Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In n-type silicon at room temperature, most of the electrons in the conduction band come from the silicon atoms.
Reason (R): In n-type silicon, the donor level \(E_D\) lies slightly below the bottom of the conduction band. So its electrons need very little energy to move into the conduction band.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is not correct but (R) is correct.
Assertion (A) In n-type silicon at room temperature, most of the electrons in the conduction band come from the silicon atoms. — False. At room temperature, most of the donor atoms are ionised, but only a very small number of Si atoms are ionised. So most conduction electrons come from the donor atoms.
Reason (R) In n-type silicon, the donor level \( E_D \) lies slightly below the bottom of the conduction band. So its electrons need very little energy to move into the conduction band. — True. The donor level sits just under \( E_C \), the bottom of the conduction band. A very small energy is enough to lift its electrons into the band.
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. At room temperature, most of the donor atoms get ionised but very few (~1012) atoms of Si get ionised. So the conduction band will have most electrons coming from the donor impurities …”