If light of energy greater than the forbidden energy gap falls on a semiconductor, what happens to the electrons?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The electrons move from the valence band to the conduction band.
In a semiconductor the filled valence band is separated from the conduction band by the energy gap \( E_g \). An electron that absorbs more energy than \( E_g \) from the light can cross this gap.
Option (1) The electrons move from the conduction band to the valence band. — Incorrect. Electrons rise to the higher band when they gain energy. A fall from the conduction band to the valence band gives out energy and does not absorb it.
Option (2) The electrons move from the valence band to the conduction band. — Correct. The absorbed energy is more than \( E_g \). So an electron of the valence band is lifted across the gap into the conduction band, leaving a hole behind. Both the free electron and the hole then add to the current.
Option (3) The electrons stay in the valence band and oscillate with higher energy. — Incorrect. Energy greater than \( E_g \) is enough to lift the electron across the gap, so it does not remain in the valence band.
Option (4) The electrons stay in the conduction band and oscillate with higher energy. — Incorrect. The electrons that absorb the light start in the valence band. They then move up to the conduction band.
NCERT (2026-27):
Summary – “… The extent of conductivity depends upon the energy gap (Eg) between the top of valence band (EV) and the bottom of the conduction band EC. The electrons from valence band can be excited by heat, light or electrical energy to the conduction band and thus, produce a change in the current flowing in a semiconductor.”