Zener Diode Interview Questions and Answers:

1. What is a zener diode?

Ans. Zener diode, also sometimes known as breakdown diode, is like an ordinary P-N junction diode except that it is properly doped so as to have a sharp breakdown voltage. It is specially designed for operation in its breakdown region.

2. Avalanche breakdown can occur at large reverse voltage whereas zener breakdown occurs at low voltage. Give reasons.

Ans. The Avalanche breakdown can occur at large reverse voltage whereas zener breakdown occurs at low voltage, the major reason for this is the difference of doping level of Avalanche and zener. The doping level of zener is high as compared to the doping level of Avalanche. Since, the depletion width is inversely proportional to the doping, the depletion width of zener is small as compared to the depletion width of the Avalanche.

Because of high doping in zener, the depletion width is smaller and the electric field, being inversely proportional to depletion width, is large. At a critical field strength, electrons participating in covalent bonds may be torn from the bonds by the field and accelerated to the N-side of the junction. The electric field required for such ionization is of the order of 106 V/cm.

For lightly doped junctions (Avalanche) electron tunneling is negligible, and instead, the breakdown mechanism involves the impact ionization of host atoms by energetic carriers.

3. What is Zener Voltage?

Ans. The voltage at which the zener diode breaks down is called the zener voltage.

4. What is meant by the temperature coefficient ?

Ans. The effect of temperature on zener voltage is given in terms of temperature coefficient which is defined as the percentage change in nominal zener voltage for each degree centigrade of change in junction temperature.

5. What happens to the series current, load current and zener current when the dc input voltage of a zener regulator increases?

Ans. Zener current and series current increase while the load current remains unchanged.

6. Why is zener diode used as voltage regulator?

Ans. Zener diode has the property of behaving like a de battery in `on’ state (i.e., when the voltage across zener diode exceeds its zener voltage rating Vz). In ‘on state’, the voltage across zener diode remains constant until the voltage across it drops less than Vz. This property of zener diode makes its use as voltage regulator.

7. Explain how zener diode maintains constant voltage across the load?

Ans. Zener diode has the property of behaving like a dc battery in `on’ state. If the zener diode is shunted across the load RL and the voltage across zener diode is more than zener voltage Vz then zener diode is in ‘on’ state, and any variation in voltage across the zener diode due to variations either in supply voltage or in load resistance is not able to change the output voltage. Thus zener diode maintains voltage constant across the load.

8. What is tunnel diode?

Ans. The tunnel diode (sometimes called the Esaki diode after its inventor, Dr. Leo Esaki) is a high conductivity, heavily doped two-terminal P-N junction. This diode differs from the normal P-N junction diode in the amount of impurity concentration. The impurity concentration in a normal P-N junction diode is about 1 part in 108, whereas in the tunnel diode the impurity concentration is about 1 part in 103. It can conduct in reverse as well as forward direction but is usually used in forward biased mode.

A tunnel diode’s characteristics exhibit a negative resistance region and it may be used as an ultra-fast switching device or in ultrahigh frequency oscillators.

9. What is tunneling?

Ans. The mechanism of conduction in a semiconductor diode in which charge carriers (possessing very little energy) punch through a barrier directly instead of climbing over it is called tunneling.

10. What are the applications of tunnel diodes?

Ans. Tunnel diodes are used as amplifiers, oscillators or switching devices, being an exclusive high-frequency component because of its very fast response of inputs.

11. What is PIN diode?

Ans. PIN diode is composed of three sections with a high resistivity intrinsic layer sandwiched between P and N regions. It offers a variable resistance (decreasing with the increase in forward current) in the forward bias mode and infinite resistance in reverse bias mode.

12. What is a varactor diode?

Ans. A varactor diode is a specially fabricated P-N junction with proper impurity concentration profile and operated under reverse-biased mode so as to give a variable junction capacitance.

13. Which device produces voltage variable capacitor? How the voltage variable capacitance varies with the change in voltage across it?

Ans. The varactor diode produces voltage variable capacitor. The junction capacitance of a varactor diode varies inversely as the square root of the reverse bias voltage in case of alloyed junction and varies inversely as the cube root of the reverse bias voltage for diffuse junction.

