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 6.2 volt Zener diode will exhibit a voltage drop of 6.2 volt 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.
The breakdown voltage can be controlled quite accurately in the doping process. Tolerances up to 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 type of diode designed to break down under reverse bias, by a different mechanism to the Zener diode, is the avalanche diode.
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