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Gallium Arsenide

GaAs inorganic
Molar Mass
144.645 g/mol
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Properties

StateSolid (crystalline)
ColorDark gray to black with metallic luster
SolubilityInsoluble in water and non-oxidizing acids; soluble in aqua regia and HF/HNO3
Melting Point1238 °C

About Gallium Arsenide

Gallium arsenide (GaAs, 144.645 g/mol) is the prototype direct-bandgap III-V semiconductor — a dark-gray brittle crystal with the cubic zinc-blende structure (Ga and As on interpenetrating fcc sublattices, every Ga tetrahedrally bonded to four As and vice versa) and a 1.43 eV direct bandgap that places its absorption edge at 870 nm in the near-infrared. The direct gap is what separates GaAs from silicon mechanistically: in silicon (1.12 eV indirect), an absorbed photon needs both an electron and a phonon to make a vertical k-space transition, which makes silicon a poor light absorber and a hopeless light emitter. In GaAs, the conduction-band minimum sits directly above the valence-band maximum at the Γ point, so absorption and emission are both first-order radiative processes — and that single property is the whole reason GaAs dominates LEDs, laser diodes, and high-efficiency solar cells. The other big GaAs advantage is electron mobility: 8500 cm²/V·s versus silicon's 1400, which lets GaAs HEMTs and pHEMTs operate cleanly into the mm-wave band where silicon transistors run out of speed. Industrial single-crystal GaAs is grown by the LEC (liquid-encapsulated Czochralski) method, pulling 4- to 8-inch boules from a Ga-rich melt under a B2O3 cap that stops volatile arsenic from escaping, then sliced into wafers and doped with Si (n-type), Zn (p-type), or C (high-resistivity) for specific device flows. Heteroepitaxial AlGaAs/GaAs structures grown by MOCVD or MBE are the foundation of every red CD/DVD laser, every multijunction satellite solar cell, and most cellphone power amplifiers.

Where you'll encounter it

If your phone connects to a 5G cell, the RF power amplifier on the antenna front-end is almost certainly a GaAs pHEMT or HBT die — Skyworks, Qorvo, and Murata between them ship billions a year. In a satellite, the solar arrays on most modern commercial buses (Starlink uses Si, but GEO comsats and JWST-class science missions use multijunction GaAs) are triple-junction InGaP/GaAs/Ge cells where each layer captures a different slice of the solar spectrum. In a teaching lab the giveaway that a wafer is GaAs and not Si is the dark gray color and the smell — broken GaAs releases trace AsH3 in humid air.

Common Uses

  • Triple-junction InGaP/GaAs/Ge solar cells for GEO comsats and Mars rovers
  • 650 nm AlGaAs laser diodes in CD/DVD pickups and supermarket barcode scanners
  • GaAs pHEMT and HBT RF power amplifiers in 5G cellphone front-end modules
  • X-band and Ka-band MMIC transmitters in radar and satellite phased arrays
  • Near-infrared LEDs at 850 nm for IR remote controls and TOF camera illuminators
  • Hall-effect sensors for current sensing and brushless DC motor commutation
  • Vertical-cavity surface-emitting lasers (VCSELs) in datacom transceivers
  • Schottky diodes for mm-wave mixers in radio astronomy receivers

Safety Information

DANGER — contains arsenic. GHS: Carcinogenicity (Cat 1A, H350 — IARC Group 1, carcinogenic to humans), Reproductive toxicity (Cat 1A, H360), Specific target organ toxicity repeated exposure (Cat 1, respiratory tract and blood, H372), Aquatic chronic toxicity (Cat 1, H410). H-codes H350, H360, H372, H410. OSHA PEL 0.01 mg/m3 (as As, 8-hr TWA), NIOSH IDLH 5 mg/m3 (as As). Bulk wafer is low hazard if intact, but any process that generates dust — sawing, lapping, CMP, breaking — releases As-containing particulates that cross the skin and lungs. Wafer fab gas streams must be scrubbed for AsH3 and As-oxide; broken wafers go into double-bagged hazardous-waste streams. Do not heat above 600 °C in air without a scrubber: GaAs decomposes liberating As4 vapor, which is acutely toxic. Industrial-hygiene controls in semiconductor fabs are extensive and well-documented — outside that environment, do not handle.

This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.

Constituent Elements

Ga — Gallium As — Arsenic

Frequently Asked Questions

What is the molar mass of gallium arsenide?
GaAs has a molar mass of 144.645 g/mol — Ga (69.723) + As (74.922). Density is 5.32 g/cm³, considerably heavier than silicon's 2.33 g/cm³, which is one reason GaAs wafers are usually thicker than the Si wafers of the same diameter — the extra mass per area helps with mechanical handling at the same fragility. A standard 6-inch GaAs wafer at 0.625 mm thickness weighs about 19 g, twice what an Si wafer of the same dimensions weighs.
Why is GaAs preferred over silicon for high-efficiency solar cells?
Two reasons. First, GaAs has a direct bandgap of 1.43 eV — almost exactly the Shockley-Queisser optimum (1.34 eV) for single-junction conversion of the AM1.5 solar spectrum, so the radiative-limit efficiency is around 33% vs 29% for silicon. Second, the direct gap means GaAs absorbs sunlight in a layer about 100× thinner than silicon — 2 µm of GaAs absorbs what 200 µm of Si absorbs, which makes thin-film and flexible cells practical. Single-junction GaAs cells from Alta Devices reach 29.1% efficiency under one-sun AM1.5; multijunction InGaP/GaAs/Ge stacks under concentrator illumination exceed 47%. For spacecraft, where every watt-per-kg matters, GaAs displaces silicon completely.
Why are GaAs transistors used for cellphone RF?
Electron mobility in GaAs is 8500 cm²/V·s — about six times higher than silicon's 1400 — which directly translates into higher transit frequency (fT) and faster switching for the same gate length. GaAs pHEMTs and HBTs operate cleanly to 100 GHz, where silicon CMOS struggles past 30 GHz without exotic process tricks. Cellphone power amplifiers also benefit from the high breakdown voltage and high power density of GaAs, letting one die push out 1-3 W of clean RF where a silicon die would need to be much larger. The cost per wafer is higher than silicon, but GaAs dies are tiny — the economics work out as long as the volume justifies a dedicated 6-inch GaAs fab.