V-I Characteristics of Zener Diode

In FORWARD bias, a Zener diode behaves just like an ordinary p-n junction diode, starting to conduct at roughly 0.7 V. Its distinctive behaviour is in REVERSE bias: as the reverse voltage is increased up to the Zener breakdown voltage $V_Z$, the reverse current stays as small as an ordinary diode's leakage current, but once $V_Z$ is reached the current rises extremely sharply while the voltage across the diode stays almost perfectly CONSTANT at $V_Z$ throughout the breakdown region -- this near-vertical, constant-voltage segment of the curve is exactly what makes the Zener diode useful as a voltage reference/regulator (9.3.6.2). Key parameters read off the reverse characteristic are: $V_Z$, the Zener breakdown voltage; $I_{Z(min)}$, the minimum reverse current needed to SUSTAIN breakdown (below this the diode drops out of regulation); and $I_{Z(max)}$, the maximum current the diode can carry, limited by its maximum power dissipation rating -- exceeding $I_{Z(max)}$ damages the diode. In practice the reverse characteristic is not perfectly vertical, meaning the diode has a small internal resistance, the Zener dynamic (or Zener) impedance -- the inverse of the curve's slope in the breakdown region -- but for an IDEAL Zener diode this resista …