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Zener diodes 
In 1934, Clarence Zener calculated the electrical breakdown of an insulator when a large electric field is applied.1 He found that there was a sudden increase in the current when a critical field is reached. At the breakdown field, electrons tunnel from the valence band into the conduction band. Zener did not do the calculation for a pn junction but the conditions for Zener tunneling arise in a reverse-biased pn junction if both sides are heavily doped.
Another breakdown mechanism is avalanche breakdown where electrons in the conduction band are accelerated to high enough energies to scatter an electron from the valence band to the conduction band. Avalanche breakdown dominates in silicon for electric fields $E < 5 \times 10^5\text{ V/cm}$ while Zener tunneling dominates in silicon for electric fields $E > 10^6\text{ V/cm}$.2 For silicon pn-diodes there is a transition range of reverse bias voltages $4 - 7\text{ V}$ where both mechanisms occur. The breakdown voltage of the Zener effect has a negative temperature coefficient while the breakdown voltage of the avalanche effect has a positive temperature coefficient. Diodes marketed as Zener diodes are designed to exhibit both breakdown effects so that the temperature coefficients cancel each other out and the breakdown voltage is temperature-independent. Zener diodes made of silicon typically have a breakdown voltage of 5.6 V. They can be used to provide a 5.6 V reference voltage in a circuit.
- Clarence Zener, A theory of the electrical breakdown of solid dielectrics, Proceedings of the Royal Society of London,
Volume 145, Issue 855
Jul 1934
- R. B. Fair and H. W. Wivell, "Zener and avalanche breakdown in As-implanted low-voltage Si n-p junctions," in IEEE Transactions on Electron Devices, vol. 23, no. 5, pp. 512-518, May 1976, doi: 10.1109/T-ED.1976.18438.
- Zener Diode - Wikipedia
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