A semiconductor is the result of which combination?

• A. A metal
• B. An n-type crystal
• C. An n-type crystal and a p-type crystal
• D. A superconducting material

Answer: (C)

A semiconductor arises when both n-type and p-type regions are brought together to form a pn junction. In an n-type crystal, donor dopants add extra electrons as the majority carriers; in a p-type crystal, acceptor dopants create holes as the majority carriers. When these regions meet, electrons diffuse into the p-region and holes diffuse into the n-region, creating a depletion zone and an internal electric field. This junction enables current to flow mainly in one direction and can be controlled by bias, which is the defining behavior of many semiconductor devices. A metal conducts due to free electrons and doesn’t rely on a pn junction. An n-type crystal alone is a doped semiconductor but lacks the junction behavior that gives semiconductors their distinctive operations. Superconductors aren’t semiconductors at all—they exhibit zero resistance under certain conditions and follow different physics.