Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo

Light-Emitting Diode, LED

Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_131

Synonyms

Definition

Light source that produces light as a result of recombination of positively charged holes and negatively charged electrons at the junction of inorganic, solid-state, p- and n-type semiconductor material.

Solid-State LED Light Sources

LEDs are solid-state radiators where the light is created inside solid-state material [2, 3, 4]. Common semiconductor diode chips, used today in so many electrical circuits, all use much the same technology. The light-radiating diode versions are called light-emitting diodes or LEDs. The name LED is commonly used for light-emitting diodes made of inorganic semiconductor material; they are point light sources. Light-emitting diodes made of organic semiconductor material are referred to as OLEDs; they are planar light sources. A separate chapter deals with OLEDs.

Until the mid-1990s of the last century, LEDs had a low lumen output and low efficiency, making them only suitable as small indicator lamps (e.g., in electrical appliances). Today, the efficacy of LEDs is comparable to that of gas discharge lamps. The lumen output of a single LED can be more than 1,000 lm. To distinguish these LEDS from the indicator type of LEDs, they are referred to as high-brightness or high-power LEDs (Fig. 1). Further improvements in high-brightness LEDs are expected, ultimately leading to efficacies of probably slightly more than 200 lm/W (for white-light LEDs). This is approximately twice the efficacy of today’s most efficient white-light gas discharge lamps. With its light-emitting surface of some 0.5–5 mm2, an individual LED chip represents the smallest artificial light source. LEDs have a long lifetime and are, given the solid-state material, extremely sturdy. They are available in white and in colored-light versions. The colored versions are extensively used in traffic signs. Colored versions were also the first ones to be used on a large scale for lighting: specifically, the exterior floodlighting of buildings and monuments. Both the efficacy and the color quality of white LEDs have been improved so much that they are now used in many different lighting applications, including road lighting, indoor accent lighting, domestic lighting, and automobile lighting. Examples of the use of LEDs for office lighting and outdoor sports lighting can also be found. Given the potential for a further increase in their efficacy, the number of LED applications is bound to increase. Their small size and their availability in many different colors and the easiness of lighting control, both in terms of dimming and color changing, are properties that permit of completely new applications.
Light-Emitting Diode, LED, Fig. 1

Indicator lamp LEDs (left) and a high-power LED (right)

Working Principle

Principle of Solid-State Radiation

Like any diode, an LED consists of layers of p-type and n-type of semiconductor material. The n-type of material has an excess of negatively charged electrons, whereas the p-type material has a deficiency of electrons, viz., positively charged holes. Applying a voltage across the p-n semiconductor layer pushes the n- and p-type atoms towards the junction of the two materials (Fig. 2). Here the n-type of atoms “donate” their excess electrons to a p-type of atom that is deficient in electrons. This process is called recombination. In doing so, the electrons move from a high level of energy to a lower one, the energy difference being emitted as light. The wavelength of the light is dependent on the energy-level difference between the p and n materials, which in turn depends on the semiconductor material used: different semiconductor materials emit different wavelengths, and thus different colors, of light. The process is very much like the process of excited electrons in a gas discharge falling back to their original orbit with a lower energy level while emitting light. In this sense it is surprising that the expression “solid-state discharge” and “solid-state discharge lamp” is hardly ever used. Not all recombinations result in light emission. Some recombinations are non-radiative and just heat up the solid material. This limits the efficiency of light creation. A further limitation of the efficiency is caused by absorption of light in the solid material of the chip itself. Improvements in radiative-recombination efficiency and light-extraction efficiency have been the most important reasons for the dramatic improvements of efficacy of LEDs during the past decade. Further improvements will be sure to further greatly increase the efficacy of LEDs over the coming decade.
Light-Emitting Diode, LED, Fig. 2

Principle of operation of solid-state radiators

Like gas discharge lamps, solid-state lamps cannot function when they are operated direct from the mains supply voltage. Solid-state light sources are low-voltage rectifiers that allow current to pass in one direction only. This means that the AC mains supply has to be transformed to low voltage and then rectified into a DC supply. Small fluctuations in supply voltage cause large variations in current that can damage the light source. High-power LEDs need an electronic driver to obtain a constant current characteristic.

