why is zener diode called 'special fabricated' diode?explain its use in rectifier.
A diode especially designed to work only in the reverse break down region is called a Zener diode. It is a special purpose semiconductor diode, named after its inventor C. Zener. The voltage across such a diode remains constant.
Zener diode is fabricated by heavily doping both p, and n sides of the junction. Due to this, depletion region formed is very thin (< 10-6m) and the electric field of the junction is extremely high (~5106 V/m) even for a small reverse bias voltage of about 5V.
It is seen that when the applied reverse bias voltage (V) reaches the breakdown voltage (Vz) of the Zener diode, there is a large change in the current. Note that after the breakdown voltage Vz, a large change in the current can be produced by almost insignificant change in the reverse bias voltage.
In other words, Zener voltage remains constant, even though current through the Zener diode varies over a wide range. This property of the Zener diode is used for regulating supply voltages so that they are constant.
Hope this helps.