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Minerals; Inorganic; Column Three
Description of the substance
Gallium arsenide (GaAs) is a chemical compound composed of gallium and arsenic. It is an important semiconductor, and is used to make devices such as microwave frequency integrated circuits (ie, MMICs), infrared light-emitting diodes and laser diodes.
Name gallium arsenide
Chemical Formula GaAs
Appearance Dark gray cubic crystals
CAS number 1303-00-0
Formula weight 144.64 u
Lattice constant 0.56533 nm
Crystal structure zincblende
State of matter at STP solid
Melting point at SP 1513 K
Boiling point at SP ?
Specific gravity 5.318
Band gap at 300 K 1.424 eV
Electron effective mass 0.067 me
Light hole effective mass 0.082 me
Heavy hole effective mass 0.45 me
Electron mobility at 300 K 9200 cm2/(V·s)
Hole mobility at 300 K 400 cm2/(V·s)
Decomposition products Highly toxic arsenic fumes
SI units were used where possible.
GaAs has some electronic properties which are superior to silicon's. It has a higher saturated electron velocity and higher electron mobility, allowing it to function at frequencies in excess of 250 GHz. Also, GaAs devices generate less noise than silicon devices when operated at high frequencies. They can also be operated at higher power levels than the equivalent silicon device because they have higher breakdown voltages. These properties have made GaAs circuitry common in mobile phones, satellite communications, microwave point-to-point links, and some radar systems.
Another advantage of GaAs is that it has a direct bandgap. This means that it can be used to emit light. Silicon has an indirect bandgap, and so is very poor at emitting light. (Nonetheless, recent advances may make silicon LEDs and lasers possible).
Its high switching speed makes GaAs seemingly ideal for computer uses, and for some time in the 1980s many thought that it was only a matter of time before the entire market switched off of silicon. The first to attempt this were the supercomputer vendors, with Cray, Convex and Alliant all running GaAs projects in order to stay ahead of the ever-improving CMOS microprocessor. The closest to production was the Cray-3, built to one example in the early 1990s, but the effort was so costly the venture failed and the company filed for bankruptcy in 1995.
Silicon has three major advantages over GaAs. First, silicon is cheap. This is for several reasons: silicon's large wafer size (maximum of ~300 mm compared to ~150 mm diameter), its higher strength allowing for easier processing, its tremendous abundance in the Earth's crust, and economy of scale from the silicon industry already in place.
The second major advantage is the existence of silicon dioxide—one of the best known insulators of any kind. Silicon dioxide can easily be incorporated into silicon circuits wherever a good insulator is required. GaAs circuits must either use the intrinsic semiconductor itself or silicon nitride; neither comes close to the extremely good properties of silicon dioxide.
The third, and perhaps most important, advantage is that silicon possesses a much higher hole mobility. This allows the fabrication of higher-speed P-channel field effect transistors, which are required for CMOS logic. A lack of a fast CMOS structure means that GaAs logic circuits have much higher power consumption, which has made them unable to compete with silicon logic circuits.
Complex layered structures of gallium arsenide in combination with aluminium arsenide (AlAs) or the alloy AlxGa1-xAs can be grown using molecular beam epitaxy (MBE). Because GaAs and AlAs have almost the same lattice constant, the layers have very little induced strain, which allows them to be grown almost arbitrarily thick.
Another important application of Gallium Arsenide is for the realisation of high efficiency solar cell. By combining gallium arsenide with germanium and indium gallium phosphide, it is possible to realise a triple junction solar cell which holds the record efficiency of over 32% and can operate also with concentrated light up to 2.000 suns. This kind of solar cell was used to power the robots Spirit and Opportunity, which are exploring Mars surface. Also many solar cars utilize GaAs in solar arrays.
Single crystals of gallium arsenide are manufactured by the Bridgeman technique, as the Czochralski process is difficult for this material.
The toxicological properties of gallium arsenide have not been thoroughly investigated. However, it is considered highly toxic and carcinogenic.