GaAs advantages

Some electronic properties of gallium arsenide are superior to those of silicon. It has a highersaturated electron velocity and higher electron mobility, allowing gallium arsenide transistors to function at frequencies in excess of 250 GHz. Unlike silicon junctions, GaAs devices are relatively insensitive to heat owing to their wider bandgap. Also, GaAs devices tend to have lessnoise than silicon devices, especially at high frequencies. This is a result of higher carrier mobilities and lower resistive device parasitics. These properties recommend GaAs circuitry inmobile phones, satellite communications, microwave point-to-point links and higher frequency radar systems. It is used in the manufacture of Gunn diodes for generation of microwaves.

Another advantage of GaAs is that it has a direct band gap, which means that it can be used to emit light efficiently. Silicon has an indirect bandgap and so is very poor at emitting light. Nonetheless, recent advances may make siliconLEDs and lasers possible.

As a wide direct band gap material with resulting resistance to radiation damage, GaAs is an excellent material for space electronics and optical windows in high power applications.

Because of its wide bandgap, pure GaAs is highly resistive. Combined with the high dielectric constant, this property makes GaAs a very good electrical substrate and unlike Si provides natural isolation between devices and circuits. This has made it an ideal material for microwave and millimeter wave integrated circuits, MMICs, where active and essential passive components can readily be produced on a single slice of GaAs.

One of the first GaAs microprocessors was developed in the early 1980s by the RCAcorporation and was considered for the Star Wars program of the United States Department of Defense. Those processors were several times faster and several orders of magnitude moreradiation hard than silicon counterparts, but they were rather expensive.[8] Other GaAs processors were implemented by the supercomputer vendorsCray Computer Corporation, Convex, and Alliant in an attempt to stay ahead of the ever-improvingCMOS microprocessor. Cray eventually built one GaAs-based machine in the early 1990s, theCray-3, but the effort was not adequately capitalized, and the company filed for bankruptcy in 1995.

Complex layered structures of gallium arsenide in combination with aluminium arsenide (AlAs) or the alloy AlxGa1-xAs can be grown usingmolecular beam epitaxy (MBE) or usingmetalorganic vapor phase epitaxy (MOVPE). Because GaAs and AlAs have almost the samelattice constant, the layers have very little inducedstrain, which allows them to be grown almost arbitrarily thick. This allows for extremely high performance high electron mobility, HEMTtransistors and other quantum well devices.

Silicon advantages

Silicon has three major advantages over GaAs for integrated circuit manufacture. First, silicon is abundant and cheap to process. Si is highly abundant in the Earth's crust, in the form ofsilicate minerals. The economy of scale available to the silicon industry has also reduced the adoption of GaAs.

In addition, a Si crystal has an extremely stable structure mechanically and it can be grown to very large diameter boules and can be processed with very high yields. It is also a decent thermal conductor, thus enabling very dense packing of transistors that need to get rid of their heat of operation, all very desirable for design and manufacturing of very large ICs. Such good mechanical characteristics also makes it a suitable material for the rapidly developing field ofnanoelectronics.

The second major advantage of Si is the existence of a native oxide (silicon dioxide), which is used as an insulator in electronic devices. Silicon dioxide can easily be incorporated onto silicon circuits, and such layers are adherent to the underlying Si. GaAs does not have a native oxide and does not easily support a stable adherent insulating layer.

The third, and perhaps most important, advantage of silicon is that it possesses a much higher holemobility. This high mobility allows the fabrication of higher-speed P-channel field effect transistors, which are required for CMOS logic. Because they lack a fast CMOS structure, GaAs logic circuits have much higher power consumption, which has made them unable to compete with silicon logic circuits.

For manufacturing solar cells, silicon has relatively low absorptivity for the sunlight meaning about 100 micrometers of Si is needed to absorb most sunlight. Such a layer is relatively robust and easy to handle. In contrast, the absorptivity of GaAs is so high that a corresponding layer would be only a few micrometers thick and mechanically unstable.[9]

Silicon is a pure element, avoiding the problems of stoichiometric imbalance and thermal unmixing of GaAs.

Silicon has a nearly perfect lattice, impurity density is very low and allows to build very small structures (currently down to 25 nm). GaAs in contrast has a very high impurity density, which makes it difficult to build ICs with small structures, so the 500 nm process is a common process for GaAs