Conceptual Questions Based On Electronics Devices

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Q.1. The resistance of the pn junction is low when forward biased and is high when it is reverse biased. Explain.
Answer: The resistance of pn junction is low when it is forward biased because force acts on charge carriers to move them across the junction whereas the force in reverse biasing acts in opposite direction which
opposes the motion of charge carriers across the junction.

Q.2. How will you test whether a transistor is spoiled or in working order?
Answer: a working transistor has low resistance when it is forward biased whereas it has high resistance when
it is reverse biased. But if the transistor is spoiled the resistance is low for both forward and reverse
biasing.

Q.3. Would you prefer to use a transistor as common base or common emitter configuration.
Answer: We prefer to use the common emitter configuration of the transistor because the current gain is more in that configuration.

Q.4. Name the p-n junction diode which emit spontaneous radiation when forward biased. How do we choose the semiconductor, to be used in these diodes, if emitted radiation lies in visible region.
Answer: The p-n junction that emits radiation when forward biased is called photodiode. In case of gallium
arsenide the emitted radiations lies in the visible region.

Q.5. In a transistor forward bias is very small as compared to reverse bias. Explain why?
Answer: If the emitter voltage is large, then the number of charges drifting from emitter to collector through
base becomes very large. It results in lot of heat generation which can damage the transistor. But if
collector reverse bias is large charges may drift quickly to collector but there is small and no heating
effect is produced.

Q.6. What is the difference between the transistor as amplifier and the step up transformer?
Answer: The difference between the transistor as an amplifier and the transformer is that in the transistor
increase in voltage does not imply the decrease in current whereas in transformer if the magnitude of
the voltage increases there is a corresponding decrease in current.

Q.7. Why is the base of transistor of made thin in comparison to the emitter or collector region.
Answer: The base of transistor is made thin in comparison to the collector and the emitter region because the recombination in the base region should be small.

Q.8. What is the effect of temperature on the conductivity of a semiconductor?
Answer: The conductivity of the semiconductor increases with the increase in temperature because as the
temperature increases more and more covalent bonds break resulting in the release of charge particles.

Q.9. In a transistor base is very lightly doped, why?
Answer: In transistor the base is lightly doped so that the recombination in the base region should be small
and the magnitude of output current is large

Q.10. Why does the thickness of depletion region in a pn junction diode increases with increase in reverse
bias?

Answer: When a pn junction is formed, a small potential difference is set up across the depletion layer. But
when it reverse biased the charges move away from the junction thus increasing the width of the
depletion region.

Q.11. Explain why the transistor starts working immediately on switching on whereas vacuum tube circuits
take some time before they starts working?

Answer: Vacuum tube circuits are based on the heating effect of the current and thermionic emission takes
place from cathode when it is heated. Thus, it takes some time to start as cathode can’t be heated
instantly.

Q.12. Why is the depletion region formed at the pn junction?
Answer: The depletion region is formed because of the recombination of electrons and the holes at the junction

Q.13. What will happen if collector as well emitter are forward biased?
Answer: In this case transistor will work as two pn diodes and it can not work as amplifier or oscillator.

Q.14. What is Fermi level and what is Fermi energy?
Answer: Fermi energy level is the highest energy level occupied by the electrons at zero-kelvin and the energy of that level is fermi energy.

Q.15. When semiconductor junction diode is formed electrons should flow from n to p region but all the
electrons do not do so? Explain why?

Answer: When electrons are transferred from p type to n type, the n type semiconductor gets positively charged and p type gets negatively charged which creates potential difference across junction. This potential
difference after some time prevents the flow of electron from n type to p type.

Q.16. Is the number of free electrons and holes equal in extrinsic semiconductors. Are they charged?
Answer: No, the number of electrons and holes are not equal in extrinsic semiconductors. Both n type and p
type semiconductors are electrically neutral.

Q.17. Is the junction diode linear or a non linear circuit element?
Answer: Junction diode is a non linear circuit element because the V-I curve is not a straight line

Q.18. What will happen if both emitter and collector junction are reverse biased?
Answer: In this situation no current will flow in the semiconductor because majority carriers cannot move
across the emitter base or base collector junctions.

Q.19. How will you detect intensity of light using diode?
Answer: Light intensity is proportional to the intensity of current which can be measured by using a photodiode in reverse biasing.

