Silicon is an element, like oxygen or gold. It is the second most abundant element in the Earth’s crust (after oxygen and ahead of aluminum). Things like sand and rocks are largely silicon dioxide. “Silicone” is rubber; silicon and silicone are not the same thing at all. (There’s a big difference between Silicon Valley and…) Computer chips start out as pure silicon, which is grown into ingots from a big vat of molten silicon, not unlike the kindergarten experiment where you dissolved salt in water and hung a string in the solution from a pencil. When the water evaporated, the salt crystals formed on the string. In the same way, the silicon ingot is a single crystal grown from a seed crystal lowered into the vat and slowly pulled out. This crystal is sliced into wafers, and the wafers are processed and cut into computer chips.

A single silicon crystal is formed from four atoms and is shaped like a three-sided pyramid (tetrahedron). If the atoms were carbon instead of silicon, the shape would be the same and we’d call the substance “diamond.” (Warning: High school chemistry ahead!) Both silicon and carbon have four electrons in their outermost shell. A shell is stable when it has the magic number of eight electrons. The easiest way for a whole lot of silicon atoms to have stable outer shells is for each atom to share one electron with each of its four neighbors, and the smallest three-dimensional shape with four corners is the tetrahedron. So in a pure silicon crystal, all the outer shells are stable, everyone’s content, and no work gets done. The secret of computer chips (semiconductors) is to “dope” the silicon, that is, to carefully introduce specific impurities. The elements phosphorus and arsenic have five electrons in their outermost shells. If a few of these atoms are forced into the stable crystal lattice, there will be extra electrons hanging around with no particular place to go. A region with extra electrons is said to be “negatively doped.” The element boron has three electrons in the outermost shell. Introducing boron into the crystal lattice means that there are missing electrons (“holes”). Holes can act as positive charge carriers just as electrons are negative charge carriers. A region with missing electrons is said to be “positively doped.” If a positively doped region and a negatively doped region are right next to each other, the extra electrons from the negatively doped region will migrate to fill the holes in the positively doped region. All the electron shells in this “depletion region” will be filled, but there will be a charge difference, that is, a voltage, between the two regions. This arrangement is the simplest semiconductor, a diode. Electric current will flow through a diode only one way, from the positively doped end to the negatively doped end. Transistors are built from slightly more complex arrangements of positively doped regions and negatively doped regions.

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