Quantum tunneling in zener diodes 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.

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?
 A: I’ll give an EE’s perspective from the RC properties from my experiences since 1975.
Zeners always have an exponentially continuous reducing resistance above knee voltage threshold that flattens to a fixed bulk electrode size dependent resistance as in all common diodes . ON semi quotes threshold resistance as Zzt and “on” knee resistance as Zzk which  is inversely proportional to the power rating (size).  The capacitance of diodes also increases with conductivity such that the conductive rise time is large relative to tunnelling effects.
Tunnelling effect diodes is similar to dielectric breakdown with negative incremental resistance after the triggering of conduction, whereby the dielectric capacitance is very small just before conduction and resistance quite high.
Although unlike the RdsOn FETs where the Rs drops as the C rises during the Vg(th) transition both inversely very rapidly as the conduction gap closes,  tunnelling does not rise in capacitance as fast so the transition appears as an incremental negative resistance near zero time (almost) It is so tiny that extremely fast DSO’s may be needed to capture it.
The tunnelling effect of an ESD arc to metal  with <100 pF with _ kV charge voltage has been captured in less than 10 ps, and Tunnelling type diodes found to approach this depend greatly on doping and geometry yet often cannot.
A: 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:

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. 
