Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
A Zener diode is used in REVERSE bias, in the breakdown region. In forward bias it behaves just like an ordinary diode (conducts after about 0.7 V for silicon), so that is not its special use. Its special job of keeping voltage constant only happens in reverse breakdown, where the voltage stays fixed at Vz while the current changes.
A normal diode is usually destroyed if you push it into reverse breakdown. A Zener diode is heavily doped on both sides, so it is DESIGNED to operate in reverse breakdown without damage (as long as current is limited by a series resistor). So the difference is not the material but the doping level and the intended operating region.
Vz is the fixed reverse voltage at which the Zener diode suddenly starts conducting a large reverse current. Beyond this point the voltage across the diode stays almost constant at Vz even if the current rises. Vz is set during manufacturing by choosing the doping level, and common values range from about 2 V to 200 V.
Zener breakdown happens in HEAVILY doped diodes with a thin depletion layer: the strong electric field directly pulls electrons out of bonds. It occurs at low voltage (below about 6 V) and Vz DECREASES as temperature rises. Avalanche breakdown happens in lightly doped diodes with a wide depletion layer, at higher voltage, where fast electrons knock out more electrons in a chain, and Vz INCREASES with temperature.
Heavy doping makes the depletion region very THIN. A thin depletion region means a very strong electric field forms even at a small reverse voltage. This strong field can directly break covalent bonds (Zener breakdown) at a low, well-defined voltage, which is exactly what we want for a stable reference voltage.
No, not by breakdown itself. Breakdown is reversible for a Zener as long as the current is kept within safe limits using a series resistor. Damage only happens if the power (Vz times current) exceeds the diode rating and it overheats. So the series resistor is essential in every Zener circuit.
An ideal Zener diode with breakdown voltage of -3 V is reverse biased with a negative input voltage Vi = -5 V. In the circuit shown (C-[Zener]-B-R-A), the magnitude of the voltage difference between points B and A is:
Statement I: Photovoltaic devices can convert optical radiation into electricity. Statement II: Zener diode is designed to operate under reverse bias in breakdown region. Choose the most appropriate answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a special p-n junction diode that can safely work in reverse breakdown and keep a nearly constant voltage (Vz) across itself, even when the current through it changes.
It is the normal diode triangle pointing at a straight bar, but the bar has small bends at its ends forming a Z shape (like the letter Z). This Z-bar tells you it is a Zener diode, not an ordinary one.
In the reverse breakdown region. Here the reverse voltage stays fixed at Vz while the reverse current can change a lot, which lets it act as a voltage reference or regulator.
The series resistor limits the current in the breakdown region. Without it, the current could rise too high, the power would exceed the rating, and the diode would overheat and be destroyed.
Its main use is as a voltage regulator, keeping the output voltage steady even when the input voltage or the load changes. This is covered in the next topic, Zener diode as a voltage regulator.
