Reverse breakdown; Zener as voltage regulator

Reverse breakdown is the sudden large increase in reverse current when the reverse bias voltage across a p-n junction diode reaches a critical value, the breakdown voltage VBR. Below VBR a reverse-biased diode carries only a very small leakage current (reverse saturation current). At VBR the current rises sharply while the voltage stays nearly constant. If the current is not limited by an external resistor, the heat produced destroys the diode.

              I ^
                |           /
                |          /  forward
      -VBR      |         /
  ------+-------+--------/-------> V
        |   small leakage  0.7 V
        |   (reverse saturation current)
        |
        |   steep rise in reverse
        v   current at VBR

There are two mechanisms.

1. Avalanche breakdown

• Occurs in lightly doped junctions with a wide depletion region, usually for VBR above about 6 V.
• The minority carriers are accelerated by the strong field and gain enough kinetic energy to knock electrons out of the atoms by collision (impact ionisation). The new carriers cause further collisions, a multiplying chain like an avalanche.
• Has a positive temperature coefficient (VBR increases with temperature).

2. Zener breakdown

• Occurs in heavily doped junctions with a very thin depletion region, for VBR below about 5 V.
• The electric field (about 3 × 10⁷ V/m) is strong enough to pull electrons directly out of the covalent bonds (field ionisation or tunnelling).
• Has a negative temperature coefficient. Around 5 to 6 V the two effects cancel, giving a diode with almost zero temperature coefficient.

Zener diode as a voltage 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 almost 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).