Silicon Properties

Engineering variability

• Elastic modulus varies with crystal orientation; [110] and [111] are stiffer than [100].
• Fracture strength is governed by surface flaws, etching, wafer thickness and fabrication process.
• Thermal conductivity depends on purity, defect density and temperature.
• Doping usually has modest effect on room-temperature thermal conductivity until carrier/impurity concentrations become high.

Selection note

Used for integrated circuits, MEMS, optical components and precision structures where crystalline anisotropy and brittle fracture must be considered.

Where a property is shown as qualitative rather than numeric, that is intentional: the selected source or the material physics does not justify a single comparable scalar.

Where it is specified

Integrated circuits, power devices, photovoltaic cells, and — less obviously — MEMS: pressure sensors, accelerometers, gyroscopes, micromirrors and microfluidics, where silicon is chosen as a structural material.

Silicon dominates electronics not because its electronic properties are the best available (gallium arsenide has higher mobility, silicon carbide a wider bandgap) but because silicon dioxide is an excellent, stable, easily grown native insulator. No competing semiconductor has an oxide that good, and the entire planar process depends on it.

Chosen over what

Against gallium arsenide: lower electron mobility, but vastly cheaper, available in larger wafers, mechanically stronger, non-toxic, and with that native oxide. GaAs wins only where speed or direct-bandgap optics demand it.

Against silicon carbide and gallium nitride in power electronics: silicon is far cheaper and mature, but limited by its narrow bandgap to lower voltage, lower temperature and lower switching frequency. The wide-bandgap materials are displacing it in traction inverters and fast chargers.

Against metals in MEMS: single-crystal silicon has effectively no mechanical hysteresis or fatigue in the classical sense, so a silicon flexure returns to exactly the same position — the reason precision sensors are built from it.

How it fails

Brittle cleavage. Single-crystal silicon is perfectly elastic to fracture at room temperature, with no plastic deformation, and cleaves preferentially on {111} planes. Wafer breakage during handling is the practical consequence, and it is why wafer thinning has limits.

Anisotropy. Young's modulus varies from roughly 130 GPa in the <100> direction to 188 GPa in <111>, so a single quoted modulus is only a directional reference. MEMS designs must account for crystal orientation explicitly.

Thermal expansion mismatch with packaging, metallisation and die-attach materials, causing stress, warpage and sensor drift — often the dominant error source in a MEMS pressure sensor.

Electromigration and gate oxide breakdown at the device level, rather than bulk material failure.

Fabrication notes

Anisotropic wet etching in KOH or TMAH proceeds far faster on {100} than {111} planes, producing the characteristic 54.74° sloped sidewalls of etched cavities. This crystallographic accident is the basis of bulk micromachining.

Deep reactive ion etching (the Bosch process) alternates etch and passivation steps to produce near-vertical walls with aspect ratios above 20:1, and is what made modern inertial sensors possible.

Wafer bonding — fusion, anodic to glass, or eutectic — allows sealed cavities and is how pressure sensors and resonators are packaged.

Purity requirements are extreme. Electronic-grade polysilicon is 9N to 11N pure; a few parts per billion of transition metal ruins minority carrier lifetime. Solar-grade silicon at 6N is a distinct and much cheaper product.