Principle of White LEDs

The spectrum of a single LED is always narrow. Consequently, its light is colored. White LED light can nevertheless be obtained by combining three (or more) differently colored LED chips. A common method is to combine red, green, and blue LED chips into a single module to produce white light (RGB LED, Fig. 3). Sometimes the three chips are mounted in the same LED package (Fig. 4). The color rendering of an “RGB white-light” system is not good, since large areas of the full color spectrum are not included in its light. Sometimes an amber color chip is added to the RGB combination to improve the color quality of the light (RGBA LED). Research is going on to produce single, multilayer LED chips, each layer producing a specific color of light. A single LED producing red, green, and blue light would therefore result in white light.
Light-Emitting Diode, LED, Fig. 3

White light by combining red, green, and blue LED light [1]

Light-Emitting Diode, LED, Fig. 4

RGB chips mounted in a same LED package

Good-quality white light, which is especially important when it comes to providing good color rendering, is obtained by using a blue LED chip in combination with fluorescent material that converts much of the blue light into light of different wavelengths spread over almost the whole visible spectrum. In LED technology it is customary to call such fluorescent materials, phosphors: hence white LEDs based on this principle are called “white-phosphor LEDs.” By mixing different phosphors in different proportions, white LEDs producing different tints of white light with different color-rendering capabilities can be made.

Materials and Construction

The LED chip is embedded in a larger structure for mechanical protection, for the electrical connections, for thermal management, and for efficient light out-coupling. The main parts of an LED are (Fig. 5):
Light-Emitting Diode, LED, Fig. 5

The main components of a high-brightness LED [1]

  • Semiconductor LED chip

  • Reflector cup

  • Supporting body

  • Electrodes and bond wires

  • Heat sink

  • Primary optics

  • Phosphors (for white LEDs)

Semiconductor Chip Material

The p-n semiconductor sandwich forms the heart of the LED and is called the LED chip or die. The semiconductor material used determines the wavelength and thus the color of the light emitted. For LEDs, compound semiconductor material is used, which is composed of different crystalline solids. These are doped with very small quantities of other elements (impurities) to give their typical n and p properties. For the colors blue, green, and cyan, the elements indium, gallium, and nitride (InGaN) are used in different compositions (Fig. 6, left). The elements aluminum, indium, gallium, and phosphide (ALInGaP) are used to produce the colors amber, orange, and red (Fig. 6, right). It was the Japanese Nakamura who, in 1993, succeeded for the first time in producing a blue LED suitable for mass production. This was the “missing link” that enabled the production of white-phosphor LEDs and the production of white LED light on the basis of mixing the light of red, green, and blue LEDS. As can be seen from Fig. 6, today a very small area of the spectrum, in greenish yellow, is still missing.
Light-Emitting Diode, LED, Fig. 6

The main semiconductor material elements used in high-brightness LEDs, with examples of the corresponding light colors [1]

Shape of the Chip

Unfortunately, the LED chip is a “photon or light trap”: that is to say, much of the light emitted within the chip is internally reflected by its surfaces (borders between the material and air) and, ultimately, after multi-reflections, absorbed in the material (heating up the material). Only light that hits the outer surface more or less perpendicularly (approximately 20°) can leave the material. By giving the chip a specific shape and by keeping it thin, the so-called light-extraction efficiency can be improved. Figure 7 gives an example of such a specifically shaped chip.
Light-Emitting Diode, LED, Fig. 7

An example of a specifically shaped LED chip that improves light-extraction efficiency. On top of the chip, the anode with its bond wire can be seen [1]

Reflector Cup

In many cases, the chip is placed in a reflector cup which, because of its shape, helps to direct the light in an upwards direction. Highly reflective material is used: for example, metal or ceramic material.

Primary Optics

The silicon lens on top of the LED chip serves as protection for the chip. More importantly, it helps in increasing the light extraction from the chip and as such is essential for a high lumen efficacy of the LED. This is because by introducing a lens medium between the chip and the air with a refractive index value between that of air and that of the chip material, the angle over which light can escape from the chip increases.

