Which of the following technique cannot be used for generating electron-hole pairs in electronic devices?

Question

UnlockIAS · Accepted Answer

Correct — D, Impurity injection. A genuine electron-hole PAIR is created only when a valence electron is excited across the band gap, leaving a matching hole behind — via thermal energy, absorbed photons, or a high-energy carrier collision. Impurity injection (doping with donor or acceptor atoms) instead adds a surplus of just ONE carrier type (extra electrons from donors, or extra holes from acceptors) without pairing — it changes majority-carrier concentration, not pair generation.

Why the other options are wrong — (a) Thermal energy can excite a valence electron across the band gap, generating an electron-hole pair — the basis of intrinsic thermal carrier generation.  (b) A high-energy carrier colliding with a lattice atom can knock a valence electron into the conduction band, generating an electron-hole pair — the basis of avalanche breakdown.  (c) A photon with energy at or above the band gap can excite an electron across the gap, generating an electron-hole pair — the basis of photodiodes and solar cells.

Concept: An electron-hole pair forms whenever a valence electron gains enough energy to jump into the conduction band, leaving a hole behind as a matched pair. This can be triggered by thermal energy, absorbed light (photons), or high-energy carrier impact. Doping (impurity injection) is a separate process — adding donor or acceptor atoms changes the crystal's fixed majority-carrier population without creating a matched pair.

State PCS 2022 Q49 — Science & Technology PYQ with Answer

State PCS 2022 Q49 — Science & Technology PYQ with Answer