A zener diode is fabricated by heavily doping both the p- and n-sides of the junction. Explain why. Briefly explain the use of a zener diode as a dc voltage regulator with the help of a circuit diagram.
Answer: Heavy doping gives a thin junction and sharp low-voltage breakdown; constant Zener voltage regulates the output.
Solution
Why heavy doping: heavy doping of both sides makes the depletion region very thin (less than about 10−6m). Even a small reverse voltage then produces a very strong electric field (∼5×106V/m) across it, which causes a sharp Zener breakdown at a well-defined low reverse voltage. As a voltage regulator: the Zener diode is connected in reverse bias across the load, in series with a resistance Rs (see circuit). It is operated in its breakdown region, where the voltage across it stays essentially constant even though the current through it changes. Any change in the input voltage appears as a changed drop across Rs, while the output voltage across the load (equal to the Zener voltage) remains constant — giving a regulated dc output.
