Zener Diode Questions and Answers for Viva

Answer-1: Zener diode is a P-N junction diode specially designed to operate in the reverse biased mode. It is acting as normal diode while forward biasing. It has a particular voltage known as break down voltage or "Zener knee voltage" or "Zener voltage", at which the diode break downs while reverse biased.

Answer-2: The device was named after Clarence Zener, who discovered this electrical property.

Answer-3: The basic principle of zener diode is the zener breakdown. When a diode is heavily doped, it?s depletion region will be narrow. When a high reverse voltage is applied across the junction, there will be very strong electric field at the junction. And the electron hole pair generation takes place. Thus heavy current flows. This is known as Zener break down. In reverse biased mode, after the break down of junction current through diode increases sharply. But the voltage across it remains constant.

Answer-4: The Zener diode's operation depends on the heavy doping of its p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material, as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of doping on both sides. The breakdown voltage can be controlled quite accurately in the doping process.

Answer-5: The Zener diode is therefore ideal for applications such as the generation of a reference voltage (e.g. for an amplifier stage), or as a voltage stabilizer for low-current applications. Zener diodes are also used in surge protectors to limit transient voltage spikes.

Answer-6: The V-I characteristics is similar as simple diode except zener has low breakdown voltage and after breakdown, voltage across it remains constant (current increases sharply).

Answer-7: Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across small circuits. When connected in parallel with a variable voltage source so that it is reverse biased, a Zener diode conducts when the voltage reaches the diode's reverse breakdown voltage. The intrinsic voltage drop of diode is stable over a wide current range and holds output voltage relatively constant even though the input voltage may fluctuate over a fairly wide range. Resistor R is used to limit current through the circuit.

Answer-8: Shunt regulators are simple, but the requirements that the resistor be small enough to avoid excessive voltage drop during worst-case operation (low input voltage concurrent with high load current) tends to leave a lot of current flowing in the diode much of the time, making for a fairly wasteful regulator with high quiescent power dissipation, only suitable for smaller loads.

Answer-9: In reverse mode (positive lead of multimeter to the cathode and negative lead to the anode), the multimeter must be in voltage mode. Now slowly increase the output of variable power supply and at the same time observes the voltage shown in the multimeter. The multimeter display increases along with the increase in power supply voltage until the breakdown voltage. Beyond that the multimeter reading stays put despite of the power supply voltage. This is because the Zener diode is now in breakdown region and the voltage across it will remain constant irrespective of the increase in supply voltage and this constant voltage will be equal to the breakdown voltage. If the reading of multimeter in this instant is equal to the breakdown voltage specified by the manufacturer, we can assume that the Zener diode is healthy.

Answer-10: Zener diode is highly dopped & conduct in reveres bias. In the case of normal diodes the diode damages at the break down voltage. But zener diode is specially designed to operate in the reverse breakdown region and have a greatly reduced breakdown voltage.