semiconductor
/ SEM-ee-kun-duk-tur /
Some materials cannot make up their minds about conducting electricity. They are stubborn like an insulator when cold and dark, yet they loosen up and pass current when warmed, lit, or chemically tweaked. This middle-of-the-road behaviour gives them their name: semiconductors, the in-between materials on which all of modern electronics is built.
A semiconductor is a material with a small band gap — roughly a fraction of an electron-volt up to a couple of electron-volts — separating its full valence band from its empty conduction band. The gap is narrow enough that a modest nudge from heat or light can lift some electrons across into the conduction band, each leaving a hole behind. Even better, adding tiny amounts of carefully chosen impurities, called doping, lets engineers dial the number of carriers up or down by enormous factors.
Semiconductors matter because that controllability is the whole foundation of the digital age: transistors, chips, sensors, solar cells, and LEDs all exploit it. The honest caveat is that a pure semiconductor at room temperature is actually a rather poor conductor; its usefulness comes almost entirely from our ability to engineer it through doping and by joining different pieces together, not from the bare material on its own.
Silicon, the workhorse of the semiconductor industry, has a band gap of about 1.1 electron-volts. By doping one region with phosphorus and an adjacent region with boron, engineers build the junctions that become diodes and transistors — billions of which sit on the chip running the device you are reading this on.
Silicon's small gap plus doping is what makes the transistor — and the digital age — possible.
Semiconductor and insulator differ only in gap size, with no sharp dividing line; a material with a one-electron-volt gap is a useful semiconductor, while the same material imagined with a five-electron-volt gap would behave as an insulator.