Class 12 Physics Chapter 14 Semiconductor Electronics – Extra Questions with Answers

These extra practice questions for Class 12 Physics Chapter 14 – Semiconductor Electronics go beyond the NCERT textbook exercises to reinforce energy bands, intrinsic and extrinsic semiconductors, p-n junction formation, and rectification — the topics that remain in the current 2026-27 syllabus. Useful for board exam revision and quick concept checks.

Very Short Answer Type Questions (1 Mark)

Q1. Define energy band gap.
Ans: The energy band gap (E_g) is the energy difference between the top of the valence band and the bottom of the conduction band in a solid — the range of energies an electron cannot have. Its size determines whether a material behaves as a conductor, semiconductor, or insulator.

Q2. Distinguish between an intrinsic and an extrinsic semiconductor in one line each.
Ans: An intrinsic semiconductor is a pure semiconductor (like pure silicon or germanium) with equal numbers of electrons and holes, generated only by thermal excitation. An extrinsic semiconductor is one deliberately doped with impurity atoms to increase either its electron (n-type) or hole (p-type) concentration.

Q3. What is a depletion region in a p-n junction?
Ans: The depletion region is the narrow zone around the junction, on both the p- and n-sides, that becomes depleted of free mobile charge carriers once diffusion creates immobile ionised donor and acceptor atoms there — it is this region’s internal field that produces the junction’s built-in potential barrier.

Q4. Name the two types of extrinsic semiconductors and state their majority charge carriers.
Ans: n-type (majority carriers: electrons, from pentavalent dopants) and p-type (majority carriers: holes, from trivalent dopants).

Short Answer Type Questions (2–3 Marks)

Q5. Explain, in terms of energy bands, why conductors, semiconductors, and insulators differ in their electrical behaviour.
Ans: In a conductor, the valence and conduction bands overlap (or the conduction band is partially filled), so electrons are freely available to conduct even without any energy input. In an insulator, the energy band gap is very large (several eV), so essentially no electrons can be thermally excited across it at room temperature. In a semiconductor, the band gap is small enough (around 1eV or less) that a modest number of electrons are thermally excited into the conduction band at room temperature, giving it conductivity between that of a conductor and an insulator — and one that increases sharply with temperature, unlike a metal.

Q6. Explain how the potential barrier forms at an unbiased p-n junction.
Ans: When a p-region and an n-region are joined, holes near the junction diffuse into the n-region and electrons diffuse into the p-region, driven by the concentration gradient across the junction. This diffusion leaves behind fixed, ionised donor atoms (positive) on the n-side and ionised acceptor atoms (negative) on the p-side near the junction, creating an internal electric field directed from the n-side to the p-side. This field opposes further diffusion, and equilibrium is reached once the field is strong enough to balance it — the resulting potential difference across the depletion region is the potential barrier.

Q7. Compare a half-wave rectifier and a full-wave rectifier in terms of the number of diodes used and their output ripple frequency for a 50Hz input.
Ans: A half-wave rectifier uses one diode and conducts for only one half of each input cycle, giving an output (ripple) frequency equal to the input frequency, 50Hz. A full-wave rectifier uses two diodes (in a centre-tap arrangement) or four diodes (in a bridge arrangement) and conducts during both halves of every cycle, giving an output ripple frequency of 100Hz — twice the input frequency, which is also easier to smooth into steady DC.

Higher Order Thinking Skills (HOTS)

Q8. Explain why the electrical conductivity of a pure semiconductor increases with rising temperature, while the conductivity of a metal conductor decreases with rising temperature.
Ans: In a semiconductor, raising the temperature thermally excites more electrons across the (relatively small) energy band gap into the conduction band, creating more electron-hole pairs and increasing the number of available charge carriers faster than any increase in scattering — so conductivity rises with temperature. In a metal, the number of free electrons is already essentially fixed (they don’t need to be thermally excited across a gap, since the conduction band is already populated); raising the temperature instead increases the thermal vibration of the lattice ions, which scatters the free electrons more frequently and reduces their mobility — so a metal’s conductivity falls as temperature rises.

Q9. A student builds a rectifier circuit intended to be full-wave, but wires it incorrectly so that it behaves as a half-wave rectifier instead, using the same 50Hz AC mains supply. Explain two practical differences this mistake would cause in the rectified output.
Ans: (1) Ripple frequency: the intended full-wave circuit should have produced an output ripple at 100Hz (two pulses per input cycle), but the miswired half-wave version produces only one pulse per cycle, giving a ripple frequency of just 50Hz. (2) Smoothness and efficiency: because the half-wave output is zero for one entire half of every cycle rather than continuously receiving pulses, it is far harder to smooth into steady DC with a given filter capacitor (requiring a much larger capacitor for the same ripple, or accepting greater ripple), and delivers a lower average DC output for the same input — a difference that would show up clearly as a much rougher, lower-voltage output on an oscilloscope or multimeter than the intended full-wave design.