Zener diodes are semiconductor devices that allow current to flow in both directions but specialize in current flowing in reverse. Also known as breakdown diodes, these diodes are the most common electronic components used as stable voltage references for electronic circuits.
Current going across the terminals in reverse bias (backward) is called the Zener effect. When voltage potential is met, this causes the Zener voltage (Vz)/breakdown voltage. Zener diodes consist of a heavily doped P-N junction that allows current to flow in reverse when reaching Vz. A well-defined Vz can conduct current continuously in reverse bias without getting damaged. Current then increases to a maximum level determined by a series resistor and stabilizes, remaining constant over a range of applied voltages. Therefore, Zener diodes are applicable for use as voltage regulators.
A Zener diode operates within the normal range of forward bias, with a turn-on voltage between 0.3 V and 0.7 V. When connected in reverse bias, the current flows backward, thus causing a small leakage of current to flow. As the reverse voltage increases to the arranged Vz, current flows throughout the diode. Current increases to a maximum decided by the series resistor and then stabilizes to remain constant over a range of applied voltages.
An understanding of the two breakdown effects of Zener diodes helps with understanding their functionality. First is the Zener effect, prevalent in voltages below 5.5 V and involving a tunneling effect to cause the breakdown. The second is the avalanche breakdown or impact ionization, which occurs in voltages above 5.5 V.
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These breakdowns both result in the same behavior and do not require different circuitry, but each does have a different temperature coefficient. The Zener effect has an anode (negative) terminal temperature coefficient, while the avalanche has a cathode (positive) terminal temperature coefficient. Both temperature effects are almost equal to 5.5 V and cancel each other out, making Zener diodes rated at around 5.5 V and stable over a wide range of temperature requirements.
