Semiconductor Basics

Semiconductor lasers are produced using crystalline inorganic semiconductor materials.  The most widely-used semiconductor in electronics is silicon, but unfortunately ordinary silicon is not useful as a light-emitting material, for reasons we’ll discuss later.  Instead, semiconductor lasers commonly use compound semiconductors, such as III-V semiconductors, which have one element drawn from column III of the periodic table and another drawn from column V.  Binary III-V compound semiconductors include GaAs, InAs, and AlAs.

Single crystals of these III-V compounds can be grown using techniques such as Metallorganic Chemical Vapor Deposition (MOCVD) and Molecular Beam Expitaxy (MBE).  In a crystal, the atoms are arranged in a periodically repeating pattern throughout the solid.  This periodic arrangement of atoms strongly influences the behavior of electrons in the solid, leading to their desirable properties for electron transport and light emission.  The arrangement of atoms common to many III-V semiconductors is called the Zincblende structure.  The 3d arrangement of atoms in crystals can be difficult to visualize, but luckily there are lots of great visualization tools to assist, for example:  (Note that they are specifically visualizing ZnS, but the same structure applies to GaAs and others).

If we compare GaAs to AlAs, we’ll see that the Ga atoms sit on the same crystal lattice sites in GaAs as the Al atoms sit on in AlAs.  And, in fact, we could replace some of the Ga atoms in GaAs with Al atoms, thus making a ternary compound: AlGaAs.  We could similarly replace some of the Ga atoms with In, thus making InGaAs.  And we could go further by replacing some of the As atoms in InGaAs with P, thus making a quaternary compound, InGaAsP.  As you can imagine, there are a very wide range of materials we could produce this way, which is useful for engineering the material properties for particular applications.