Electrodes and Bond Wires

In order to be able to apply power to the chip, the p and n parts of the chip have metal contacts called electrodes. Bond wires connect the electrodes with the electrical connections. They are usually gold wires. Since the electrodes intercept light leaving the chip, the dimensioning of the electrodes and bond wires, especially on the side of the main light-escape route, is one of the factors that determines the light efficiency of the LED.

Heat Sink

LEDs do not radiate infrared radiation and consequently give a cool beam of light. However, this does not mean that they do not generate heat. Non-radiative recombinations of electrons and holes in the p-n sandwich, and light trapped in the chip, do heat up the chip. The larger the power of the chip and the lower its luminous efficacy, the higher is this heating effect. The higher the temperature of the p-n junction in the chip, the lower the light output of the chip. A too-high chip temperature also seriously shortens LED life, and it also slightly shifts the emitted wavelength and thus the color of the LED. Effective thermal management is therefore critical for a proper functioning of LEDs. All high-power high-brightness LEDs therefore have a heat sink of high-thermal-conductivity material (like aluminum or copper) on their rear side to conduct the heat away from the chip towards the outside world through the luminaire housing. LED luminaires must therefore incorporate in their design thermal conduction and convection features (such as cooling fins Fig. 8) to dissipate the heat to the immediate surroundings. For retrofit LED lamps (LED bulbs), the size of the heat sink is limited by the size of the bulb. The heat sink therefore has a limited capacity, thus limiting the power of the retrofit LED bulbs.
Light-Emitting Diode, LED, Fig. 8

Examples of luminaires with cooling fins

Phosphors

As mentioned above, the most important method for producing white light with LEDs is by applying a phosphor coating to a blue-light LED that converts part of the blue light into longer-wavelength, green, yellow, and red light. Different compositions of different phosphors are used to produce white light of different color tints. Since the basis of the process is the blue light of the chip, the final efficacy will become higher as more blue is kept in the light. However, this implies a high color temperature (cool-white light) and relatively poor color rendering. If, with a different phosphor composition, more blue light is converted, the color quality will improve at the expense of a somewhat-lower efficacy.

Phosphors Applied Direct to the Chip
The phosphor is often applied on or very near to the blue LED chip (Fig. 9). The thickness of the layer has to be very uniform as a variation in thickness of the phosphor layer causes a variation of the color temperature in the light beam.
Light-Emitting Diode, LED, Fig. 9

Principle of creating white light with a blue-light chip covered with phosphor

Remote Phosphor
In the case of multi-LED units, the phosphor is sometimes applied at a greater distance from the LEDs. Such modules are called remote-phosphor LED modules (Fig. 10). Here, a number of blue LEDs are placed inside a mixing chamber of high and diffuse reflective material. The phosphor layer, positioned remotely from the LEDs on the bottom of the chamber, converts the blue light of the chips into white light. In this way, thanks to the mixing process, small differences in light output and or color of individual chips are not visible. The risk for disturbing glare is also reduced because the light intensity from the large-sized phosphor layer is much lower than the intensities of small, individual LEDs. In normal phosphor LEDs the hot LED heats up the phosphor that is applied on the LED itself, reducing its conversion efficiency. Heating of remote positioned phosphor is much less and thus the conversion efficiency of remote-phosphor systems is higher. In normal phosphor LEDs a relatively large amount of light from the LED is reflected back from the phosphor towards the LED and absorbed there. In the remote-phosphor situation, most of the light reflected back from the phosphor layer takes part in the mixing process without reaching the LEDs, thus further increasing the final efficiency of the system. The phosphors used for the blue-light conversion appear yellow when they are not activated, that is to say, when the LED is not switched on (see Fig. 10 right). This sometimes makes people think, erroneously, that the blue light is filtered through a yellow filter. It is really wavelength conversion and not light filtering that takes place.
Light-Emitting Diode, LED, Fig. 10

Principle of a remote-phosphor LED module creating white light [1]

