Read the following passage and answer the questions.
A student studies a slab of pure silicon. It contains about \(5\times10^{28}\) atoms per cubic metre. The atoms are arranged in a diamond-like structure, and each silicon atom has four valence electrons. Germanium has the same kind of structure. First, the slab is kept in a cold chamber at a temperature very close to 0 K. It is then taken out and allowed to warm up to room temperature (300 K). At this temperature, the intrinsic carrier concentration of silicon is about \(1.5\times10^{16}\ \mathrm{m^{-3}}\). Finally, the student connects a battery and an ammeter in series with the slab. The ammeter shows a small but steady current. The slab is kept at room temperature throughout this test.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): In the warmed slab, a hole moves because the free electron that left the bond keeps jumping back into nearby vacancies.
Reason (R): The motion of a hole is a convenient way of describing the motion of bound electrons from neighbouring bonds into an empty bond.
In the light of the above statements, choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) is false but (R) is true.
A hole moves when an electron from a neighbouring bond jumps into the empty bond. The empty place then appears at the site that this electron has left.
Assertion (A) In the warmed slab, a hole moves because the free electron that left the bond keeps jumping back into nearby vacancies. — False. The electron set free from the bond takes no part in this process. It moves on its own as a conduction electron and gives the electron current.
Reason (R) The motion of a hole is a convenient way of describing the motion of bound electrons from neighbouring bonds into an empty bond. — True. Only bound electrons of nearby bonds move into the vacancy, one jump at a time. Calling this the motion of a hole is a convenient picture of these jumps.
NCERT (2026-27):
Intrinsic Semiconductor – “… Note that the electron originally set free … is not involved in this process of hole motion.”
Intrinsic Semiconductor – “… Remember that the motion of hole is only a convenient way of describing the actual motion of bound electrons, whenever there is an empty bond anywhere in the crystal.”