Silicon (Si, atomic number 14) is a chemical element classified as a metalloid, exhibiting properties of both metals and nonmetals. It is remarkably abundant, ranking as the second most common element by mass in the Earth’s crust, second only to oxygen. Silicon is a foundational material for geology, construction, and modern technology, forming the basis of sand, glass, and the semiconductor chips that power the digital world. Tracing its path reveals a story that spans from the hearts of dying stars to the highly refined materials used in microelectronics.
The Elemental Birth of Silicon
Silicon is a product of stellar nucleosynthesis, the fusion process occurring within massive stars. These stars, at least eight to eleven times the mass of our sun, generate increasingly heavier elements as they exhaust lighter fuels like hydrogen and helium. Silicon is formed during the late stages of a star’s life through the alpha process, where helium nuclei (alpha particles) are added to lighter nuclei such as magnesium. This process demands high temperatures and pressures within the star’s core. The element is also produced during the oxygen-burning process. Once silicon is created, it is dispersed across the cosmos when the star explodes in a Type II supernova, seeding new star systems and planets, including Earth.
Where Silicon Resides on Earth
Despite its abundance, silicon is rarely found in its pure elemental form on Earth. The element has a strong chemical affinity for oxygen, meaning it almost always exists in a combined state. This combination forms the backbone of the Earth’s crust and mantle, making up approximately 27.7% of the crust by weight. The two main terrestrial forms are silicon dioxide (\(\text{SiO}_2\)) and silicate minerals. Silicon dioxide, commonly known as silica, is the primary component of quartz, sand, agate, and opal. Silicate minerals are compounds of silicon, oxygen, and other metals like aluminum, iron, and magnesium. These silicates, which include feldspar, mica, and granite, constitute over 90% of the Earth’s crustal rocks. High-purity quartz rock and vast deposits of sand serve as the most common raw material source for human use.
From Quartz to Chips: Industrial Refinement
The path from naturally occurring quartz to the hyper-pure material used in electronics is a multi-stage industrial process. The journey begins with the carbothermal reduction of high-purity quartz sand (\(\text{SiO}_2\)) in a submerged electric arc furnace. Quartz is mixed with a carbon source, such as coal, coke, and wood chips, and heated to temperatures between \(1500^{\circ}\text{C}\) and \(2000^{\circ}\text{C}\). This process removes the oxygen, resulting in Metallurgical Grade Silicon (MGS), which is typically about \(98\%\) to \(99\%\) pure and is primarily used for metal alloys.
To achieve the higher purity required for semiconductors and solar cells, the MGS undergoes a chemical purification route, often starting with the Siemens process. First, the silicon is reacted with gaseous hydrogen chloride (\(\text{HCl}\)) to form a volatile liquid called trichlorosilane (\(\text{SiHCl}_3\)). This liquid is purified through fractional distillation, separating it from nearly all contaminants.
The highly purified trichlorosilane is then decomposed in the presence of hydrogen gas at temperatures above \(1100^{\circ}\text{C}\), depositing Polycrystalline Silicon (polysilicon) onto thin silicon rods. This polysilicon reaches purities of \(99.99999\%\) or higher, which is essential for its function as a semiconductor material. For integrated circuits, this polysilicon is melted and grown into a single, flawless crystal structure using the Czochralski process, creating the monocrystalline silicon wafers that form the foundation of modern computer chips.
Silicon Versus Silicone
The element silicon and the compound silicone are often confused. Silicon is the naturally occurring element, a hard, brittle metalloid used as a semiconductor in electronics. When purified, it forms a rigid, crystalline structure. Silicone, conversely, is a synthetic polymer, a man-made compound derived from elemental silicon. It is characterized by a siloxane backbone—a chain of alternating silicon and oxygen atoms—with organic groups like carbon and hydrogen attached. This chemical structure gives silicone its unique properties, such as low toxicity, high heat resistance, and flexibility, allowing it to take the form of oils, sealants, or rubber-like plastics used in cookware and medical implants.