The energy required to set an electron free in an n-type semiconductor made by doping a silicon crystal with a pentavalent element is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 0.05 eV
The fifth valence electron of a pentavalent dopant is only weakly bound to its parent atom. Its ionisation energy in silicon is about 0.05 eV (for germanium about 0.01 eV), which is why the donor level lies just below the conduction band and these electrons are freed even at room temperature.
NCERT (2026-27):
Extrinsic Semiconductor – “… As a result the ionisation energy required to set this electron free is very small and even at room temperature it will be free to move in the lattice of the semiconductor. For example, the energy required is ~ 0.01 eV for germanium, and 0.05 eV for silicon, to separate this electron from its atom. This is in contrast to the energy required to jump the forbidden band (about 0.72 eV for germanium and about 1.1 eV for silicon) at room temperature in the intrinsic semiconductor.”