A semiconductor is a material with conductivity between a conductor and an insulator. Silicon is the most common. It conducts electricity under some conditions and blocks it under others. That property makes it possible to build transistors, diodes, and integrated circuits. Without semiconductors, there would be no computers.

Semiconductors are doped with impurities to change their behavior. N-type has extra electrons. P-type has holes. Putting them together creates a p-n junction, which allows current to flow in one direction. Transistors use this to switch and amplify signals. Billions of transistors fit on a single chip.

Common semiconductor materials

• Silicon: dominant, abundant, well-understood
• Germanium: early transistors, now niche
• Gallium arsenide: high-speed, optoelectronics
• Silicon carbide: high-power, high-temperature
• Gallium nitride: efficient power and RF

Semiconductor manufacturing is concentrated. Taiwan, South Korea, and China dominate. The U.S., Europe, and Japan are investing to bring fabrication back. The chip supply chain is a geopolitical fault line. Semiconductors are the new oil.