Which of the following is a semiconductor?

Correct answer: D (Germanium). Germanium is a semiconductor. It is a group 14 element with a band gap of about 0.67 eV, so at room temperature it conducts electricity better than an insulator but far worse than a metal. Its conductivity can be controlled by adding small amounts of impurities (doping), which is the basis of all semiconductor devices.

• A) Plastic: A polymer with covalent bonds and no free electrons. It is an insulator.
• B) Aluminium: A metal with a valence band that overlaps the conduction band, so it is a good conductor, not a semiconductor.
• C) Wood: Dry wood is made of cellulose and lignin and is an insulator. Moisture can make it conduct a little, but that is ionic conduction through water, not semiconduction.

The concept here is band theory: insulators have a large band gap, conductors have overlapping bands, and semiconductors have a small gap. This links to the General Science and modern technology sections, and to Mains questions on electronics and the materials behind the digital economy.

Current note: germanium was used in the first transistors (1947), but silicon has replaced it in most chips because it works at higher temperatures and forms a stable oxide. Germanium remains important in fibre optics, infrared optics and SiGe devices.

Trap: Candidates pick aluminium because metalloids and metals are often grouped together in lists. Remember that the semiconductor category is defined by band gap, not by whether an element is a metal.