Science Fair Project Encyclopedia
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.
|Appearance||Dark gray cubic crystals|
|Structure||Formula weight||144.64 amu|
|Lattice constant||0.56533 nm|
|State of matter at STP||solid|
|Melting point at SP||1513 K|
|Boiling point at SP||?|
|Band gap at 300K||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||Precautions|
|Decompostion products||Highly toxic arsenic fumes||SI units were used where possible.|
The electronic properties of GaAs 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. These properties have made GaAs circuitry common in mobile phones, satellite communications, microwave point-to-point links, and some radar systems.
GaAs devices also require less power than those made from silicon, an important consideration for low-power or high-density applications. This is another reason GaAs is popular in cell phone applications; less power is being fed into the amplifier circuitry as opposed to the resulting signal.
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).
The combination of high switching speed and low power consumption 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 two major advantages over GaAs. First, silicon is cheap. This is for several reasons: silicon's large wafer size (maximum of ~300mm compared to ~150mm diameter), high strength allowing for easier processing, and of course the scale of the economy.
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.
Complex layered structures of gallium arsenide in combination with Aluminum 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.
The toxicological properties of gallium arsenide have not been thoroughly investigated. However, it is considered highly toxic and carcinogenic.
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