ZENER DIODE: It is a two terminal semiconductor junction device with a very sharp voltage breakdown at reverse bias applied. The device is utilized to give a voltage reference. It is named after C. Zener, who first proposed electronic tunnelling as a mechanism of electrical breakdown in insulators. A Zener diode is a type diode that permits currents to flow in the forward direction like a normal diode, but also in the reverse direction in the voltage is larger than the rated breakdown voltage or “Zener Voltage”. It is sometimes called the breakdown diode. The breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating temperature and current capability. The knee point is also sharper in case of a silicon diode. A zero diode exhibits almost the same properties as an ordinary diode except the device is especially designed so as to have a greatly reduced breakdown voltage, the so called Zener voltage. A Zener diode contains a heavily doped P-N junction allowing electrons to tunnel from the valance band of the P-type material to the conduction band of the N-type material. A reverse biased Zener diode will exhibits a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage of 3.2v will exhibits a voltage drop of 3.2v if reversed biased. However the current is not limitless, thus the Zener diode is characteristically used to produce a reference voltage for an amplifier stage, or as a voltage stabilizer for low current applications. The breakdown voltage can be controlled rather precisely in the doping process. Another mechanism that generates a like effect is the avalanche effect as in the avalanche diode. The two types of diodes in the fact constructed the same way and both effects are present in diode of this type. In silicon diodes up to approximate 5.6v the Zener effect is the predominant effect and shows a marked negative temperature coefficient. At above 5.6v, the avalanche predetermined and exhibits a positive temperature coefficient. In a 5.6v diode the two effects occur together and then temperature coefficient nearly cancel each other out, thus 5.6v diode is the part of choice in temperature critical applications.
APPLICATIONS: Zener diode has numerous applications in transistor circuitry. Some of their general applications are:
(1) Zener diode used as a Meter protection
(2) Zener diode used as a peak clipper
(3) Zener diode used as a reference element
(4) Zener diode in switching operation has made possible an externally fast performance in computer applications.
The fundamental parameters of a Zener diode are:
The most ordinary range of Zener voltage is 3.3v to 75v. A tolerance of the specified voltage must be started. While the most popular tolerances are 5% and 10% more precision tolerances as low as 0.05% are available. The power managing capability should be specified for the Zener diode popular power ranges are 1/4, 1/2, 1, 5,10 and 50 watts.