gallium arsenide (thing)

Gallium arsenide (GaAs) is the most important non-silicon semiconductor. GaAs has many electrical properties superior to those of silicon. However, GaAs suffers from a few drawbacks that ensure that silicon (Si) will maintain its dominance in the future.

Useful properties of gallium arsenide

• GaAs is a direct bandgap semiconductor, while Si is an indirect bandgap semiconductor. Direct bandgap semiconductors can be used to make semiconductor lasers and light emitting diodes (LED's).
• GaAs can form high-quality heterojunctions with a large number of alloys, such as InGaAs, AlGaAs, GaAsP, and GaAsSb. These heterojunctions can be used to construct quantum wells, which are used for semiconductor lasers.
• GaAs has a peak saturation velocity of 2x10⁷ cm/sec (twice that of Si) and an electron mobility of 8000 to 9000 cm²/Vsec (10-20 times that of Si). These properties suggest that GaAs devices could switch faster than their Si counterparts.
• GaAs has a larger bandgap than Si, so it is more immune to the radiation effects present in space and military applications.

Unfortunate properties of gallium arsenide

• No high-quality insulators (counterparts to silicon dioxide) grow on GaAs. The insulator/GaAs interfaces have high levels of defects called "traps" that make GaAs MOSFET's impossible. This is probably the main reason that GaAs will never supplant Si as the semiconductor-of-choice for integrated circuits. GaAs transistors are usually either MESFET's or HEMT's. These transistors have some advantages over MOSFET's but their disadvantages outweigh the advantages.
• GaAs is extremely brittle. For this reason, GaAs wafers are typically smaller than three inches in diameter, as opposed to silicon wafers, which commonly have diameters of eight or more inches. Since the area on a wafer is proportional to the square of its diameter, Si has a huge cost advantage over GaAs.
• The hole mobility in GaAs is 15-20 times smaller than the electron mobility, which makes CMOS-like circuits unattractive.

Some material constants of gallium arsenide

• Energy bandgap: 1.40eV
• Undoped conductivity: 10^-6 siemen/meter
• Lattice constant: 0.565nm
• Electron effective mass (fraction of free electron mass): 0.067
• Hole effective mass: 0.650

I used these two references:

• Digital Integrated Circuits by Jan Rabaey
• CRC Handbook of Electromagnetic Materials by Perambur Neelakanta