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Well Zener diodes operate as a voltage regulator because of the Zener effect-Quantum tunneling . Quantum tunneling is the effect when an electron faces an energy barrier but after the energy barrier there is a energy downhill and it has a propability of "passing through " the energy hill.

enter image description here

But as far as I can understand the energy barrier in a zener diode is bigger than the energy downhill so the electron must be excited to even have the propability of passing through the energy hill . Do only excited electrons can move through the energy hill and become mobile charge carriers?

What am I missing?

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I think you are confused by the concept of tunneling. Suppose that the potential barrier an electron faces is greater than the energy of the electron. Classically, we would not find the electron behind the potential barrier, but when the electron tunnels, then the electron can pass the barrier even when its energy is too low in a classical sense. Therefore, even electrons which are not excited have a nonzero probability to be beyond the barrier. I'll include an image which, for me, clarifies tunneling by displaying the wave function of the electron:

Barrier

The second graph represents the case in which the electron has a reasonable chance to be found beyond the barrier. I hope this resolves your question.

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  • $\begingroup$ Yes but electrons only tunnel to same energy levels:) $\endgroup$ – Altair Mar 15 at 13:46
  • $\begingroup$ The downhill is less tall than the barrier $\endgroup$ – Altair Mar 15 at 13:46
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    $\begingroup$ Yes, as your image shows, the energy of the electron (or the rock in that analogy) stays the same. It has nothing to do with the barrier itself. The shape/width of the barrier only affects the probabilty of tunneling. So even electrons which are not excited can tunnel. $\endgroup$ – Thibeau Wouters Mar 15 at 13:53
  • $\begingroup$ Yes but my question says that the barrier height is bigger than the downhill height so an electron which will have to cross the whole barrier wont be at the bottom of the downhill $\endgroup$ – Altair Mar 15 at 13:58
  • $\begingroup$ Do you mean the energy of the electron (or rock in your image) when you say downhill height? Because as you pointed out yourself, the energy of the electron is unchanged by crossing the barrier. The barrier height does not change its energy, it only affects the probability of tunneling. $\endgroup$ – Thibeau Wouters Mar 15 at 14:01

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