Zener breakdown and Zener regulator

Zener breakdown

When a heavily doped p-n junction is reverse biased, the depletion region is very narrow (below about 10 nm). At a reverse voltage Vz (below about 5 V) the electric field across it (about 3 × 10⁷ V/m) is strong enough to pull valence electrons directly out of their covalent bonds (field emission). This produces a large number of carriers, and the reverse current rises sharply while the voltage across the diode stays almost constant at Vz. This is Zener breakdown. (Above about 6 V the avalanche effect takes over, but diodes using either effect are called "Zener diodes".)

V-I characteristic

                    I (forward)
                     ^
                     |       /
                     |      /   forward region
                     |     /    (conducts above about 0.7 V)
    -Vz              |    /
 ----+---------------+---+-----------> V
     |               0  0.7 V
     |   reverse: only a tiny leakage
     |   current until -Vz is reached
     |
     |   breakdown region: the current
     |   rises steeply while the
     |   voltage stays at Vz
     v
  I (reverse)

• Forward bias: it behaves like a normal diode (conducts above about 0.7 V).
• Reverse bias (up to Vz): only a very small leakage current flows.
• At Vz the current rises sharply along an almost vertical line (the dynamic resistance rz is a few ohms). The current must be limited by a series resistor and must not exceed Iz(max) = Pz(max)/Vz.

Zener as a voltage regulator (shunt regulator)

 Vin o----[ Rs ]-----+-------+----o Vo = Vz
                     |       |
                   (Dz)     [RL]
                     |       |
                    GND     GND

The Zener is reverse biased and connected in parallel with the load, with a series resistance Rs.

• Line regulation: If Vin rises, the current through Rs increases. The extra current is taken by the Zener (Iz rises), and the drop across Rs increases by the same amount as the increase in Vin, so Vo = Vz stays almost constant.
• Load regulation: If the load current IL increases, the Zener current Iz decreases by the same amount, so Is = Iz + IL stays constant and Vo remains nearly constant.

Design condition: Rs = (Vin(min) − Vz)/(Iz(min) + IL(max)); Iz(min) must be at least the knee current Izk, and the Zener power (Vz Iz(max)) must not exceed its rating.

Limits: it is only suitable for small currents (tens of mA), its efficiency is low, and the output changes slightly because of rz (ΔVo ≈ rz ΔIz).