12 - Visible-spectrum LEDs

Summary

Originally, LEDs were exclusively used for low-brightness applications such as indicator lamps. In these applications, the efficiency and the overall optical power of the LED are not of primary importance. However, in more recent applications, for example traffic light applications, the light emitted by LEDs must be seen even in bright sunlight and from a considerable distance. LEDs with high efficiency and brightness are required for such applications.

In this chapter, low-brightness as well as high-brightness LEDs are discussed. GaAsP and nitrogen-doped GaAsP LEDs are suitable only for low-brightness applications. AlGaAs LEDs are suitable for low- as well as high-brightness applications. AlGaInP and GaInN LEDs are used in high-brightness applications.

The GaAsP, GaP, GaAsP:N, and GaP:N material systems

The GaAs_1−xP_x and GaAs_1−xP_x:N material system is used for emission in the red, orange, yellow, and green wavelength range. The GaAsP system is lattice mismatched to GaAs substrates, resulting in a relatively low internal quantum efficiency. As a result these LEDs are suitable for low-brightness applications only.

GaAs_1−xP_x was one of the first material systems used for visible-spectrum LEDs (Holonyak and Bevacqua 1962; Holonyak et al. 1963, 1966; Pilkuhn and Rupprecht 1965; Wolfe et al., 1965; Nuese et al. 1966). In the early 1960s, GaAs substrates were already available. Bulk growth of GaAs substrates was initiated in the 1950s and epitaxial growth by LPE and VPE started in the 1960s.