be operated be operated continuously in that breakdown mode. Commonly available Zener diodes are available with breakdown voltages (“Zener voltages”) anywhere from 1.8 to 200 V.

If we hook up a diode in a simple circuit with a variable voltage source and a current-limiting resistor, we can measure the current I through the diode when a given voltage V is applied across it. For an ideal diode, no current at all passes when the voltage is less than zero: the diode completely prevents reverse current flow. For small positive voltage (“forward bias,” or sometimes “foward voltage”), a tiny amount of current may flow, and a very large amount of current will flow above a given threshold. The amount of current that flows is actually exponential with increasing voltage.

The threshold where an appreciable amount of current flows is typically around 0.7 V for simple semiconductor diodes, but may be as low as 0.15 V for Schottky diodes, or as high as 4 V for certain types of LEDs.Of course, no diode is truly ideal. In real diodes, when the voltage is reversed, a very small amount of current (leakage) may flow. And, more significantly, each diode is rated for a certain maximum amount of reverse voltage. If you apply voltage more negative than that limit, the diode will undergo “reverse breakdown”

and begin to conduct a significant amount of current, but backwards from the normal direction of diode current flow. For a regular diode, we would say that the diode has failed if it begins to conduct current in that direction.

Aside: The actual physics of what happens at breakdown is quite interesting; two separate effects, the Zener effect and Avalanche breakdown both contribute to this behavior.

To understand how Zener diodes are different from other diodes, let’s first review the properties of regular diodes. And, while there are many different types of diodes– see herefor a long list –we’re going to focus on so-called “normal” semiconductor diodes, most commonly constructed with a p-n silicon junction.Diodes usually come in glass or plastic cylindrical packages, marked with a stripe on one side to indicate polarity. In a perfectly ideal diode, current flows in one direction only, from the anode (positive side) to the cathode (negative side) which is marked with the stripe. The schematic symbol is a triangle pointing towards a bar, where the current flows in the same direction, towards the barred (striped) end. Surface mount versions of diodes tend to follow the same labeling convention, where the cathode end is marked with a broad stripe.

Zener diodes are a special type of semiconductor diode– devices that allow current to flow in one direction only –that also allow current to flow in the opposite direction, but only when exposed to enough voltage. And while that sounds a bit esoteric, they’re actually among the handiest components ever to cross an engineer’s bench, providing great solutions to a number of common needs in circuit design.

In what follows, we’ll show you how (and when) to use a Zener, for applications including simple reference voltages, clamping signals to specific voltage ranges, and easing the load on a voltage regulator.