What is silicon, and why are computer chips made from it?

It might seem like a silly question, one that could be answered in just a few short words: Silicon is the 14th element on the Periodic Table. It's one of the fundamental constituents of the universe, one proton heavier than aluminum, one proton lighter than phosphorus. Yet silicon, more than almost any other element, comes up all too often on a site like ExtremeTech -- it is a major component in the building materials that make up your home, it's the basis of all current computer processors, and it's even the most likely candidate to underlie alien, non-carbon-based life. What exactly makes silicon so special?

Silicon as building block

Foremost among silicon's distinguishing features is that, quite simply, there is a hell of a lot of it. After oxygen, it's the second most abundant element in the Earth's crust -- but don't expect to find it just lying around. Silicon is almost never found in a pure state in nature, and virtually always comes as a compound with other elements. It's most commonly found as a silicate (SiO₄, or one silicon atom bound to four oxygen atoms) and silica (SiO₂, or one silicon atom bound to two oxygen atoms). Silica, in a rough and highly contaminated form, is the primary component of sand. Feldspar, granite, quartz, and more are all based on silicon-oxygen compounds.

Silicon compounds have a wide variety of useful properties, mostly because they can bind other atoms very tightly, and in complex arrangements. Various silicates, like calcium silicate, are a primary component of Portland cement, the main binder in concrete, mortar, and even stucco. Some silicate-rich materials can be heated to produce hardened ceramics like porcelain, while others will fuse to form the world's primary form of glass, soda-lime glass. And silicon can also be useful as a trace additive in other substances, like cast iron, which uses both carbon and silicon to make iron more resilient and less brittle.

Silicon as computer chip

When selecting an element to use as the basis of a computer transistor, the key word is resistance. Conductors have low resistance, and pass along electric current very easily, while insulators have (predictably) high resistance, and slow or block the flow of electrons. For a transistor, which must be able to switch on and off at will, we require in a semi-conductor, a substance with resistance between that of a conductor and an insulator. The best semiconductors for industry can be treated with a wide variety of "dopants" to finely adjust their resistance, as needed.

Silicon isn't the only semiconducting substance on Earth -- it's not even the best semiconductor on Earth. What it is, is by far the most abundant semiconductor on Earth. Silicon is readily available, all over the world; you don't need to import it from special African mines, or do months of expensive and polluting treatment just to get some. It's easy to work with and, most importantly, scientists have come up with reliable ways of growing it into perfectly ordered crystals. These crystals are to silicon as diamond is to carbon.

Growing enormous, near-perfect silicon crystals is one of the primary skills in modern computer chip manufacturing. These crystals are then sliced into thin wafers, then engraved, processed, and treated in sometimes hundreds of different ways before being diced into the individual die and packaged into commercial processors. It's possible to make superior transistors out of things like carbon, and even more exotic materials like germanium, but none of them allow the sort of bulk manufacturing silicon allows through large crystal growth -- at least, not yet.

Silicon as alien life

The phrase "carbon-based life" gets thrown around a lot, but what does it really mean? It means that the core structural molecules that make up our bodies (proteins, amino acids, nucleic acids, fatty acids, and more) are built on skeletons of carbon atoms. That's because carbon has the great property of being "tetravalent." Oxygen can only form two stable chemical bonds at once (thus leading to water, or H₂O), and nitrogen only three (thus leading to ammonia, or NH₃), but carbon can stably hold onto up to four different atoms at once (thus giving us methane, or CH₄). Tetravalency is a powerful basis for building molecules that are both strong and geometrically complex, and that duo of chemical virtues has allowed the evolution of all life currently known in the universe.

Yet, if we know how the Periodic Table is organized, we know that elements in a vertical column have similar chemical properties -- and right below carbon, is silicon. This is why science fiction authors have spent so much time and ink of the idea of silicon-based life; being tetravalent itself, silicon is the most plausible alternate structural element in totally novel forms of life. Silicon is also happy to bond powerfully to other silicon atoms (just like carbon to carbon) and can thus double-lock certain conformations into place. Both are presumed to be crucial to allowing the development of life.

Of course, with silicon being so much more abundant on Earth than carbon, there has to be a reason that we are organic (carbon-based), rather than silicon-based -- and that reason comes back to the Periodic Table. Without going into too much detail, elements that are vertically lower on the Periodic Table have heavier nuclei and larger electron shells; silicon is physically larger and heavier than carbon, making it less well-suited to super-fine tasks like, for instance, recombinant DNA. Silicon is also less widely reactive than carbon, meaning silicon-based life could be less chemically diverse, or require a much wider array of reaction-driving silicon enzymes to force chemically less-desirable compounds into existence.

Silicon and you