9.11.1 Semiconductor Properties and Composition

A pure semiconductor such as silicon has uniform properties, with the same band gap and lattice properties. A simple binary semiconductor such as GaAs or InP also has uniform properties.

You can get other properties by adding one or two other elements from the same groups of the periodic table to a compound semiconductor. This is equivalent to blending different materials to get a new compound with intermediate characteristics. For example, a mixture of 20% AlAs and 80% GaAs has band gap energy and lattice spacing partway between pure AlAs and pure GaAs. The formula for such a compound can be written Ga_0.8Al_0.2As or simply GaAlAs. (The order of the two elements that replace each other-aluminum and gallium-is arbitrary; some people write AlGaAs.)

Things get more complex if the compound semiconductor contains four elements. Depending on the relative concentrations of the elements, such "quaternary" semiconductors can have properties somewhere in a broad range. For example, as shown in Figure 9-20, InGaAsP (more precisely In_xGa_{1 x}As_{1 y}P_y) can have lattice spacing and bandgap within an area defined by four possible binary compounds: InAs, InP, GaAs, and GaP. The material's properties can vary within the entire space because the In Ga and As P ratios can be adjusted independently. (The odd shape of the area in the figure arises because pure GaP is an indirect band gap material, unsuitable for lasers and somewhat different from the other binary compounds. The dashed line indicates the indirect band gap region.) Plots like Figure 9-20 are very helpful in understanding semiconductor properties; they are commonly drawn with lattice spacing on the bottom, but some have lattice spacing on the sides and band gap on the bottom.

The band gaps of different layers in a semiconductor device do not have to match, but the lattice spacing must be close to avoid flaws that degrade device performance. This means that all compositions in a bulk InGaAsP structure must fall roughly on a vertical line in Figure 9-20. The use of thin strained layers relaxes this constraint somewhat, as mentioned earlier.

Semiconductor laser structures are grown on substrates of simple-to-produce binary materials such as InP or GaAs, which

account for most of the device volume. The choice of substrate restricts composition of other layers to materials with similar lattice spacing.