What does the V-I characteristic of a Zener diode look like and what do its key points mean?

In one line

The Zener V-I characteristic is like a normal diode in forward bias, but in reverse bias the current stays almost zero until \( V_Z \), then rises steeply at an almost constant voltage between \( I_{Z(min)} \) and \( I_{Z(max)} \).

Explained simply

On a graph of current against voltage, the right side (forward) looks like any silicon diode: nothing much until about 0.7 V, then a steep rise. On the left (reverse), the line hugs zero until it hits a "wall" at \( V_Z \), then drops almost straight down. That nearly vertical portion is where the Zener diode is useful: a large change in current gives only a tiny change in voltage.

Technically, the reverse breakdown region is bounded by the knee current \( I_{Z(min)} \) (minimum current to stay in regulation) and \( I_{Z(max)} \) (set by maximum power). The slope in this region gives the dynamic (Zener) resistance \( r_Z \).

How it works

1. Forward region: cut-in voltage about 0.7 V for silicon; current then rises exponentially.
2. Reverse region before breakdown: only a very small reverse saturation (leakage) current flows.
3. Knee: at \( V_Z \) the curve bends sharply; \( I_{Z(min)} \) (also called \( I_{ZK} \)) is the minimum current for stable regulation.
4. Breakdown region: nearly vertical; voltage stays close to \( V_Z \) over a wide current range.
5. Maximum: beyond \( I_{Z(max)} = P_{Z(max)}/V_Z \) the diode overheats and can be destroyed.
6. The test circuit uses a variable DC supply, series resistor, voltmeter across the diode and milliammeter in series, first in forward then in reverse bias.

Key formula

\( r_Z = \dfrac{\Delta V_Z}{\Delta I_Z} \)

\( r_Z \) = dynamic (Zener) resistance (\( \Omega \)), small, typically a few ohms to tens of ohms; \( \Delta V_Z \) = small change in Zener voltage (V); \( \Delta I_Z \) = corresponding change in Zener current (A). A smaller \( r_Z \) means a better regulator.

Quick example

For a typical 5.1 V Zener, changing the reverse current from 10 mA to 30 mA changes the voltage only slightly (by a fraction of a volt, set by \( r_Z \)). This flat behaviour is exactly what a voltage reference needs.

Common mistakes

• Drawing the breakdown region as gradually sloping like a resistor.
• Forgetting to mark \( I_{Z(min)} \) and \( I_{Z(max)} \) on the graph.
• Drawing the reverse current axis in the same scale and direction as forward current without marking quadrants.

How MU asks it

Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Draw and explain the V-I characteristics of a Zener diode" (5 marks), often combined with the regulator question. For full marks draw both quadrants clearly (forward 0.7 V knee, reverse breakdown at \( V_Z \)), label \( V_Z \), \( I_{Z(min)} \), \( I_{Z(max)} \), leakage current, and explain each region in a line, with the test circuit.