What Is Silicon?

Silicon is a tetravalent elemental semiconductor occupying group IV of the periodic table, with atomic number 14 and the chemical symbol Si. It is the second most abundant element in Earth's crust by mass and the dominant material in microelectronics, photovoltaics, and semiconductor device manufacturing. In its pure crystalline form, silicon has a diamond cubic structure with a lattice constant of 5.431 Å and an indirect band gap of 1.12 eV at room temperature, making it an intrinsic semiconductor with conductivity properties that can be precisely tuned through doping.

Silicon's prominence in electronics stems from several coincidental material advantages: the ease with which it forms a stable, electrically insulating native oxide (SiO₂) through thermal oxidation; its mechanical strength and compatibility with photolithographic processing; and the relative abundance and low cost of its raw feedstock, silicon dioxide in quartz sand. These properties, combined with decades of manufacturing refinement, have made crystalline silicon the foundational substrate of the global integrated circuit industry.

Elemental Semiconductor Properties

Pure silicon is a Group IV elemental semiconductor whose valence electrons participate in covalent sp³ bonding with four neighboring silicon atoms. The intrinsic carrier concentration at room temperature is approximately 1.45×10¹⁰ cm⁻³, several orders of magnitude below practical device requirements. Doping with phosphorus or arsenic (n-type) introduces donor electrons, while boron doping (p-type) creates holes as charge carriers, allowing resistivity to be controlled from near-metallic to near-insulating across many decades.

Amorphous silicon, which lacks long-range crystalline order, exhibits different electrical behavior: its higher defect density traps carriers, reducing mobility and increasing resistivity. However, hydrogenated amorphous silicon (a-Si:H) passivates many of these defects and is widely used in large-area thin-film transistors and solar cells where single-crystal wafers would be prohibitively expensive.

The NIST Enriched Silicon and Devices for Quantum Information program uses isotopically purified ²⁸Si, depleted of the ²⁹Si isotope (which carries nuclear spin), to create substrates where spin-based qubits experience far longer coherence times than in natural silicon.

Si-Based Devices

The transistor architectures that power modern digital logic, analog circuits, and memory are built almost exclusively on silicon substrates. The metal-oxide-semiconductor field-effect transistor (MOSFET) relies on the Si/SiO₂ interface, whose low interface trap density enables the precise threshold voltage control that digital switching requires. Bipolar junction transistors in silicon dominated high-speed analog and RF designs before SiGe heterojunction transistors extended performance into millimeter-wave frequencies.

Power semiconductor devices exploit silicon's combination of breakdown voltage, thermal conductivity, and processability. Silicon PIN diodes, thyristors, and insulated-gate bipolar transistors (IGBTs) control kilowatt to megawatt power levels in industrial drives, railway traction, and grid inverters. Silicon avalanche diodes, designed to operate in the reverse-breakdown regime, function as voltage references, transient suppressors, and single-photon detectors (Geiger-mode avalanche photodiodes). Physical and thermodynamic property data for elemental silicon, including heat capacity, melting point, and vapor pressure, are catalogued in the NIST WebBook silicon entry.

Crystal Growth and Wafer Production

Device-grade silicon begins with metallurgical-grade silicon reduced from quartz, which is then refined by the Siemens process (chemical vapor deposition of trichlorosilane) to polysilicon with impurity levels below one part per billion. Single-crystal ingots are grown by either the Czochralski method, which pulls a rotating seed crystal from a melt to produce the large-diameter wafers (300 mm and above) used in high-volume CMOS manufacturing, or the float-zone method, which yields higher purity material for power devices and specialized applications.

ScienceDirect's overview of crystalline silicon details the relationship between growth conditions, defect density, and the electrical performance of both monocrystalline and multicrystalline wafers used in photovoltaics.

Applications

Silicon has applications in a wide range of fields, including:

• Digital logic and memory integrated circuits
• Photovoltaic solar cells and modules
• Power electronics for motor drives and grid converters
• MEMS sensors (accelerometers, pressure sensors, gyroscopes)
• Silicon avalanche diodes for voltage regulation and photodetection