Silicon

Silicon (Si, atomic number 14) is a Group 14 metalloid with a diamond-cubic crystal structure, a melting point of 1,414 °C, and an indirect bandgap of 1.12 eV — the physical properties that make it the foundation of the global semiconductor industry and the most economically important non-metallic element. Silicon is the second most abundant element in Earth's crust (~27.7 wt%), occurring as silica (SiO₂) and silicate minerals; it is never found free in nature. Elemental Si is produced by carbothermic reduction of quartz in electric arc furnaces (~8 million tonnes/year metallurgical grade), then refined to electronic grade by the Siemens process (chemical vapor deposition of trichlorosilane) or Czochralski crystal pulling to produce the ultra-high-purity single-crystal boules from which semiconductor wafers are cut. Silicon forms a self-passivating native SiO₂ oxide (2–4 nm thick at room temperature, thermally grown to precise thicknesses in device fabrication) that provides excellent chemical stability and is central to CMOS device physics as the gate dielectric in all pre-high-κ transistor generations.

Silicon's 1.12 eV indirect bandgap, mature processing technology, and earth abundance make it the dominant material for both microelectronics and photovoltaics — silicon CMOS accounts for essentially all logic, memory, and mixed-signal ICs produced worldwide, and silicon solar cells represent ~95% of global photovoltaic capacity. In microelectronics, the International Technology Roadmap for Semiconductors (now IRDS) has driven Si transistor gate lengths from 10 µm (1971, Intel 4004) to 2 nm node (2024, TSMC/Intel), with each generation approximately doubling transistor density per Moore's Law; modern 2 nm node chips contain ~100 billion transistors on a die the size of a fingernail. In photovoltaics, monocrystalline Si cells achieve record efficiencies of 26.7% (single junction, UNSW/Kaneka) and passivated emitter rear contact (PERC) cells dominate commercial production at ~22–24% efficiency; multi-junction Si-based tandems with perovskite top cells now exceed 33%. Silicon's indirect bandgap — while disadvantageous for light emission — means it is transparent to near-IR light above ~1.1 µm, enabling Si photonics for on-chip optical interconnects and LiDAR.

Beyond semiconductors, silicon is the essential alloying element in aluminum casting alloys (Al-Si, 4–22% Si), the monomer for the entire silicone polymer industry, a critical reductant in steelmaking, and the substrate for emerging quantum computing platforms based on spin qubits in isotopically purified ²⁸Si. Al-Si casting alloys (e.g. A380, A356) are the largest volume use of metallurgical Si, providing fluidity, low shrinkage, and high strength-to-weight ratio for automotive engine blocks, transmission housings, and structural castings. Silicone polymers (polysiloxanes, –Si(R₂)–O–) are made from chlorosilane monomers derived from Si metal and span applications from medical implants and cookware coatings to sealants, lubricants, and electrical insulators. In quantum computing, electron and nuclear spin qubits in isotopically enriched ²⁸Si (depleted in ²⁹Si to <50 ppm, eliminating the dominant nuclear spin decoherence source) have demonstrated coherence times exceeding 30 seconds — orders of magnitude longer than superconducting qubits — making Si spin qubits a leading platform for scalable fault-tolerant quantum computing.