What are III-V semiconductors?
III-V semiconductors are compounds created by combining elements from Group 3 (B, Al, Ga, In) and Group 5 (N, P, As, Sb). They have gained prominence as semiconductor materials due to their bandgap structure. Silicon, the most important semiconductor materials, has what is called an “indirect” bandgap. This essentially means there are multiple pathways which an excited electron must move through to reach its ground state in the valence band. The time scale for this process is very slow, making the silicon a poor choice for devices which emit light, like LEDs and lasers. III-V semiconductors, like GaN, GaAs, and InP, have a “direct” bandgap. Direct bandgaps require only one pathway for radiative recombination, making the process quite fast and giving these materials the ability to directly emit a photon. This makes III-V semiconductors well-suited for optoelectronic applications.
The most common III-V semiconductors are GaN, GaAs, and InP. Below are some examples of optoelectronics applications for each of these materials.
GaN – Bringing blue
Gallium nitride (GaN) has a hard structure and bandgap energy of 3.4 eV. This material, and materials derived from it, have been the bedrock for devices emitting short wavelengths for decades. Pure GaN is the basis for violet-emitting laser diodes, and combinations of other elements, like Al and In, allow from blue-green to red. The introduction of AlGaN blue-emitting diodes was a breakthrough in LED technology to allow full color displays. GaN is also an important material for replacing silicon in power devices.
GaAs – Into the IR
Of all the III-V semiconductors, GaAs has the highest electron mobility, which has placed it at the top of go-to materials for integrated circuits operating at high frequencies, like those in RF devices. For optoelectronics, GaAs is the basis for devices operating in the infrared region with a bandgap of 1.42 eV (~890 nm). The ternary (three-element) compounds, InGaAs and AlGaAs are used for IR-emitting lasers and IR photodetectors, respectively. Interestingly, compound semiconductors based on GaAs have been engineered to emit photons via subband transitions, allowing photon emission in the far IR, up to wavelengths of 8 microns.
InP – Beyond LEDs and lasers
InP is another small bandgap (1.34 eV) III-V semiconductor that operates in long wavelength regions. Unlike GaN and GaAs, it is not typically used for emitting devices, but rather as an amplifier or modulator of incoming waves. Often, InP is the epitaxial substrate which other systems can be grown on, like InGaAsP or InAlGaAs, which get denoted InGaAsP/InP and InAlGaAs/InP. These quaternary systems have been engineered to modulate 1.3 micrometer and 1.5 micrometer light, respectively.
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