compound semiconductor
/ KOM-pownd SEM-ee-kun-DUK-ter /
Silicon is a one-element semiconductor — pure silicon, all the way through. But you can also build a semiconductor by pairing two or more different elements that together imitate silicon's bonding. The result is a compound semiconductor: a crystal made of more than one kind of atom, yet behaving as a single well-ordered semiconducting material.
The classic recipe combines an element with three outer electrons and one with five, so that on average each atom shares four bonds, just as silicon does. Gallium arsenide, made of gallium and arsenic, is the famous example. Other families pair elements from the second and sixth columns of the periodic table. By choosing the partners — and by blending three or four elements together — engineers can dial in band gaps and properties that no single element offers.
Compound semiconductors matter because they do jobs silicon cannot: gallium arsenide and gallium nitride emit light efficiently for LEDs and lasers and switch fast for high-frequency radios, while silicon, with the wrong kind of band gap, makes a hopeless light emitter. The honest trade-off is cost and difficulty — compounds are harder to grow defect-free, more expensive, and often scarcer in raw materials, so silicon still rules wherever its weaknesses do not matter.
Gallium arsenide combines gallium (three outer electrons) with arsenic (five) so the pair averages four bonds per atom, like silicon. It runs the lasers in DVD players and fibre-optic links, where silicon simply cannot emit light.
A group-III plus a group-V element together mimic silicon's four-bond structure.
The deep reason compounds like gallium arsenide outshine silicon as light sources is that they have a 'direct' band gap, letting an electron and hole recombine into a photon easily, whereas silicon's 'indirect' gap makes that same step clumsy and rare.