Q.20. Why gallium arsenide solar cells are preferred over silicon solar cells?
Answer: Gallium arsenide solar cells are used because they can operate with visible energy whereas the
silicon diodes works with infrared energies.

Q.21. Define input and the output resistance of a transistor?
Answer: Input resistance of transistor is the ratio of change in base voltage to the change in base current at
constant collector voltage .

Q.22. What is the phase relation between the input and the output signal in an amplifier?
Answer: Input and output current are in phase in common base transistor amplifier and in transistor as common emitter the input and the output are out of phase by 1800

Q.23. What is zener breakdown and what is the zener breakdown voltage?
Answer: Zener breakdown takes place when a strong reverse bias electric field is applied across the
semiconductor. The covalent bonds in the structure break simultaneously resulting in large increase
in the magnitude of the current. The voltage at which zener breakdown occurs is zener breakdown
voltage

Q.24. What is the avalanche breakdown voltage of a junction diode?
Answer: When the reverse bias voltage is increased to a large value large number of covalent bonds break
near the junction resulting in large reverse currents flowing in the transistor. The voltage at which
breakdown occurs is avalanche breakdown voltage

Q.25. What will happen if the input circuit is reverse biased and the output circuit is forward biased?
Answer: In this case the collector starts acting as emitter and the emitter as collector with transistor functioning as usual.

Q.26. What type of charge carriers flow during reverse biasing of the diode?
Answer: In reverse biasing the charge carriers flowing in a semiconductor are minority charge carriers.

Q.27. Transistor is a temperature sensitive device. Explain.
Answer: Transistor is a temperature sensitive device because the number of charge carriers increases with the
increase in current but if the current increases to a very large magnitude it may damage the transistor.

Q.28. Why an extrinsic semiconductor gets permanently damaged if temperature is increased beyond a
certain limit?

Answer: If temperature is increased beyond a limit large number of covalent bonds break in the structure
resulting in release of charge carriers and making it highly conducting

Q.29. Explain why input resistance of transistor is low and the output resistance is high.
Answer: Input resistance of the transistor is low because input circuit is forward biased whereas the output
circuit is reverse biased.

Q.29. How can you increase the current gain in an amplifier circuit?
Answer: Current gain can be increased by decreasing the doping level in the base and keeping it thin.

Q.30. What is the potential barrier of pn junction in a silicon transistor?
Answer: Potential barrier for silicon transistor is of the order of 0.3V and for germanium transistor it is of the
order of 0.7V.

Q.31. For what particular application the common base configuration is preferred over the common emitter configuration?
Answer: Common base is preferred over common emitter if we want the voltage amplification without the
phase difference between input and the output.

Q.32. By increasing load resistance can we increase or not the gain of transistor indefinitely.
Answer: No, because increases RL will decrease the net output voltage. If the output voltage becomes less
than input then it cannot act as amplifier.

Q.33. In the depletion region of pn junction what are the charge carriers in its unbiased state?
Answer: In depletion region there are no charge carriers present

Q.34. What type of feedback is required in transistor as an oscillator?
Answer: In transistor as an oscillator positive feedback is required i.e. emf is induced in such a way that if current in output increases emf is induced in input to support the forward bias and if the current in output decreases emf is induced in input to oppose the forward bias.

Q.35. Can we measure the potential barrier of a pn junction by putting voltmeter across is?
Answer: In the depletion layer, there are no free charges present, thus it offers infinite resistance to the flow of current through it. Therefore potential barrier across a p-n junction cannot be measured using voltmeter.

Q.36. Fill in the blanks:
[1] When p-n junction if forward biased, then the motion of charges across the barrier is due to……..
and when it is reverse biased then the motion of charge carriers is due to……..
[2] An ideal pn junction diode conducts, when _ and does not conduct when ___
Answer: [1] diffusion, drift [2] forward biased, reverse biased

Q.37. State two disadvantages of semiconductor devices.
Answer: [1] The semiconductor devices can’t withstand high temperature and an get damaged
[2] the semiconductor devices gets damaged by power surge and can’t withstand high power.

Q.38. NAND and NOR gate may be considered as digital building blocks. Why?
Answer: The repeated use of NAND or NOR can produce all 3 basic gates i.e. OR, AND and NOT. Thus they
are called basic building blocks.

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