Zener diode

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Zener diode:

see diodediode
, two-terminal electronic device that permits current flow predominantly in only one direction. Most diodes are semiconductor devices; diode electron tubes are now used only for a few specialized applications.
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Zener diode

[′zē·nər ′dī‚ōd]
A semiconductor breakdown diode, usually constructed of silicon, in which reverse-voltage breakdown is based on the Zener effect.

Zener diode

A two-terminal semiconductor junction device with a very sharp voltage breakdown as reverse bias is applied. The device is used to provide a voltage reference. It is named after C. Zener, who first proposed electronic tunneling as a mechanism of electrical breakdown in insulators. See Semiconductor

A classic circuit to define a very stable current uses an operational amplifier and three stable resistors (see illustration). The voltage across the Zener itself defines a higher level from which the current is drawn. Thus, a stable noise-free Zener defines its own stable noise-free current. See Operational amplifier

The effect of temperature on the breakdown voltage can be nulled by having a second forward-biased junction, which has a small negative temperature coefficient, in series with the Zener junction. Such a device is called compensated Zener and has a breakdown voltage of 6.2 V rather than the normal 5.6 V (for the smallest possible temperature coefficient). Alternatively, a Zener junction can be part of an integrated circuit which adds a whole temperature controller to keep the silicon substrate at a constant temperature. For the very best performance, only four components are integrated into the silicon: the Zener, a heater resistor, a temperature-sensing transistor, and a current-sensing transistor. A separate selected dual operational amplifier then completes the current-and-temperature-control circuit. Such a circuit sets the chip temperature at, say 122°F (50°C), and the junction condition is then largely independent of ambient temperature. See Integrated circuits

The compact, robust Zener diode, with the circuits described to set its current and temperature, makes a fine portable voltage standard. This is used to disseminate the voltage level from national or accredited calibration laboratories to industry and to research laboratories. See Voltage measurement