Editing Zener diode (section)

==Construction==
The Zener diode's operation depends on the heavy [[Doping (semiconductor)|doping]] of its [[p–n junction]]. The depletion region formed in the diode is very thin (<&nbsp;1&nbsp;μm) and the electric field is consequently very high (about 500&nbsp;kV/m) even for a small reverse bias voltage of about 5&nbsp;V, allowing [[electron]]s to [[Quantum tunneling|tunnel]] from the valence band of the p-type material to the conduction band of the n-type material.

At the atomic scale, this tunneling corresponds to the transport of valence-band electrons into the empty conduction-band states, as a result of the reduced barrier between these bands and high electric fields that are induced due to the high levels of doping on both sides.<ref name=Dorf93/> The breakdown voltage can be controlled quite accurately by the doping process. Adding impurities, or doping, changes the behaviour of the semiconductor material in the diode. In the case of Zener diodes, this heavy doping creates a situation where the diode can operate in the breakdown region. While tolerances within 0.07% are available, commonly available tolerances are 5% and 10%. Breakdown voltage for commonly available Zener diodes can vary from 1.2&nbsp;V to 200&nbsp;V.

For diodes that are lightly doped, the breakdown is dominated by the avalanche effect rather than the Zener effect. Consequently, the breakdown voltage is higher (over 5.6&nbsp;V) for these devices.<ref>Rakesh Kumar Garg, Ashish Dixit, Pavan Yadav, ''Basic Electronics'', p. 150, Firewall Media, 2008 {{ISBN|8131803023}}.</ref>

===Surface Zeners===
The emitter–base junction of a bipolar [[NPN transistor]] behaves as a Zener diode, with breakdown voltage at about 6.8&nbsp;V for common bipolar processes and about 10&nbsp;V for lightly doped base regions in [[BiCMOS]] processes. Older processes with poor control of doping characteristics had the variation of Zener voltage up to ±1&nbsp;V, newer processes using ion implantation can achieve no more than ±0.25&nbsp;V. The NPN transistor structure can be employed as a ''surface Zener diode'', with collector and emitter connected together as its cathode and base region as anode. In this approach the base doping profile usually narrows towards the surface, creating a region with intensified electric field where the avalanche breakdown occurs. [[Hot carrier]]s produced by acceleration in the intense field can inject into the oxide layer above the junction and become trapped there. The accumulation of trapped charges can then cause 'Zener walkout', a corresponding change of the Zener voltage of the junction. The same effect can be achieved by [[radiation damage]].

The emitter–base Zener diodes can handle only low currents as the energy is dissipated in the base depletion region which is very small. Higher amounts of dissipated energy (higher current for longer time, or a short very high current spike) causes thermal damage to the junction and/or its contacts. Partial damage of the junction can shift its Zener voltage. Total destruction of the Zener junction by overheating it and causing migration of metallization across the junction ("spiking") can be used intentionally as a 'Zener zap' [[antifuse]].<ref>{{cite journal|doi=10.1155/1996/23706|title=Zener Zap Anti-Fuse Trim in VLSI Circuits |year=1996 |last1=Comer |first1=Donald T.|journal=VLSI Design|volume=5|page=89|doi-access=free}}</ref>

===Subsurface Zeners===
[[File:Buried zener structure-en.svg|thumb|Buried Zener structure]]
A subsurface Zener diode, also called a ''buried Zener'', is a device similar to the surface Zener, but the doping and design is such that the avalanche region is located deeper in the structure, typically several micrometers below the oxide. Hot carriers then lose energy by collisions with the semiconductor lattice before reaching the oxide layer and cannot be trapped there. The Zener walkout phenomenon therefore does not occur here, and the buried Zeners have stable voltage over their entire lifetime. Most buried Zeners have breakdown voltage of 5–7 volts. Several different junction structures are used.<ref>{{cite book|first=Alan |last=Hastings |title=The Art of Analog Layout |edition=Second |year=2005 |isbn=9780131464100 |publisher=Prentice Hall}}</ref>

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