A diode usually is thought of as a device that lets current flow through it in only one direction; however Zener diodes are made to permit current to also flow in the reverse direction if the voltage is larger than the rated breakdown or "Zener voltage".
A conventional solid-state diode will not let current flow if reverse-biased below its reverse breakdown voltage. By exceeding the breakdown voltage, a conventional diode is destroyed in the breakdown because of excess current, which brings about overheating. The process is however reversible, if the device is operated within limitation. In case of forward-bias (in the direction of the arrow), the diode exhibits a voltage drop of roughly 0.6 volt for a typical silicon diode. The voltage drop depends on the type of the diode.
A Zener diode exhibits almost the same properties, except the device is especially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a 3.2-volt Zener diode will exhibit a voltage drop of 3.2 volts if reverse biased. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.
The breakdown voltage can be controlled quite accurately in the doping process. Tolerances to within 0.05% are available, though the most widely used tolerances are 5% and 10%.
The effect was discovered by the American physicist Clarence Melvin Zener.
Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way, and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.
In a 5.6-volt diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6-volt diode is the part of choice in temperature-critical applications.
Modern manufacturing techniques have produced devices with voltages lower than 5.6 volts with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75-volt diode has 10 times the coefficient of a 12-volt diode.
All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of 'zener diode'.
Zener diodes are widely used in electronic circuits. Their commonest function is to regulate the voltage across a circuit. When connected in parallel with a variable voltage source so that it is reverse biased, a zener diode acts as a short circuit when the voltage reaches the diode's reverse breakdown voltage, and therefore limits the voltage to a known value. A zener diode used in this way is known as a shunt voltage regulator (shunt meaning connected in parallel, and voltage regulator being a class of circuit that produces a fixed voltage).
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