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.

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 GaAs1−xPx and GaAs1−xPx: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.

GaAs1−xPx 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.

LEDs are used in communication systems transmitting low and medium data rates (< 1 Gbit/s) over short and medium distances (< 10 km). These communication systems are based on either guided light waves (Keiser, 1999; Neyer et al., 1999; Hecht, 2001; Mynbaev and Scheiner, 2001; Kibler et al., 2004) or free-space waves (Carruthers, 2002; Heatley et al., 1998; Kahn and Barry, 2001). In guided-wave communication, individual optical fibers or fiber bundles are used as the transmission medium and LED-based optical communication links are limited to distances of a few kilometers. Optical fiber systems include silica and plastic optical fibers. Free-space communication is usually limited to a room, even though longer distances are possible. In this chapter we discuss the characteristics of transmission media used for LED communication.

Types of optical fibers

The cross section of optical fibers consists of a circular core region surrounded by a cladding region. The core region has a higher refractive index than the cladding region. Typically, the core refractive index is about 1% higher than the cladding refractive index. Light propagating in the core is guided inside the core by means of total internal reflection. The condition of total internal reflection can be inferred from Snell's law. A light ray is totally internally reflected whenever it is incident at the core–cladding boundary.

The temperature of the active region crystal lattice, frequently referred to as the junction temperature, is a critical parameter. The junction temperature is relevant for several reasons. Firstly, the internal quantum efficiency depends on the junction temperature. Secondly, hightemperature operation shortens the device lifetime. Thirdly, a high device temperature can lead to degradation of the encapsulant. It is therefore desirable to know the junction temperature as a function of the drive current.

Heat can be generated in the contacts, cladding layers, and the active region. At low current levels, heat generation in the parasitic resistances of contacts and cladding layers is small due to the I2R dependence of Joule heating. The dominant heat source at low current levels is the active region, where heat is created by non-radiative recombination. At high current levels, the contribution of parasitics becomes increasingly important and can even dominate.

There are several different ways to measure the junction temperature, which include micro-Raman spectroscopy (Todoroki et al., 1985), threshold voltage (Abdelkader et al., 1992), thermal resistance (Murata and Nakada, 1992), photothermal reflectance microscopy (Epperlein, 1990), electroluminescence (Epperlein and Bona, 1993), photoluminescence (Hall et al., 1992) and a non-contact method based on the peak ratio of a dichromatic source (Gu and Narendran, 2003). Most methods are indirect methods that infer the junction temperature from an easily measurable parameter.