Zener breakdown occurs in heavily doped p-n junctions which have very narrow depletion layers.

• Because the layer is so thin, even a relatively small reverse voltage creates a very high electric field $(\sim 10^6\text{ V/m})$.
• This field is strong enough to pull electrons directly out of their covalent bonds in the valence band and transfer them to the conduction band.
• This process is called internal field emission or field ionization (a quantum tunneling effect).