Zener Breakdown

Zener breakdown occurs in HEAVILY doped p-n junctions, which have very narrow depletion layers, of order $<10^{-6}$ m. Because the layer is so thin, even a moderate reverse voltage sets up an extremely strong electric field across it -- of order $3\times10^{7}\ \text{V m}^{-1}$ at the breakdown limit -- strong enough by itself to rupture covalent bonds directly in the lattice and generate electron-hole pairs; this direct field-driven bond-breaking is called the Zener effect, or internal field emission / field ionization. Once triggered, even a small further increase in reverse voltage produces a large number of new charge carriers, so the junction's resistance in the breakdown region becomes very low. The electric field needed for this mechanism is of order $10^{6}\ \text{V m}^{-1}$. As a rule of thumb, for a reverse voltage u …