Semiconductor Electronics, Materials, Devices and Simple Circuits

Assertion and Reason Questions Chapter-14 Semiconductor Electronics, Materials, Devices and Simple Circuits

Assertion (A) and other labelled Reason (R). Select the correct answer to these questions from the options as given below.

A. Both A and R are true, and R is the correct explanation of A.

B. Both A and R are true, but R is not the correct explanation of A.

C. A is true, but R is false.

D. A is false, but R is true.

1. Assertion (A): Semiconductors have electrical conductivity between conductors and insulators.

Reason (R): The conductivity of semiconductors increases with temperature.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Semiconductors are materials that exhibit electrical conductivity values that fall between those of conductors and insulators, and their conductivity increases as temperature rises due to the increased thermal energy that excites electrons.

2. Assertion (A): Doping a semiconductor increases its conductivity.

Reason (R): Doping introduces free charge carriers in the semiconductor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Doping involves adding impurities to a semiconductor, which introduces additional free charge carriers (electrons or holes), thereby enhancing its conductivity.

3. Assertion (A): Intrinsic semiconductors are pure semiconductors without any impurities.

Reason (R): Intrinsic semiconductors have a fixed number of charge carriers.

• (b) Both A and R are true, but R is not the correct explanation of A.
• Answer: (b)
  Explanation: Intrinsic semiconductors are indeed pure, but they have a varying number of charge carriers depending on temperature, as thermal excitation creates electron-hole pairs.

4. Assertion (A): P-type semiconductors are formed by doping with trivalent elements.

Reason (R): Trivalent elements create more electrons than holes.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: P-type semiconductors are created by doping with trivalent elements, which produce holes (positive charge carriers) rather than additional electrons.

5. Assertion (A): The energy gap in semiconductors is smaller than that in insulators.

Reason (R): A smaller energy gap allows easier excitation of electrons.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The smaller energy gap in semiconductors allows electrons to be thermally excited to the conduction band more easily than in insulators, which have a larger energy gap.

6. Assertion (A): A diode allows current to flow in one direction only.

Reason (R): Diodes have low resistance in the forward bias and high resistance in reverse bias.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Diodes conduct current easily when forward biased due to low resistance, but block current in reverse bias due to high resistance, allowing current to flow in only one direction.

7. Assertion (A): The Zener diode is used for voltage regulation.

Reason (R): Zener diodes operate in reverse bias and maintain a constant voltage across them.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Zener diodes can maintain a steady output voltage in reverse bias, making them ideal for voltage regulation in circuits.

8. Assertion (A): The PN junction diode conducts current only in the forward direction.

Reason (R): In forward bias, the depletion region is widened.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: In forward bias, the depletion region is narrowed, allowing current to flow, while in reverse bias, the depletion region widens, preventing current flow.

9. Assertion (A): Photodiodes are used to convert light into electrical energy.

Reason (R): Photodiodes operate in reverse bias to generate photocurrent.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Photodiodes generate photocurrent when exposed to light in reverse bias, making them suitable for light detection and conversion into electrical signals.

10. Assertion (A): The Hall effect is used to determine the type of semiconductor material.

Reason (R): The Hall voltage is proportional to the current and magnetic field strength.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The Hall effect allows for the determination of charge carrier type (positive or negative) based on the sign of the Hall voltage, which depends on the current and magnetic field.

11. Assertion (A): A transistor can act as both an amplifier and a switch.

Reason (R): Transistors can control a larger current with a smaller input current.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Transistors amplify current and can switch larger loads with minimal input, enabling their dual function in circuits.

12. Assertion (A): The working of a transistor is based on the control of charge carriers.

Reason (R): A small base current controls a large collector current in a transistor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: In a transistor, the small input current in the base allows for the control of a much larger current flowing from collector to emitter, demonstrating its amplifying capabilities.

13. Assertion (A): The common emitter configuration provides high voltage gain.

Reason (R): In this configuration, the output is taken from the collector.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The common emitter configuration is known for its ability to produce significant voltage gain, primarily because the output is derived from the collector where a larger voltage can be obtained.

14. Assertion (A): The output characteristic curves of a transistor show the relationship between collector current and collector-emitter voltage.

Reason (R): These curves help determine the operating point of a transistor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The output characteristic curves illustrate how the collector current varies with collector-emitter voltage, enabling the analysis of the transistor's operating conditions.

15. Assertion (A): The cutoff region in a transistor is where no current flows.

Reason (R): In this region, the base-emitter junction is reverse biased.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: In the cutoff region, the transistor is effectively off, and no current flows through it because the base-emitter junction is reverse biased, preventing charge carrier movement.

16. Assertion (A): The saturation region of a transistor occurs when it is fully on.

Reason (R): In this region, the transistor acts as a closed switch.        

