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A zener diode is fabricated by heavily doping both p- and -sides of the junction. Explain why ? Breifly explain the use of zener diode as a d.c. voltage regulator with the help of a circuit diagram. |
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Answer» Solution :Zener diode is icated by heavily doping both p- andn- sides of the p-n JUNCTION. As a result, the depletion region formed is very thin, even less than `10^(6)` m, and consequently the electric field at the junction is extremely HIGH (of the order of `5 XX 10^(6)` V/m or even more) even for a small reverse bias voltage of about 5 V and breakdown voltage has a low value of about 5 V. A zener diode is used to GET a constant d.c. voltage from the unregulated filtered output of a full-wave rectifier. The circuit diagram is shown in . The unregulated d.c. voltage is connected to zener diode through a series resistance `R_(s)`, . Of course zener diode is connected in reverse bias arrangement. Output voltage is obtained across load resistor `R_(L)` which is joined in parallel to zener diode . If the zener voltage increases, the current through `R_(s)` and zener diode also increases. This increase the voltage drop across `R_(s)` but voltage drop across zener diode (or load `R_(I)`) remains constant. This is because in the breakdown region, zener voltage remains constant even though the current through the zener diode changes. Again if the input d.c. voltage decreases, current through `R_(s)` and zener diode decreases. The voltage drop across `R_(s)` decreases but voltage drop across zener diode (or load `R_(I)`) remains practically constant. Thus, any increase/decrease in the input voltage `V_(i)` results in an increase/decrease of the voltage drop across `R_(s)` without any change in voltage across the zener diode. Hence, the zener diode acts as a voltage regulator, in which the output d.c. voltage is constant at the zener (breakdown) voltage `V_(z)` of the zener diode. 
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