Zener diode

A Zener diode is a Diode engineered to undergo Reverse breakdown at a precisely controlled voltage $V_Z$ and to operate safely and reliably in that region. While in breakdown its terminal voltage is almost independent of the current through it: change the current by a factor of ten and the voltage moves only a few percent. That near-constant voltage is the property exploited by the Zener voltage regulator and the diode Voltage reference.

How it is used

A Zener is always operated reverse biased — the breakdown polarity is the working polarity, so its symbol and orientation are drawn so that the operating current flows cathode-to-anode (the opposite of an ordinary diode). It needs a series current-limiting resistor; the resistor sets the current and keeps the $V_Z\times I$ power dissipation within the safe limit. Below the breakdown voltage it behaves like an ordinary reverse-biased diode (open circuit); above it, it clamps to $V_Z$.

Which mechanism, and why ~5–6 V is special

Breakdown happens by one of two physical mechanisms (detailed in Reverse breakdown): the Zener effect — field-driven tunnelling — dominates below about $5\text{V}$, while avalanche multiplication dominates above about $6\text{V}$. The two have opposite temperature coefficients: the Zener effect gives $V_Z$ a negative tempco, avalanche a positive one. Around $5$–$6\text{V}$ they roughly cancel, so a Zener rated there drifts least with temperature — which is why precision references cluster near $\approx 5.6\text{V}$, not because that voltage is otherwise special. (Despite the name, most “Zener” diodes above ~6 V actually break down by avalanche.)

Datasheet parameters

Zener parameters: $V_{ZT}$ at test current $I_{ZT}$, knee $I_{ZK}$, power-limited max current; linearised model $V_{Z0}+r_z$.

• $V_{ZT}$ — the rated breakdown voltage, specified at a particular test current $I_{ZT}$ (because the voltage does drift slightly with current, you have to say where you measured it). Commercial Zeners span roughly $2\text{V}$ to $200\text{V}$; the most common parts sit in the few-volt to tens-of-volts range.
• $I_{ZT}$ — the test current at which $V_{ZT}$ is quoted.
• $I_{ZK}$ — the knee current: the current at the onset of breakdown, the bottom of the curve where the breakdown bend is. Below $I_{ZK}$ the device is not properly in breakdown and the regulation is poor, so a good design keeps the current above $I_{ZK}$.
• Maximum current — set by the device’s safe power dissipation. For a 0.5 W, 6.8 V Zener the maximum current is about $P_{max}/V_Z=0.5\text{W}/6.8\text{V}\approx 70$ mA. Exceed it and the device overheats and fails.

Linearised model

For hand analysis the breakdown region is modelled as a straight line: an ideal source $V_{Z0}$ in series with a small dynamic resistance $r_z$. $V_{Z0}$ is the voltage-axis intercept you get by extrapolating the steep breakdown line back to zero current — it is not the same as $V_{ZT}$, which is the voltage at a real operating current. $r_z$ is the Zener dynamic resistance, the slope of the breakdown line: $V_Z=V_{Z0}+I_Z{r}_z$. A small $r_z$ means the line is nearly vertical, so the voltage barely changes with current — that is what makes a good regulator.

In this course the Zener in breakdown is usually treated simply by its DC value $V_Z$; the $r_z$ refinement is brought in only when you need to predict how much the regulated voltage moves under load. The Zener’s role as a voltage source feeds directly into the Zener voltage regulator and the broader Voltage reference.