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.
When the battery is connected, which of the following describes the motion of the charge carriers in the slab?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Free electrons move towards the positive terminal, and holes move towards the negative terminal.
The battery sets up an electric field inside the slab. A free electron has a negative charge, while a hole acts as a particle with a positive charge.
Option (1) Free electrons move towards the positive terminal, and holes move towards the negative terminal. — Correct. The free electrons give the electron current \( I_e \). The holes move towards the negative potential and give the hole current \( I_h \).
Option (2) Free electrons and holes both move towards the positive terminal. — Incorrect. Only a negative charge is pulled towards the positive terminal. A hole behaves as a positive charge, so the field pushes it the other way.
Option (3) Only the free electrons move; the holes stay fixed at their sites. — Incorrect. In a semiconductor the holes also move. A bound electron from a nearby bond jumps into the vacancy, so the hole shifts from one site to the next.
Option (4) Free electrons and holes both move towards the negative terminal. — Incorrect. Free electrons carry a negative charge, so the battery pulls them towards its positive terminal.
NCERT (2026-27):
Intrinsic Semiconductor – “… Under the action of an electric field, these holes move towards negative potential giving the hole current, Ih. …”