The forward-biased diode has a temperature coefficient of −2 mV/☌, causing the TCs to cancel out for a net nearly zero temperature coefficient. An alternative, which is used for voltage references that need to be highly stable over long periods of time, is to use a Zener diode with a temperature coefficient (TC) of +2 mV/☌ (breakdown voltage 6.2–6.3 V) connected in series with a forward-biased silicon diode (or a transistor B-E junction) manufactured on the same chip. In a 5.6 V diode, the two effects occur together, and their temperature coefficients nearly cancel each other out, thus the 5.6 V diode is useful in temperature-critical applications. Above 5.6 volts, the avalanche effect dominates and exhibits a positive temperature coefficient. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. The two types of diode are in fact constructed in a similar way and both effects are present in diodes of this type. Īnother mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. for an amplifier stage), or as a voltage stabilizer for low-current applications. The Zener diode is therefore well suited for applications such as the generation of a reference voltage (e.g. For example, a diode with a Zener breakdown voltage of 3.2 V exhibits a voltage drop of very nearly 3.2 V across a wide range of reverse currents. By contrast with the conventional device, a reverse-biased Zener diode exhibits a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the Zener breakdown voltage. A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a reduced breakdown voltage, the so-called Zener voltage. Unless this current is limited by external circuits, the diode may be permanently damaged due to overheating. When the reverse bias breakdown voltage is exceeded, a conventional diode will conduct a high current due to avalanche breakdown. Temperature coefficient of Zener voltage against nominal Zener voltage.Ī conventional solid-state diode allows significant current if it is reverse-biased above its reverse breakdown voltage. Operation Current-voltage characteristic of a Zener diode with a breakdown voltage of 3.4 V. Later, his work led to the Bell Labs implementation of the effect in form of an electronic device, the Zener diode. The device is named after American physicist Clarence Zener who first described the Zener effect in 1934 in his primarily theoretical studies of breakdown of electrical insulator properties. They are also used to protect circuits from overvoltage, especially electrostatic discharge. They are used to generate low-power stabilized supply rails from a higher voltage and to provide reference voltages for circuits, especially stabilized power supplies. Both breakdown types are present in Zener diodes with the Zener effect predominating at lower voltages and avalanche breakdown at higher voltages. Diodes with a higher Zener voltage have lighter doped junctions which causes their mode of operation to involve avalanche breakdown. Some Zener diodes have an abrupt, heavily doped p–n junction with a low Zener voltage, in which case the reverse conduction occurs due to electron quantum tunnelling in the short distance between p and n regions − this is known as the Zener effect, after Clarence Zener. Zener diodes are manufactured with a great variety of Zener voltages and some are even variable. A Zener diode is a special type of diode designed to reliably allow current to flow "backwards" (inverted polarity) when a certain set reverse voltage, known as the Zener voltage, is reached.
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