Zener diodes are designed to specifically operate in the reverse-bias region. At certain voltages, they begin to conduct, and the current flow increases when the voltage does. Regular diodes behave like closed switches when they are reverse-biased, and will self-destruct when they begin conducting in this region.
When forward-biased, Zener diodes behave normally.
The symbol for a diode is an arrow with a bar to indicate the current flow is in the direction of the arrow. It is called forward-biased when the current is flowing this way, and reverse-biased when the current flow is in the opposite direction. A regular diode will allow some when there is too much the wrong way, but will self-destruct. A Zener diode will not unless its specifications are exceeded. They are typically made using silicon, and can operate between 2 to 200 volts.
Their drawback is that they are current and temperature sensitive, and are noisy. The lower end ones tend to have a poor voltage tolerance, so it is best use precision Zener diodes where the voltage must be very accurate.
The Zener Effect
The Zener Effect was discovered by physicist Clarence M. Zener. Diodes form what is called a p-n junction when p-type and n-type semiconductors are combined. P means positive, and n means negative. Semiconductors have holes which have positive charges, and free electrons, which have negative charges.
In a Zener diode, the p-n junction is heavily doped, which causes it to become very narrow and to generate an intense electric field. When the diode is reverse-biased, this intense electric field causes the electrons to become ionized. They are pulled away from their valence orbits, so that they become free and can flow.
A popular use is as voltage regulators for low current circuits. For example, if you have a 9-volt battery but need to power a 3 V hobby motor, a Zener diode may be used to reduce the voltage. When they are used as voltage regulators, they must be combined with a current limiting diode in order to prevent their destruction.
They are also used as voltage regulators in unregulated power supplies, where they are placed across the output.
Voltage references are useful in precision regulator power supplies, and in circuits that require a precision constant current supply. A Zener diode needs a constant current in order for it to function as a voltage reference, in order to avoid unstable performance. They are often used as voltage references in opamp circuits, such as in linear regulators.
They are found in devices where precision voltage references are needed, such as ohmmeters, ammeters, and voltmeters. Zener diodes are also found in power supplies and surge protectors. Another useful function of theirs is as a voltage clamp, where they control the amount of input voltage to a circuit that would otherwise become unstable or self-destruct.