Zener (a.ka. avalanche or breakdown) diodes are p-n junction semiconductor devices designed to operate in the reverse breakdown region of their VI characteristic curve. By maintaining their reverse current within certain limits, the voltage drop across the diode will remain constant. If maintained in this mode of operation, the diode will act as a voltage reference.
Examine the spec sheet for a typically avalanche diode – Vz is the Zener voltage measured at a given test current Izt. The knee current Izk, is the minimum current through the device to maintain a constant Vz. Since the dynamic resistance at the knee of the of the response curve is high, if current through the device is below Izk, regulation will be poor. Izm is the maximum Zener current that can be passed without exceeding the maximum permissible power dissipation. Violate this and the device will turn into smoke before your eyes.
For operation as a voltage reference, a Zener Diodes must be reverse biased. When the reverse voltage is smaller than Vz only the normal diode reverse saturation current is allowed to flow. When forward biased it behaves like an ordinary silicon diode – a large forward current flows, and the forward diode voltage is typically 0.7 V. Avalanche diodes are available with maintaining voltages from several voltages to several hundreds volts and with power dissipation ratings up to 50 Watts.
The temperature sensitivity of a Zener Diodes (temperature coefficient) is given as a percentage change in reference voltage per centigrade degree change in diode temperature. This number is usually in the range of +/- 0.1 percent per deg C. The direction of the change is related to the mechanism of breakdown (avalanche multiplication versus Zener breakdown). Generally, if the reference voltage is above 6V the coefficient is positive, if below, negative. Some manufacturers have produced temperature compensated reference diodes by combining a positive temperature coefficient breakdown diode with a forward biased, negative temperature coefficient, silicon diode in a single package (i.e. the generic 1N829, a 6.2V reference diode with a temperature coefficient of +/- 0.0005 % per deg C over a range of -55 to +100 deg C). Rather than use a single larger diode it is often better to place multiple breakdown diodes in series when designing a high voltage reference. This combination allows higher voltage, higher power dissipation, lower temperature coefficient, and lower dynamic resistance (the reciprocal slope of the volt-amp curve in the operating region). Of course, this is a more costly solution than a single diode.