Zener reverse breakdown as voltage reference

Reverse breakdown region of a Zener diode

A Zener diode is a heavily doped p-n junction designed to operate in the reverse breakdown region. For a small reverse voltage, only a tiny leakage current flows. When the reverse voltage reaches the Zener voltage Vz, the strong electric field in the thin depletion region breaks covalent bonds (the Zener effect, with avalanche multiplication at higher Vz). The reverse current then increases sharply. The diode stays safe as long as a series resistor limits the current so that Iz ≤ Iz(max) = Pz(max)/Vz.

V-I characteristic

                I ^
                  |          /
                  |         /  forward
       -Vz        |        /
  ------+---------+-------/-------> V
        |     leakage    0.7 V
        +-- Izk (knee)
        |
        |   breakdown region: almost
        |   vertical, V stays ≈ Vz
        v   Iz(max)

(In the reverse breakdown region the curve is almost vertical.)

Why the Zener acts as a voltage reference

1. In the breakdown region, the I-V curve is almost vertical. A large change in current ΔIz (from Izk to Iz(max)) causes only a very small change in voltage ΔVz = rz ΔIz, where the dynamic resistance rz = ΔV/ΔI is small (a few ohms to tens of ohms).
2. Hence the voltage across the diode stays almost constant at Vz, whatever the current through it, and so whatever the input voltage or the load.
3. Vz is fixed by the doping level (Zeners are available from 2.4 V to 200 V), so the diode provides a fixed reference voltage to compare against or to supply to a circuit.

Example: Vz = 6.8 V, rz = 20 Ω. If the current changes by 10 mA, the voltage changes by only 20 Ω × 10 mA = 0.2 V (3 %).

Therefore, when it is reverse biased with a series resistor, it holds its terminal voltage constant at Vz and acts as a voltage reference element (in shunt regulators, series regulators and clippers).