Which material is considered a good semiconductor?

• A. Copper
• B. Gold
• C. Silicon
• D. Aluminum

Answer: (C)

Semiconductors are materials whose electrical conductivity sits between conductors and insulators and can be precisely controlled by introducing impurities (doping) and by applying electrical fields. Silicon fits this role well because it has a moderate band gap of about 1.12 eV, which means it is not a perfect conductor but can create electron-hole pairs when energized and, more importantly, its conductivity can be finely tuned by doping with donors or acceptors. The four valence electrons in silicon form a stable crystal lattice, and adding impurities creates free carriers (electrons or holes) that adjust current flow. This controllability makes silicon ideal for devices like diodes and transistors, especially since silicon readily forms a stable native oxide, silicon dioxide, which serves as an excellent insulating layer in layered devices. In contrast, copper, gold, and aluminum are metals with many free electrons and no practical band gap to support predictable, controllable junction behavior, so they do not function as semiconductors.