IV.14 - Zener Diodes?

Every pn junction will break down in reverse bias if enough voltage is applied. A typical medium current discrete bipolar transistor has a collector-base junction which is doped fairly lightly (on the collector side) and will break down at a reasonably high voltage (perhaps 30V to 50V). This type of breakdown is called avalanche breakdown. It happens when thermally generated carriers in the depletion layer are accelerated by the electric field therein. If the field is high enough, the carriers are accelerated to high energies and they become capable of ionizing Si atoms in the depletion layer. The charge carriers from these secondary ionizations are in turn accelerated by the same electric field, and can cause additional chain-reaction ionization. The process resembles an avalanche (eg. on a snow covered mountain) hence the term. All junctions will exhibit avalance breakdown with sufficient reverse bias.

The emitter-base junction on the other hand, is generally very heavily doped. Before the field becomes high enough to cause avalanching, the junction will breakdown by another mechanism, called band-to-band tunneling, or Zener breakdown. In this case, the depletion layer is thin because of the much heavier doping levels. As the reverse bias is applied, a situation occurs where the conduction band on the n-side aligns and then drops below the valence band on the p-side. The exact voltage at which this is achieved depends upon the doping. When it happens, conditions are right for electrons to tunnel through the barrier. They suddenly appear on the other side of the junction, if the depletion layer is thin enough. This is called direct tunneling. There is another type of tunneling called indirect tunneling, or trap-assisted tunneling in which the electron tunnels into an intermediate trap level (or series of levels) before making it all the way throught the barrier. This one is less common in standard pn junction devices.

A "Zener diode" is made to break down at a specific voltage with a sharp reproducible characteristic. The diodes are designed to conduct the breakdown current evenly, and nondestructively. The breakdown mechanism may be avalanche breakdown or Zener breakdown, or a mixture of the two. If the diode breaks down at voltages of about 5.6V at room temperature, the two mechanisms are in equal measure. If the breakdown voltage is higher, the avalanche process dominates, and if lower, the tunneling or Zener mechanism dominates.

The temperature coefficient of the avalanche mechanism is positive, that is, at higher temperature, the avalanche breakdown voltage increases. The temperature coefficient of the tunneling breakdown is negative. At just the right doping level, the breakdown is a mixture of the two types in just the right proportion that the temperature coefficients cancel. This voltage is around 6 V.

For more, info, check out Microsemi's Zeners and Zero TC Reference Diodes application notes.

Practically everyone makes zener diodes, so I'll only list two:

• Motorola (many distributors)
  

  Just from personal experience, Motorola diodes tend to have sharper I-V knees than the competing generic diodes.

• Julie Research Laboratories, Inc., 508 W.26 Str., NY, NY. 10001
  Phone (212) 633-6625
  Fax (212) 691-3320
  

  Julie Research Labs makes ultra-stable zener diodes, with very low temperature coefficients. That typically means stabilities with orders of magnitude of roughly 1 ppm/day, 1 ppm/celcius.