Zener breakdown occurs in a $$p - n$$ junction having $$p$$ and $$n$$ both:

Zener breakdown is a phenomenon that occurs in heavily doped p-n junctions under reverse bias conditions.

When both the p-side and n-side are heavily doped, the depletion layer becomes very narrow because the high concentration of charge carriers on both sides confines the space charge region to a thin layer.

In this narrow depletion region, even a modest reverse bias voltage creates a very strong electric field (since field strength $$E = V/d$$, where $$d$$ is the width of the depletion layer). When this electric field is strong enough (typically around $$10^6$$ V/m), it can directly pull electrons out of the covalent bonds of the semiconductor atoms. This quantum mechanical tunnelling of electrons across the narrow depletion layer is called Zener breakdown.

This is distinct from avalanche breakdown, which occurs in lightly doped junctions with wider depletion layers, where charge carriers gain enough kinetic energy to ionise atoms through collisions.

The correct answer is that Zener breakdown occurs when both p and n sides are heavily doped and have a narrow depletion layer.