Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits Assertion-Reason Questions | physics Class 12 CBSE

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 1

Logic Check

Assertion (A): In an intrinsic semiconductor, the number of free electrons equals the number of holes. Reason (R): Each time a valence electron is thermally excited to the conduction band, it leaves behind one hole in the valence band.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 2

Logic Check

Assertion (A): Semiconductor devices have replaced vacuum tubes in most electronic applications. Reason (R): Semiconductor devices are smaller, consume less power, operate at lower voltages, and have longer life compared to vacuum tubes.

Reasoning Options

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Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 3

Logic Check

Assertion (A): Carbon (diamond) is an insulator while germanium is a semiconductor. Reason (R): The energy band gap of carbon is much larger (5.4 eV) than that of germanium (0.7 eV), making thermal excitation across the gap impossible in carbon at room temperature.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 4

Logic Check

Assertion (A): In a metal, electrical conductivity decreases with increasing temperature. Reason (R): At higher temperatures, increased lattice vibrations scatter conduction electrons more frequently, reducing their mobility.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 5

Logic Check

Assertion (A): Phosphorus (P) is used as a donor impurity in silicon to create n-type semiconductor. Reason (R): Phosphorus is a trivalent element that creates acceptor levels just above the valence band of silicon.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 6

Logic Check

Assertion (A): The depletion region in a p-n junction contains no free charge carriers. Reason (R): The depletion region consists of immobile ionised donor and acceptor atoms whose charges balance each other, and all mobile carriers have diffused away.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 7

Logic Check

Assertion (A): In a reverse-biased p-n junction, the depletion region width increases. Reason (R): Under reverse bias, the applied voltage adds to the built-in potential, increasing the barrier height and extending the space-charge region on both sides.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 8

Logic Check

Assertion (A): The forward current in a p-n junction diode is in the milliampere range. Reason (R): Forward bias causes minority carrier injection, creating large concentration gradients that drive substantial diffusion currents through both regions of the diode.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 9

Logic Check

Assertion (A): An extrinsic semiconductor maintains overall electrical neutrality despite having unequal concentrations of electrons and holes. Reason (R): The charge of the majority carriers is exactly balanced by the opposite charge on the immobile ionised dopant cores in the crystal lattice.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 10

Logic Check

Assertion (A): The output frequency of a half-wave rectifier is 50 Hz when the input AC frequency is 50 Hz. Reason (R): A half-wave rectifier uses both half-cycles of the AC input, doubling the output frequency.

Reasoning Options

ByteNotes

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 11

Logic Check

Assertion (A): A p-n junction cannot be formed by simply pressing a p-type slab against an n-type slab. Reason (R): Physical surfaces have surface roughness much larger than inter-atomic crystal spacing (~2–3 Å), preventing atomic-level contact necessary for junction formation.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 12

Logic Check

Assertion (A): In an n-type semiconductor at room temperature, holes are the minority carriers. Reason (R): Doping with pentavalent impurity adds many electrons; by the mass-action law, the equilibrium hole concentration decreases below the intrinsic level.

Reasoning Options

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Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 13

Logic Check

Assertion (A): In a forward-biased diode, the applied voltage mostly drops across the depletion region. Reason (R): The depletion region has very high resistance compared to the p-side and n-side because it contains no free charge carriers.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 14

Logic Check

Assertion (A): The reverse saturation current in a p-n junction is essentially independent of the applied reverse voltage. Reason (R): The reverse current is limited by the concentration of minority carriers on each side, not by the magnitude of the applied voltage.

Reasoning Options

ByteNotes

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 15

Logic Check

Assertion (A): A capacitor connected across the load of a rectifier reduces the AC ripple in the output. Reason (R): The capacitor charges during the peak of the rectified pulse and discharges slowly through the load between pulses, smoothing the output voltage.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 16

Logic Check

Assertion (A): Tin (Sn) is a metal even though it belongs to Group IV of the periodic table like Si and Ge. Reason (R): Tin has zero energy band gap, so its conduction band and valence band overlap, providing a large number of free electrons for conduction.

Reasoning Options

ByteNotes

Assertion-Reason

Hard difficulty • Check the statement and reason

Hard

Question 17

Logic Check

Assertion (A): In a p-type semiconductor, the Fermi level shifts closer to the valence band compared to an intrinsic semiconductor. Reason (R): The large number of acceptor states just above the valence band increases hole concentration, lowering the effective Fermi level.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 18

Logic Check

Assertion (A): In a full-wave rectifier using a centre-tap transformer and two diodes, both diodes conduct simultaneously during any given half-cycle. Reason (R): During each half-cycle, one diode is forward biased and the other is reverse biased, so only one conducts at a time.

Reasoning Options

ByteNotes

Assertion-Reason

Easy difficulty • Check the statement and reason

Easy

Question 19

Logic Check

Assertion (A): The conductivity of an intrinsic semiconductor increases with temperature. Reason (R): At higher temperatures, more covalent bonds are broken, generating more electron-hole pairs and thus increasing charge carrier concentration.

