“How a Rock Ended Up Running Your Phone” spent most of its length on purity: quartz turned into silicon, the silicon turned into a gas, distilled, and grown back into a single crystal so pure that fewer than one atom in a billion is out of place. Then it explained why: chipmakers deliberately add other atoms to silicon, in precise amounts, and they need a clean start so that the only impurities are the ones they chose. “Nine nines is not perfectionism,” it said. “It’s a blank page.” This is what gets written on it.
Start with pure silicon. Each silicon atom has four outer electrons, and in a crystal it shares one with each of its four neighbours, forming four bonds. Every outer electron is busy holding the crystal together, so almost none are free to move, and pure silicon conducts electricity very poorly. Heat it, or shine light on it, and a few electrons shake loose, which is why silicon is called a semiconductor: somewhere between a conductor and an insulator, and very sensitive to conditions.
Now replace one silicon atom in, say, every million with phosphorus. Phosphorus has five outer electrons. Four go into bonds with its silicon neighbours, exactly as a silicon atom would. The fifth has nothing to do. It’s only loosely held, and at room temperature it wanders off through the crystal, free to carry current. Silicon with added phosphorus has a supply of spare electrons. It’s called n-type, for negative, and even a tiny dose makes it conduct many thousands of times better.
Do the opposite with boron, which has only three outer electrons. It can bond with three of its four neighbours, but the fourth bond is left one electron short: a gap. A nearby electron can hop into that gap, but then it leaves a gap where it came from, which another electron can fill, and so on. The electrons each shuffle one step; the gap travels steadily the other way. Physicists find it simplest to treat that travelling gap as a particle in its own right, called a hole, which moves through the crystal behaving exactly like a positive charge. Silicon with boron is called p-type, for positive.
The magic happens where the two kinds meet. Put n-type silicon right next to p-type, in the same crystal, and at the boundary the spare electrons from one side drift across and fill holes on the other. That leaves a thin layer at the junction with no free charges at all, an insulating strip, held in place by the electric charges left behind on the fixed phosphorus and boron atoms. Push current one way and the strip narrows and current flows. Push it the other way and the strip widens and almost nothing gets through. That’s a diode: a one-way valve for electricity, made of nothing but carefully placed atoms. It’s how the charger for your phone turns the back-and-forth current from the wall into the one-way current the battery needs.
A transistor goes one step further and makes a switch. In the most common kind in modern chips, two islands of n-type silicon sit side by side in p-type silicon, with a tiny gap between them. Normally, current can’t cross the gap, because it would have to pass through p-type silicon where there are no spare electrons. But above the gap, separated by a whisker-thin layer of insulating glass, is a metal plate called the gate. Put a positive voltage on the gate, and it attracts electrons into the thin layer of silicon right beneath it, forming a narrow channel that connects the two islands. Current flows: the switch is on. Remove the voltage and the channel vanishes: off. No moving parts, nothing to wear out, and it can switch billions of times a second.
One switch, on or off, is one bit: a one or a zero. Connect switches so that one controls another and you can build circuits that add numbers, compare them and store them, and from those, everything a computer does. A modern phone processor contains more than ten billion transistors, each made by placing atoms of phosphorus or boron into the silicon with machines that fire them in as a beam, at precisely controlled depths and doses.
This is why the purity matters so much. In the parts of a chip that are lightly doped, the chosen impurity may be just one atom in a hundred million silicon atoms. If the starting silicon already contained stray atoms at a similar level, the chipmakers would never know what they had. That nine-nines purity, built from coal, quartz and a lot of electricity, is what makes the next step possible: the step where a single atom in a million, placed on purpose, turns a crystal of rock into a switch.