This chapter is from the book Properties of Gallium Arsenide (3rd Edition), which is comprised of 150 specially commissioned articles contributed by experts from the USA, Europe and Japan. Gallium arsenide is revolutionizing the semiconductor industry. It is a major competitor to silicon in the push for faster, higher frequency and greater bandwidth circuits. GaAs has a much higher electron mobility, has greater thermal stability and provides higher resistivity IC (integrated circuit) substrates than silicon. Moreover, it is a key material in some areas where silicon is of only minor significance; namely, the burgeoning field of heterostructures which permit combinations of digital, microwave, millimetre wave and optical circuits. Gallium arsenide is also widely used in LEDs (light emitting diodes) and solar cells.