Which of the following technique cannot be used for generating electron-hole pairs in electronic devices?
- (a)Thermal excitation
- (b)Impact ionization
- (c)Photo excitation
- (d)Impurity injection
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.
- (a)Thermal excitation — Thermal energy can excite a valence electron across the band gap, generating an electron-hole pair — the basis of intrinsic thermal carrier generation.
- (b)Impact ionization — 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)Photo excitation — 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.
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.
The exam trap is treating doping as just another way to 'add' charge carriers and conflating it with true pair generation. The distinguishing test: does the mechanism break a covalent bond to release a matched electron and hole (a pair), or does an impurity atom simply contribute one spare carrier on its own?
- Electron-hole pair generation mechanisms: thermal excitation, photo excitation (light), impact ionization (collision)
- Impurity injection (doping) adds only ONE carrier type per dopant atom — donors add electrons, acceptors add holes — no pair is formed
- Photo excitation is the working basis of photodiodes and solar cells
- Impact ionization is the basis of avalanche breakdown
Doping is the odd one out — it changes carrier concentration but does not create a matched electron-hole pair.
- Treating doping as a pair-generation mechanism just because it also 'adds carriers'
- Mixing up impact ionization (collision-triggered) with photo excitation (light-triggered)
A reverse/odd-one-out format ('which technique CANNOT do X') — MPPSC electronics MCQs test the underlying physics distinction between pair generation and carrier addition via doping.
No directly related past PYQ was found.
- practice — not a real PYQ
In semiconductor doping, a donor impurity atom (e.g., Phosphorus in Silicon) contributes:
- (a)An electron-hole pair
- (b)An extra free electron only
- (c)An extra hole only
- (d)No change in carrier concentration
Answer(b) An extra free electron only — doping does not create a paired carrier.
- practice — not a real PYQ
The generation of electron-hole pairs by light absorption is the working principle of:
- (a)Zener diode
- (b)Photodiode/solar cell
- (c)Varactor diode
- (d)Zener regulator
Answer(b) Photodiode/solar cell — based on photo-excitation generating electron-hole pairs.