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: When a transistor is in saturation, it allows maximum current to flow from collector to emitter, effectively functioning as a closed switch.

17. Assertion (A): The gain of a transistor is defined as the ratio of output current to input current.

Reason (R): The current gain is usually expressed as beta (β).

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Transistor gain, typically represented by β, quantifies how much the input current is amplified to produce the output current.

18. Assertion (A): A BJT can be used as a current amplifier.

Reason (R): BJTs have a high current gain factor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Bipolar Junction Transistors (BJTs) are designed to amplify input current to a higher output current, leveraging their high current gain.

19. Assertion (A): The forward bias condition reduces the width of the depletion region in a diode.

Reason (R): The external voltage opposes the built-in potential of the diode.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: In forward bias, the applied voltage effectively counteracts the built-in potential, causing the depletion region to shrink and allowing current to flow.

20. Assertion (A): The reverse bias condition increases the depletion region width in a diode.

Reason (R): Reverse bias applies a voltage that enhances the built-in electric field.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: When a diode is reverse biased, the applied voltage increases the depletion region's width, which prevents current flow except for a minimal leakage current.

21. Assertion (A): A MOSFET operates on the principle of an electric field controlling the conductivity of a semiconductor.

Reason (R): MOSFETs have a gate terminal that controls the current flow through the channel.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The operation of MOSFETs relies on the electric field created by the gate terminal, which modulates the conductivity of the channel between the source and drain terminals.

22. Assertion (A): The depletion layer in a PN junction diode is formed due to the diffusion of carriers.

Reason (R): The diffusion leads to the recombination of electrons and holes at the junction.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The diffusion of carriers across the PN junction leads to recombination, resulting in a depletion region devoid of charge carriers, thereby creating an electric field.

23. Assertion (A): The output voltage of a Zener diode remains constant even with varying load currents.

Reason (R): Zener diodes are designed to operate in the breakdown region.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Zener diodes maintain a constant output voltage across varying load currents by utilizing their breakdown characteristics, making them effective for voltage regulation.

24. Assertion (A): A phototransistor is more sensitive to light than a photodiode.

Reason (R): Phototransistors provide amplification of the photocurrent generated.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Phototransistors are designed to amplify the current generated by light exposure, making them more sensitive than photodiodes, which do not provide amplification.

25. Assertion (A): The input impedance of a common collector amplifier is high.

Reason (R): High input impedance allows for minimal loading on the preceding stage.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: A common collector amplifier (emitter follower) has a high input impedance, ensuring that it does not load the previous stage, thereby preserving signal integrity.

26. Assertion (A): The emitter current in a transistor is approximately equal to the sum of base current and collector current.

Reason (R): This is due to the conservation of charge in the transistor operation.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The emitter current (I_E) is the total current flowing into the transistor, which is the sum of the base current (I_B) and collector current (I_C), demonstrating charge conservation.

27. Assertion (A): The input characteristic of a transistor shows how base current varies with base-emitter voltage.

Reason (R): The input characteristic is a straight line indicating ohmic behavior.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: The input characteristic is indeed a plot of base current versus base-emitter voltage, but it does not represent purely ohmic behavior; it typically exhibits nonlinear characteristics.

28. Assertion (A): The reverse current in a diode under reverse bias is due to minority carriers.

Reason (R): Minority carriers are generated by thermal excitation.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The reverse current in a diode under reverse bias is primarily due to the movement of minority carriers, which are thermally generated in small quantities.

29. Assertion (A): A transistor in the active region can amplify an AC signal.

Reason (R): The transistor maintains a constant operating point in the active region.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The active region allows the transistor to amplify AC signals while maintaining a stable operating point, crucial for linear amplification.

30. Assertion (A): The Fermi level in an intrinsic semiconductor lies near the middle of the energy gap.

Reason (R): This indicates equal probability of occupation by electrons and holes.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The Fermi level's position reflects the equal likelihood of finding electrons and holes in an intrinsic semiconductor at thermal equilibrium.

31. Assertion (A): In a semiconductor, the increase in temperature generally increases the number of charge carriers.

Reason (R): Thermal energy excites electrons from the valence band to the conduction band.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Increasing temperature provides thermal energy, which excites electrons, resulting in a higher number of charge carriers available for conduction.

32. Assertion (A): An NPN transistor has two N-type regions and one P-type region.

Reason (R): The configuration affects the direction of current flow in the transistor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The structure of an NPN transistor, with two N regions and one P region, determines the direction of current flow and the transistor's operation characteristics.

33. Assertion (A): The majority carriers in a P-type semiconductor are holes.

Reason (R): Holes are created when trivalent atoms are added to the semiconductor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: In P-type semiconductors, trivalent atoms create holes that serve as majority charge carriers, facilitating conduction.

