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The Zener Effect
An American physicist, Clarence Melvin Zener, described how electrical insulators breakdown (1934). His work went on to aid Bell Labs in the development of the first Zener Diode. A Zener diode has a stable reverse breakdown voltage over a large range of currents. The phenomena responsible for this behavior is called quantum tunneling.
Quantum tunneling refers to a situation where a particle tunnels through a barrier that is could not get passed using classical or Newtonian mechanics. Often this is explained using the Heisenberg uncertainty principle and the wave/particle nature of matter.
Caution! Avalanche danger
For Zener diodes below 5.6V the main effect is quantum tunneling as described above, but above 5.6 V the main effect is due to avalanche breakdown. In a semiconductor, an electron that is bound to an atom can become free due to a thermal fluctuations and then it is subject to acceleration by an electric field. Normally the electron is swept across the PN junction where it recombines with a hole (atom needing an electron). However, under large electric fields, the electron may accelerate to high enough speeds to knock other electrons free. These free electrons repeat the cycle, hence the term avalanche.
A reverse-biased diode is said to be stable (with respect to temperature) when its voltage to current ratio does not change with changes in temperature. In silicon diodes below 5.6V the temperature coefficient is negative. That is the current will decrease with increases in temperature. However, above 5.6 V, where the avalanche phenomenon dominates, the diode has a positive temperature coefficient. That is, the current will increase with increases in temperature. At 5.6V the two effects occur in roughly the same proportions and they cancel each other out.
You might think that Zener diodes are yesterday’s technology. There are far more accurate voltage references today. And yet they continue to dominate the shunt regulator market. The buried Zener diode reference provides a high-performance level and is adequate for many applications. Zener diodes are also used as surge protectors to limit voltage spikes. They can be used to lower or shift a voltage. Another use is as a waveform clipper. This both reshapes the waveform and protects the power supply from voltage spikes (positive and negative spikes).
Sometimes Random is good
One of the more interesting uses of a Zener diode is as a random number generator. More specifically, the avalanche effect causes random spikes in the voltage of the Zener around its operating point. These spikes can be converted into a digital signal and used to represent a number of any bit length.
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