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From Wikipedia (the basis of my lectures):

Under a high reverse-bias voltage, the p-n junction's depletion region widens which leads to a high-strength electric field across the junction. Sufficiently strong electric fields enable tunneling of electrons across the depletion region of a semiconductor, leading to numerous free charge carriers.

I'd expect the widening of the depletion zone and the increasing potential barrier to reduce the chance of tunnelling, not to increase it. How is the opposite possible?

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  • $\begingroup$ maybe this will help hyperphysics.phy-astr.gsu.edu/hbase/Solids/zener.html $\endgroup$
    – anna v
    Commented Feb 23, 2021 at 13:13
  • $\begingroup$ @annav I am a bit confused by the article. In the last part (with the title "Tunnel Diode Characteristic") about tunnelling, the author considers the case of forward voltage, but as far as I know, the Zener effect is observed under reverse voltage. $\endgroup$
    – Filippo
    Commented Feb 23, 2021 at 16:21
  • $\begingroup$ The zener effect is not a tunneling effect in the sense that you could model it with a potential barrier, as far as I understand the article. on the Zenner, in contrast with the wiki article. lets hope a solid state expert decides to answer (wiki articles are not the last word, after all) $\endgroup$
    – anna v
    Commented Feb 23, 2021 at 17:00
  • $\begingroup$ @annav That's another point that I found confusing. The first part ("The Zener Effect") seems like an explanation of the avalanche effect, especially the following sentence: "electrons which break free under the influence of the applied electric field can be accelerated enough that they can knock loose other electrons and the subsequent collisions quickly become an avalanche" $\endgroup$
    – Filippo
    Commented Feb 23, 2021 at 17:04
  • $\begingroup$ @annav But I think that you are right that this has little to do with the tunnelling process one knows from introductory QM lectures/books. $\endgroup$
    – Filippo
    Commented Feb 23, 2021 at 17:15

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I don't think that the Wikipedia article is correct. The widening doesn't cause tunneling; the shifting of the bands does. Maybe this figure helps (also from wikipedia):

Band diagram for Zener tunneling

(Zener tunneling is the right-most subfigure.)

In fact, I've never heard of this widening. I guess that it could happen, but I've never seen it in any models of Zener tunneling, so I don't think widening is important if it does happen.

I should add that the hyperphysics link conflates the Zener effect and avalanche breakdown. The two are different things altho they have a similar effect and can happen in the same device. (In fact, many "zener" diodes that you can buy at electronics suppliers don't really rely on Zener tunneling; they use avalanche breakdown.) Zener tunneling is in fact quantum tunneling.

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    $\begingroup$ I'll edit the wikipedia article when I have a chance (which may be a while). The reference it cites is both no longer available, and the archived versions (available thru archive.org) don't say anything about widening. $\endgroup$
    – lnmaurer
    Commented Feb 23, 2021 at 17:52
  • $\begingroup$ Thank you! As far as I understand, the Zener effect is caused by electrons moving from the valence band on the p side to the conduction band on the n side (this makes sense, because this contributes to the drift current). However, in the right-most subfigure, the valence band on the p side is higher than the conduction band on the n side. In this case, I don't see why it would make sense to call this a tunneling process. $\endgroup$
    – Filippo
    Commented Feb 23, 2021 at 18:14
  • $\begingroup$ I wonder if it would be better to call the subfigure in the middle "unbiased", the left subfigure "reverse bias, Zener voltage" (the Zener voltage is the voltage where the Zener effect kicks in) and the right one "strong reverse bias". In this case, if we look at the subfigure in the middle, we can imagine raising the right half until the valence band on the p side is close enough to (but still below) the conduction band on the n side for the tunnelling to start. $\endgroup$
    – Filippo
    Commented Feb 23, 2021 at 18:14
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    $\begingroup$ It's tunneling because the band gap is a "forbidden" region; electrons should never be in the gap --- much like, classically, you'd never expect to see electrons in a potential barrier. In fact, you can model Zener tunneling quite well by treating it as tunneling thru a trapezoidal barrier. The figure in the middle is not unbiased because the Fermi level is not constant thru the structure. (A constant Fermi level is basically the definition of unbiased.) $\endgroup$
    – lnmaurer
    Commented Feb 23, 2021 at 19:25
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    $\begingroup$ Yes, exactly. You can even treat it as tunneling thru a rectangular barrier to keep things even simpler. If you want a reference, you can check out J.M. Ziman's "Principles of the Theory of Solids" 2nd edition section 6.8 (title "Zener breakdown: tunneling"). I'm sure some other solid state physics textbooks cover this, but you're more likely to find it in device physics books aimed at electrical engineers. $\endgroup$
    – lnmaurer
    Commented Feb 23, 2021 at 20:11

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