Semiconductor heterojunctions and nanostructures have been investigated for their applications in electronic and optoelectronic devices. This chapter is directed toward optoelectronic devices such as detectors and emitters. There is a myriad of applications for the optoelectronic devices including 1.31- and 1.55-μm optical communications where the silica fibers exhibit the lowest losses, terahertz application, infrared and long wavelength infrared detectors, multijunctions solar cells, and so on. One of the mechanisms used to generate light from a semiconductor is the radiative recombination of electrons and holes across the fundamental band gap, which gives rise to photon emission. Soon after the invention of laser, p–n junction GaAs lasers were demonstrated with an emission in the 0.827–0.886-μm spectral range, which is basically limited by the band gap of the GaAs material. This spectral range is outside the energy spectrum that is visible to the human eye. Substantial research has been performed since then toward the development and production of emitters in the visible, ultraviolet, and infrared spectral regions (see Fig. 7.1 for various spectral region limits). For example, recent research efforts in III-nitride semiconductor materials led to the production of blue and green light-emitting diodes (LEDs) and diode lasers. Further research has pushed the performance of the III-nitride materials to the ultraviolet and far ultraviolet ...