14. What is point contact diode?

Ans. Point contact diode consists of an N-type germanium or silicon (preferably germanium) wafer about 1.25 mm square by 0.5 mm thick, one face of which is soldered to a metal and the other face has a phosphor bronze or tungsten spring pressed against it. Because of very low capacitance, point contact diode is very much suitable for high frequency applications (of the order of 10 GHz).

15. What is step-recovery diode and why is it so called?

Ans. Step-recovery diode is a voltage-dependent variable capacitor diode with graded droping profile (concentration of charge carriers decreasing near the junction). Because of step or sudden recovery from the reverse current ON to reverse current OFF, it is called the step-recovery diode.

16. What is Schottky diode?

Ans. Schottky diode is quite different in construction from the normal P-N junction diode. It has metal (such as gold, silver, platinum, molybdenum, chrome or tungsten) on one side and N-type doped silicon on the other side of the junction. It has no storage charge. The junction barrier is called the Schottky barrier.

17. Why is Schottky diode called hot-carrier diode?

Ans. Since in forward bias operation of the Schottky diode, the electrons on the N-side gain enough energy to cross the junction and plunge into the metal with very large energy, they are usually called hot carriers and the diode is called the hot-carrier diode.

18. What is back diode?

Ans. Back diode is similar to a tunnel diode except that tunneling effect is large but only in the reverse direction. This is also called a unilateral diode.

19. What are power diodes?

Ans. The power diodes are similar to P-N junction signal or low-power diodes but have large power-, voltage- and current-handling capabilities than those of conventional P-N junction diodes. Power diodes find many applications in electronics and electrical engineering circuits.

20. Why the current in power diodes varies linearly rather than exponentially with voltage?

Ans. The large magnitude of current in power diodes leads to ohmic drop that hides the exponential part of the V-I characteristic curve.

21. What are thermistors?

Ans. The word thermistor is a combination of thermal and resistor. A thermistor is a resistor with definite thermal characteristics. Most thermistors have a negative temperature coefficient, but positive temperature coefficient devices are also available.

22. What are the applications of negative temperature coefficient (NTC) thermistors?

Ans. Thermistors are widely used for temperature compensation i.e., for cancelling the effect of temperature on other electronic devices. They are also used for measurement and control of temperature, liquid level, gas flow etc.

23. The temperature coefficient of resistance in thermistor is negative? Explain.

Ans. The reason of negative temperature coefficient of resistance of a semiconductor is that when its temperature is increased, the concentration of charge carriers increases resulting in a decrease in resistance.

24. Give the applications of thermistors.

Ans. The thermistors have the following applications:

  1. As a thermistor develops a large change in resistance with a small change in temperature, it has large sensitivity, and therefore, good accuracy and resolution. So these find wide use in temperature measurement.
  2. Thermistors are widely used in temperature control systems because thermal control systems are inherently sensitive. stable and fast acting and need relatively simple circuitry.
  3. Thermistors are widely used for temperature compensation e. for cancelling the effect of temperature on other electronic devices.

The other applications of thermistors are: (i) Measurement of thermal conductivity (ii) Measurement of composition of gases (iii) Measurement of power at high frequencies (iv) Vacuum measurement (v) Measurement of level, flow and pressure of liquids.

25. How does negative resistance develop in transit time devices?

Ans. Negative resistance develops due to the following two processes:

  • Avalanche delay-time taken for building-up of avalanche currents.
  • Transit time delay-time taken to cross the drift region. If the sum of above delay times is approximately one half cycle of the operating frequency, negative resistance occurs.

26. Name a few transit time devices.

Ans. Transit time devices include (i) IMPATT (ii) BARITT (iii) DOVETT and (iv) TRAPATT diodes.

27. What are the applications of transit time devices?

Ans. Transit time devices can convert dc to microwave signals with high efficiency and are very useful in the generation of microwave power for many applications.

28. What are Gunn diode?

Ans. The Gunn diodes are named after J.B. Gunn, who first demonstrated one of the forms of oscillation, are microwave devices which operate by the transferred electron mechanism. These devices are called diodes only because they are two-terminal devices, though no P-N junction is involved. However, the Gunn diode and other related devices use bulk instabilities which do not require junctions.

29. Why power dissipation is very important in Gunn devices?

Ans. Power dissipation is very important in Gunn devices because considerable heating (roughly 10,000 kW/cm3) and the increase in temperature results in change of device characteristics.