For a heavily doped n-type semi-conductor, donor level lies
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → a little below the conduction band
A pentavalent donor atom has one extra electron that is only loosely bound; about 0.01 eV (Ge) or 0.05 eV (Si) frees it into the conduction band. So the donor energy level lies just below the bottom of the conduction band, and even at room temperature almost all donor electrons move up into the conduction band.
Why the other options are incorrect
Option (2) a little above the valence band — Incorrect. A level a little above the valence band is the acceptor level of a p-type semiconductor.
Option (3) a little inside the valence band — Incorrect. A donor level inside the valence band could not supply electrons to the conduction band.
Option (4) at the center of the band gap — Incorrect. A level at the middle of the gap would need far more energy than the small ionisation energy of a donor.
NCERT (2026-27):
Extrinsic Semiconductor – “… In the energy band diagram of n-type Si semiconductor, the donor energy level ED is slightly below the bottom EC of the conduction band and electrons from this level move into the conduction band with very small supply of energy.”