Doped silicon

Pure silicon conducts electricity poorly. Doped silicon is silicon with a tiny, carefully controlled sprinkle of other atoms, sometimes just one in millions, which changes how it conducts. Regions doped one way are called n-type, and the other way p-type. Where they meet, the useful electronics happen.

Each silicon atom shares four outer electrons with its neighbors. Phosphorus and arsenic have five, so each one placed in the crystal brings a spare electron that can move freely: n-type. Boron has only three, leaving a gap called a hole that acts like a moving positive charge: p-type. To put them exactly where they're needed, the silicon wafer is coated with a light-sensitive mask and patterned. Then an ion implanter, a kind of particle accelerator, fires dopant atoms into the exposed areas, with the beam energy setting how deep they go. A quick, intense heating, called annealing, repairs the damaged crystal and settles the dopants into place.

A boundary between p-type and n-type is a p–n junction, which lets current flow only one way: that's a diode. Arrange the regions as a source, channel and drain, and you have a transistor or a power transistor.