Q.How is a Zener diode different than an ordinary diode?

[!TLDR] A Zener diode is heavily doped so it can safely and repeatedly enter reverse breakdown at a fixed voltage $V_Z$ without damage, whereas an ordinary diode is destroyed if driven into breakdown. [!ANSWER] A Zener diode is designed (via heavy doping and a narrow depletion layer) to be deliberately and safely operated in reverse breakdown at a constant voltage $V_Z$; an ordinary diode is designed only for forward-bias use and is damaged if reverse breakdown occurs.

An ordinary p-n junction diode and a Zener diode are both p-n junctions, and both show a sharp reverse breakdown if the reverse voltage across them is increased far enough -- but they differ fundamentally in how that breakdown is meant to be used. An ORDINARY diode is normally operated only in forward bias (as a rectifier or switch); if its reverse voltage is pushed beyond its breakdown voltage, the resulting breakdown current is large and uncontrolled, and the diode is typically DAMAGED or destroyed. A ZENER diode, by contrast, is specifically manufactured with HEAVY doping on both sides of the junction, giving it a NARROW depletion layer; this is precisely what allows Zener (or, at higher voltages, avalanche) breakdown to occur at a well-defined, reproducible voltage $V_Z$, and to do so WITHOUT damaging the device -- the diode is engineered to operate safely and indefinitely in this reverse-breakdown regime. Functionally, this difference in intended use shows up in the I-V characteristic: past $V_Z$, a Zener diode's voltage stays essentially PINNED at $V_Z$ over a wide range of reverse currents, a controlled and useful property exploited for voltage regulation; an ordinary diode's reverse breakdown, by contrast, is simply a failure mode to be avoided by keeping the reverse voltage well below its (uncontrolled, destructive) breakdown value. The circuit symbols also differ -- the Zener diode's cathode bar is drawn bent/Z-shaped, distinguishing it visually from an ordinary diode's straight cathode bar. [!ANSWER] A Zener diode is designed (via heavy doping and a narrow depletion layer) to be deliberately and safely operated in reverse breakdown at a constant voltage $V_Z$; an ordinary diode is designed only for forward-bias use and is damaged if reverse breakdown occurs.

Contrast the doping/depletion-layer design and the INTENDED operating region of a Zener diode (safe, controlled reverse breakdown at $V_Z$) against an ordinary diode (forward-bias use only, damaged by reverse breakdown).

Describing only the I-V-curve difference (constant voltage in breakdown) without explaining WHY that difference exists -- it is the heavy doping and consequently narrow depletion layer that is the underlying physical cause, not an incidental detail.