Reasoning Options

ByteNotes

Assertion-Reason

Medium difficulty • Check the statement and reason

Medium

Question 20

Logic Check

Assertion (A): In an n-type semiconductor, the donor energy level ED lies very close to (just below) the conduction band. Reason (R): The fifth electron of the pentavalent dopant atom is very loosely bound and requires only ~0.01–0.05 eV to be set free into the conduction band.

Reasoning Options

ByteNotes

Chapter sections

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

Assertion (A): In an intrinsic semiconductor, the number of free electrons equals the number of holes. Reason (R): Each time a valence electron is thermally excited to the conduction band, it leaves behind one hole in the valence band.

Assertion (A): Semiconductor devices have replaced vacuum tubes in most electronic applications. Reason (R): Semiconductor devices are smaller, consume less power, operate at lower voltages, and have longer life compared to vacuum tubes.

Assertion (A): Carbon (diamond) is an insulator while germanium is a semiconductor. Reason (R): The energy band gap of carbon is much larger (5.4 eV) than that of germanium (0.7 eV), making thermal excitation across the gap impossible in carbon at room temperature.

Assertion (A): In a metal, electrical conductivity decreases with increasing temperature. Reason (R): At higher temperatures, increased lattice vibrations scatter conduction electrons more frequently, reducing their mobility.

Assertion (A): Phosphorus (P) is used as a donor impurity in silicon to create n-type semiconductor. Reason (R): Phosphorus is a trivalent element that creates acceptor levels just above the valence band of silicon.

Assertion (A): The depletion region in a p-n junction contains no free charge carriers. Reason (R): The depletion region consists of immobile ionised donor and acceptor atoms whose charges balance each other, and all mobile carriers have diffused away.

Assertion (A): In a reverse-biased p-n junction, the depletion region width increases. Reason (R): Under reverse bias, the applied voltage adds to the built-in potential, increasing the barrier height and extending the space-charge region on both sides.

Assertion (A): The forward current in a p-n junction diode is in the milliampere range. Reason (R): Forward bias causes minority carrier injection, creating large concentration gradients that drive substantial diffusion currents through both regions of the diode.

Assertion (A): An extrinsic semiconductor maintains overall electrical neutrality despite having unequal concentrations of electrons and holes. Reason (R): The charge of the majority carriers is exactly balanced by the opposite charge on the immobile ionised dopant cores in the crystal lattice.

Assertion (A): The output frequency of a half-wave rectifier is 50 Hz when the input AC frequency is 50 Hz. Reason (R): A half-wave rectifier uses both half-cycles of the AC input, doubling the output frequency.

Assertion (A): A p-n junction cannot be formed by simply pressing a p-type slab against an n-type slab. Reason (R): Physical surfaces have surface roughness much larger than inter-atomic crystal spacing (~2–3 Å), preventing atomic-level contact necessary for junction formation.

Assertion (A): In an n-type semiconductor at room temperature, holes are the minority carriers. Reason (R): Doping with pentavalent impurity adds many electrons; by the mass-action law, the equilibrium hole concentration decreases below the intrinsic level.

Assertion (A): In a forward-biased diode, the applied voltage mostly drops across the depletion region. Reason (R): The depletion region has very high resistance compared to the p-side and n-side because it contains no free charge carriers.

Assertion (A): The reverse saturation current in a p-n junction is essentially independent of the applied reverse voltage. Reason (R): The reverse current is limited by the concentration of minority carriers on each side, not by the magnitude of the applied voltage.

Assertion (A): A capacitor connected across the load of a rectifier reduces the AC ripple in the output. Reason (R): The capacitor charges during the peak of the rectified pulse and discharges slowly through the load between pulses, smoothing the output voltage.

Assertion (A): Tin (Sn) is a metal even though it belongs to Group IV of the periodic table like Si and Ge. Reason (R): Tin has zero energy band gap, so its conduction band and valence band overlap, providing a large number of free electrons for conduction.

Assertion (A): In a p-type semiconductor, the Fermi level shifts closer to the valence band compared to an intrinsic semiconductor. Reason (R): The large number of acceptor states just above the valence band increases hole concentration, lowering the effective Fermi level.

Assertion (A): In a full-wave rectifier using a centre-tap transformer and two diodes, both diodes conduct simultaneously during any given half-cycle. Reason (R): During each half-cycle, one diode is forward biased and the other is reverse biased, so only one conducts at a time.

Assertion (A): The conductivity of an intrinsic semiconductor increases with temperature. Reason (R): At higher temperatures, more covalent bonds are broken, generating more electron-hole pairs and thus increasing charge carrier concentration.

Assertion (A): In an n-type semiconductor, the donor energy level ED lies very close to (just below) the conduction band. Reason (R): The fifth electron of the pentavalent dopant atom is very loosely bound and requires only ~0.01–0.05 eV to be set free into the conduction band.

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