34. Assertion (A): In a PN junction diode, the depletion region is narrower in forward bias.

Reason (R): Forward bias reduces the barrier potential.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The application of forward bias reduces the barrier potential, resulting in a narrower depletion region, which enhances current flow.

35. Assertion (A): The junction capacitance of a diode is influenced by the applied voltage.

Reason (R): The width of the depletion layer changes with the applied voltage.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The junction capacitance is inversely related to the width of the depletion layer, which varies with the applied voltage across the diode.

36. Assertion (A): The biasing of a transistor affects its operation mode.

Reason (R): Different biasing configurations lead to different amplification characteristics.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The method of biasing determines whether the transistor operates in active, cutoff, or saturation regions, each with distinct amplification properties.

37. Assertion (A): The load line of a transistor represents the relationship between collector current and collector-emitter voltage.

Reason (R): The load line is a graphical representation of the external circuit connected to the transistor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The load line indicates how the collector current and collector-emitter voltage change in response to the external circuit's load, helping to determine the operating point.

38. Assertion (A): In a common emitter configuration, the output is inverted.

Reason (R): The phase relationship between the input and output signals is 180 degrees.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The common emitter configuration inverts the input signal due to the nature of current flow, resulting in a 180-degree phase shift.

39. Assertion (A): Zener diodes are used for voltage regulation.

Reason (R): They can maintain a constant output voltage across a wide range of load currents.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Zener diodes are designed to operate in breakdown regions, allowing them to provide stable voltage regulation regardless of variations in load current.

40. Assertion (A): The temperature coefficient of a semiconductor is negative.

Reason (R): Increased temperature leads to a decrease in resistance.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: While the temperature coefficient is indeed negative, increased temperature actually leads to increased conductivity (not decreased resistance) in semiconductors due to more charge carriers.

41. Assertion (A): Photodiodes operate effectively in reverse bias.

Reason (R): Reverse bias improves the speed of response and sensitivity.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: Photodiodes are reverse-biased to increase their response speed and sensitivity to light, allowing for efficient operation in photodetection applications.

42. Assertion (A): The output impedance of a common collector amplifier is low.

Reason (R): Low output impedance allows for effective coupling to the load.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: The common collector amplifier actually has a high output impedance, which does not effectively couple to low-impedance loads without additional components.

43. Assertion (A): A diode can be used as a rectifier in circuits.

Reason (R): Diodes allow current to flow in one direction only.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The unidirectional current flow characteristic of diodes makes them ideal for converting AC to DC, thus functioning effectively as rectifiers.

44. Assertion (A): An intrinsic semiconductor has a very low conductivity.

Reason (R): The absence of free charge carriers in intrinsic semiconductors results in low conductivity.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: In intrinsic semiconductors, the limited number of thermally generated charge carriers leads to very low conductivity.

45. Assertion (A): A PNP transistor has two P-type regions and one N-type region.

Reason (R): The configuration influences the direction of current flow through the transistor.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The structure of a PNP transistor, with two P regions and one N region, affects the current flow direction, making it distinct from an NPN transistor.

46. Assertion (A): A common base amplifier has a low input impedance.

Reason (R): The low input impedance allows for better coupling with the preceding stage.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: The common base amplifier does have low input impedance, but this does not necessarily correlate with better coupling, as it can lead to increased loading of the previous stage.

47. Assertion (A): The width of the depletion layer in a PN junction varies with the applied bias voltage.

Reason (R): Increased voltage leads to a higher electric field across the junction.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: As the bias voltage changes, the electric field across the PN junction modifies the depletion layer width, narrowing in forward bias and widening in reverse bias.

48. Assertion (A): The cut-off frequency of a transistor is influenced by its current gain.

Reason (R): Higher current gain results in a higher cut-off frequency.

• (c) A is true, but R is false.
• Answer: (c)
  Explanation: While it is true that the cut-off frequency is affected by the transistor's characteristics, the relationship is not straightforward as other factors also play a significant role.

49. Assertion (A): An N-channel MOSFET has lower on-resistance than a P-channel MOSFET.

Reason (R): N-channel devices generally have higher mobility of charge carriers.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: N-channel MOSFETs have higher electron mobility compared to hole mobility in P-channel devices, resulting in lower on-resistance and better performance.

50. Assertion (A): The working of a diode is based on the properties of the PN junction.

Reason (R): The PN junction creates an electric field that controls charge carrier movement.

• (a) Both A and R are true, and R is the correct explanation of A.
• Answer: (a)
  Explanation: The PN junction's characteristics create an electric field that governs the flow of charge carriers, thus enabling the diode's functionality as a rectifier and switch.