The crystal is purified, then deliberately contaminated

Electronic-grade silicon is refined to roughly one stray atom in a billion. The behaviour of a chip comes from putting a chosen impurity back, at a dose of one atom in millions to billions.

Sand is silicon dioxide. The path from sand to a wafer is a sequence of reductions and distillations that end in a crystal pulled from a melt, with impurities driven down to levels that would be a rounding error in any other industry. At that purity the crystal is a poor, and very predictable, conductor. Then phosphorus, boron, or arsenic is introduced in a measured dose. Those atoms have one electron too many, or one too few, and they decide whether a region conducts, and which way current prefers to flow. A transistor is a few such regions arranged so a voltage on one of them opens or closes a path through another.

The dose is the whole art. Too little and nothing switches. Too much and the region behaves like a wire. The industry’s decades of shrinking are decades of placing that dose in a smaller volume, and of keeping the unwanted atoms out while the wanted ones go in. Purity is not the product. Controlled impurity is the product.