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New for February 2006!
Zener diodes really have no use as a "microwave device", except maybe as a voltage reference or protection circuit, named after Clarence Melvin Zener. Note that the "Melvin" is a one form of the "wedgie". They are the poor-man's voltage regulator!
The schematic symbol for a Zener diode is shown below:
Zeners are made on silicon. In the forward direction (anode at a higher potential than cathode) a Zener diode behaves similar to a standard diode. (i.e., it conducts current with a typical 0.7V drop at 25C). When reverse-biased they also conduct when breakdown voltage is reached, and the I-V curve is remarkably vertical. The breakdown voltage vcan be set by the manufacturer to be anywhere from a fe volts up to 75 volts or more. Yes they can be used as a poor-man's voltage reference, but don't expect anything highly accurate. The reference voltage will vary with current, with temperature, and even somewhat between two devices with the same part number. In general, the higher the voltage value, the worse the temperature coefficient of voltage.
Zeners are also sometimes used as over-voltage protection devices. One thing to be aware of is that power dissipation is often a limiting design factor. 100mA through a forward-biased diode will cause ~70mW of dissipation. But 100mA through a 12V zener causes 1.2W of dissipation, and a physically large diode may be required.
Although yo won't read this anywhere else, we have heard engineers use "Zene" as the verb form of Zener. As in "crank up the curve tracer and see where that baby starts to Zene!"
Some advice: buy a linear regulator you cheapskate!