LEDs can be used for either free-space communication or for fiber communication applications. Free-space communication applications include the remote control of appliances such as television sets and stereos, and data communication between a computer and peripheral devices. LEDs used in optical fiber communication applications are suited for distances of a few km and bit rates up to about 1 Gbit/s. Most fibers used with LEDs are multimode (step-index and graded-index) fibers. However, some applications employ single-mode fibers.

LEDs for free-space communication

Free-space communication LEDs are commonly made with GaAs or GaInAs active regions and are grown on GaAs substrates. The GaInAs layer is pseudomorphic, i.e. sufficiently thin that it is coherently strained, and no dislocations are generated. The emission wavelength of GaAs and coherently strained GaInAs LEDs is limited to wavelengths in the IR ranging from 870 nm (for GaAs active regions) to about 950 nm (for GaInAs active regions).

The wavelength of free-space communication LEDs is in the infrared so that the light emitted is invisible to the human eye and does not distract. Since free-space communication usually involves transmission distances of less than 100 m, the transmission medium (air) can be considered, to a good approximation, to be lossless and dispersionless.

The total light power is an important figure of merit in free-space communication LEDs, so that the internal efficiency and the extraction efficiency need to be maximized. The emission pattern (far field) is another important parameter.

As the trend of higher efficiencies in LEDs continues, the number of possible applications increases as well. A highly interesting application with a very large potential market size is general daylight illumination in homes and offices. The field of solid-state lighting (SSL) is concerned with the development of solid-state sources for illumination applications. LEDs are inherently monochromatic emitters. However, there are several ways to generate white light using LEDs. Approaches to white-light generation based on LEDs will be covered in the current chapter, whereas approaches based on LEDs and wavelength-converting materials will be discussed in the following chapter. A pivotal discussion of the promise of solid-state lighting was given by Bergh et al. (2001). A comprehensive introduction to lighting technology using solid state sources was given by Zukauskas et al. (2002a).

In the field of general daylight illumination, devices should have the following properties (i) high efficiency, (ii) high power capability, (iii) good color-rendering capabilities (iv) high reliability, (v) low-cost manufacturability, and (vi) environmental benignity. Such properties would allow LEDs to compete with conventional illumination sources, in particular incandescent and fluorescent lamps.

Generation of white light with LEDs

Light is perceived as white light if the three types of cones located on the retina of the human eye are excited in a certain ratio, namely with similar intensity. For the case of white light, the tristimulus values are such that the location of the chromaticity point is near the center of the chromaticity diagram.

The assessment and quantification of color is referred to as colorimetry or the “science of color”. Colorimetry is closely associated with human color vision. Both colorimetry and human vision have attracted a great deal of interest that spans many centuries. For a thorough and entertaining review of the history of colorimetry including early attempts to understand color, we recommended the collection of historical reprints complied by MacAdam (1993).

The human sense of vision is very different from the human sense of hearing. If we hear two frequencies simultaneously, e.g. two frequencies generated by a musical instrument, we will be able to recognize the musical tone as having two distinct frequencies. This is not the case for optical signals and the sense of vision. Mixing two monochromatic optical signals will appear to us as one color and we are unable to recognize the original dichromatic composition of that color.

Color-matching functions and chromaticity diagram

Light causes different levels of excitation of the red, green, and blue cones. However, the sensation of color and luminous flux caused a particular light source varies slightly among different individuals. Furthermore, the sensation of color is, to some extent, a subjective quantity. For these reasons, The International Commission for Illumination (Commission Internationale de l'Eclairage, CIE) has standardized the measurement of color by means of color-matching functions and the chromaticity diagram (CIE, 1931).

LEDs may be grown on conductive and insulating substrates. Whereas the current flow is mostly vertical (normal to the substrate plane) in structures grown on conductive substrates, it is mostly lateral (horizontal) in devices grown on insulating substrates. The location and size of ohmic contacts are relevant to light extraction, because metal contacts are opaque. The current chapter discusses the current flow patterns of different device structures aimed at high extraction efficiency.

Current-spreading layer

In LEDs with thin top confinement layers, the current is injected into the active region mostly under the top electrode. Thus, light is generated under an opaque metal electrode. This results in a low extraction efficiency. This problem can be avoided with a current-spreading layer that spreads the current under the top electrode to regions not covered by the opaque top electrode.

The current-spreading layer is synonymous with the window layer. The term window layer is occasionally used to emphasize the transparent character of this layer and its ability to enhance the extraction efficiency.

The usefulness of current-spreading layers was realized during the infancy of LEDs. Nuese et al. (1969) demonstrated a substantial improvement of the optical output power in GaAsP LEDs by employing a current-spreading or window layer. The window layer is the top semiconductor layer located between the upper cladding layer and the top ohmic contact. The effect of a current-spreading layer is illustrated in Fig. 8.1.