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Zener Diode Experiment Viva Questions

 

Zener Diode Experiment Viva Questions and Answers

 

1. What is a Zener diode?

 

A junction diode which can operate in the reverse breakdown voltage region is called a Zener diode.

 

2. Is Ohm's law obeyed in a semiconductor or not?

 

In a semiconductor, Ohm's law is valid only for low electric fields.

 

3. What is meant by a junction diode?

 

When a p-type semiconductor is fixed with n-type semiconductor, it forms a junction diode.

 

4. What is meant by forbidden gap?

 

The band separating the valance band and the conduction band is called forbidden gap.

 

5. What is meant by reverse current?

 

The reverse current is the current in the p-n junction diode circuit, when the junction is reverse biased.

 

6. What is meant by reverse breakdown?

 

Sudden increase in reverse current of a p-n junction diode when a definite reverse voltage is applied is called reverse breakdown.

 

7. What is the cause of reverse breakdown?

 

The cause of reverse breakdown will rupture a very large number of covalent bonds due to high reverse voltage applied.

 

8. What is zener current?

 

The value of reverse current after the zener breakdown is called zener current.


9. What is meant by the break down voltage of a diode


Materials are known as Conductors, Semiconductors, and Insulators, based on their electrical features. Materials that can easily conduct electricity are conductors. In comparison, insulators are categorised as materials that do not conduct any electricity. Among conductors and insulators lie the features of semiconductor materials. Researchers have observed during their work with insulators that when a certain amount of electricity is applied to them, insulator material can be made to act as a conductor. Breakdown was named for this phenomenon, and the minimum voltage at which this happens is known as Breakdown Voltage. For various materials, these voltage levels are distinct and often depend on their physical properties.


The main use of zener diodes in electronic circuits is they can be used as basic building blocks. In order to provide a reference voltage to the electronic circuits, zener diodes are used. Zener diodes works in the breakdown regions of the diode. 


Generally zener diodes are huge diodes, so that they can work in the reverse biased regions.  Due to the zener effect, the breakdown takes place. When the electric field of the reverse-biased P-N diode is increased in the Zener effect, tunnelling of the valence electrons into the conduction band occurs. This results in an improvement in the reverse current of the minority charge carriers. The Zener effect is known as this phenomenon, and the minimum voltage at which this phenomenon begins is known as the Zener voltage breakdown.


10. What are the differences between an ordinary diode and zener diode?


The main differences between ordinary diode and zener diodes are:


(i) The operation of diode is uni-directional, while the zener diode operates bi-directionally in both forward and reverse biased directions.


(ii) For an ordinary diode and a zener diode, the doping characteristics is also different. The conventional diode is moderately doped, while the zener diode is sharply doped.


(iii) In normal P-N Junction diodes, the breakdown voltage is high, but in case os zener diodes, the breakdown voltage is sharp.


(iv) It is not possible to operate a conventional diode in reverse biased mode, but it is possible to operate a zener diode in reverse biased mode.


(v) The main use of conventional diodes are they can be used in clipper, clamper, rectifiers, etc. The main use of zener diode is in voltage regulator circuits.


11. Distinguish between zener break down and avalanche break down?


Avalanche Breakdown:


(i) The condition of occurrence of Avalanche breakdown is when both sides of the PN junction are lightly doped and high depletion layer.


(ii) In Avalanche breakdown, it has a weak electric field across the depletion region.


(iii) Covalent bonds are broken due to the collision with valence electrons and electron-holes are created.


(iv) In Avalanche breakdown, from the applied potential,  the charge carriers acquires energy and produces  more carriers. This process is termed as Avalanche multiplication and the breakdown is termed as the avalanche breakdown.


Zener Breakdown


(i) The depletion layer is narrow and zener breakdown occurs when both sides of the PN junction are strongly doped.


(ii) Zener breakdown produces a a strong electric field by applying a small reverse bias voltage.


(iii) In Zener Breakdown, the covalent bonds are braked by the field, hence producing large number of electrons and holes.


(iv) The electrons and holes yields to the reverse saturation current (Zener current). This zener current and the applied voltage is independent.


12. What is the use of a zener diode


The main uses of zener diodes are:

(a) Zener diode can be used as constant - voltage diodes

(b) It can be used as voltage detection devices.#

(c) It provides voltage clipping


13. Give the Symbol of Zener Diode

14. Aim of Zener Diode Experiment

 

(i) To draw I—V characteristic of a zener diode under reverse biased condition.

(ii) To determine the zener breakdown voltage from the graph.

 

15. Apparatus of Zener Diode Experiment

 

A zener diode (ex: FZ 6.2 A), milliammeter, multimeter or dc voltmeter, dc regulated supply which can be varied from 0 to 10 V in steps of IV, 100 Ω resistor, etc.

 

16. Procedure of Zener Diode Experiment

Connections are made as shown in the figure. The type of zener diode is noted. Its breakdown voltage Vz, maximum current rating and maximum power rating are noted from the data book. (This helps us to fix the value of current limiting resistor R and the range of voltmeter or multimeter).


The regulated supply is switched on. The voltage V across the zener diode is measured between the points C and D by the multimeter and the zener current I is noted in the milliammeter. The zener voltage V is varied from zero in steps of, say, IV by varying the input voltage Vi of the regulated till the zener current I is about 50% of the maximum rated value of the zener current. In each step the zener current I is noted. The input voltage Vi is also measured in each step across the points A and B by the multimeter. Once the break down occurs zener voltage remains almost constant.

A graph is plotted between I and V in the third quadrant of the graph paper This gives the I—V characteristic of the zener diode. From the graph the breakdown voltage Vz of the zener diode is noted at any point on the steep portion of the graph. The dynamic resistance of the diode is also calculated at the point.

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