LED Cluster Modules

The luminous flux of one individual LED is quite low compared to that of most conventional light sources. Multiple LEDs are therefore often mounted on a printed circuit board (PCB) to obtain an LED module emitting a high luminous flux (Fig. 11). The PCB establishes the electrical connections between all components and the external electrical driver. The PCB must also conduct the heat from the heat sinks of the LEDs to the outside world.
Light-Emitting Diode, LED, Fig. 11

LED cluster module with multiple LEDs mounted on a printed circuit board

Properties

Temperature of the Chip Junction

It has already been mentioned that with rising temperature of the p-n chip junction, the performance of LEDs decreases: particularly the light output and lifetime. The performance data are usually specified for a junction temperature (Tj) of 25 °C. However, under normal operating conditions, a junction temperature of 60–90 °C is easily obtained. Depending on LED type, the lumen output falls to 60–90 % when the junction temperature increases from 25 to 80 °C. Amber and red LEDs are the most sensitive to changes in junction temperature, and blue LEDs the least.

Binning

The mass production of LEDs results in LEDs of the same type varying in color, light output, and voltage. In order to ensure that LEDs nevertheless conform to specification, LED manufacturers use a process called binning in their production process. At the end of the manufacturing process, LED properties are measured and LEDs are subsequently sorted into subclasses or “bins” of defined properties. As far as the color quality is concerned, the tolerances in these definitions (based on MacAdam ellipses) are such that visible differences in color between LEDs from the same bin are minimized. As the definition of a bin does not change with time, the same quality is also assured from production run to production run.

With the advancement of knowledge concerning LED materials and the mass-production process, it may be expected that binning will ultimately no longer be required (“binning-free LEDs”).

Energy Balance

The energy balance of an LED is much easier to specify than that of conventional light sources. This is because no energy is radiated in the UV and infrared region of the spectrum, which means that the energy balance comprises only visible radiant energy and heat energy. Today, white LEDs transform 20–30 % of the input power into visible light and the remaining part into heat. The light percentage comes close to that of fluorescent lamps and will soon supersede it.

System Luminous Efficacy

Sometimes, luminous efficacies are specified for the bare chip. It is evident that the ancillary devices described in the previous sections, which are essential for a proper functioning of LEDs, absorb light. The only realistic thing to do, therefore, is to specify luminous efficacy (and light output as well) for the total LED package. As with most conventional lamps, the luminous efficacy of LEDs is dependent on the power of the LED and on the color quality of the light it produces. Higher-power LEDs have higher efficacies, while those with better color rendering have lower efficacies. Today cool-white LEDs are commercially available in efficacies up to some 150 lm/W and warm-white LEDs with color-rendering indices larger than 80 in efficacies of around 130 lm/W (all lm/W values include driver losses). Retrofit LED bulbs with warm-white light (around 2,700 K) and color-rendering index better than 80 are available in efficacies up to 80 lm/W. Here, too, cooler-white versions are slightly more efficient.

As discussed at the beginning of this chapter, in coming years it may be expected to see further important improvements in luminous efficacies for white LEDs – even up to slightly more than 200 lm/W.

Lumen-Package Range

Today, single LEDs exist in lumen packages varying from a few lumen (indicator lamps) to more than 1,000 lm. In the latter case, severe screening is called for to restrict glare, because so much light comes from such a very small light-emitting surface. By mounting multi-LEDs on a printed circuit board, LED modules can be achieved with much larger lumen packages.

Color Characteristics

Principally, LEDs have a quasi-monochromatic, narrow-band spectrum. Figure 12 shows the spectra of blue, green, and red LEDs. The half-maximum width of the spectrum bands is smaller than ca. 50 nm. It has been already been shown what colors can be produced by using different semiconductor materials (see Fig. 6). With some LED modules, different colored LEDs can be controlled (dimmed) individually. With such LED modules (e.g., making use of RGB LEDs) the color of the light can be dynamically changed from white to all colors of the spectrum.
Light-Emitting Diode, LED, Fig. 12

The relative spectral power distributions of a typical blue, green, and red LED

White phosphor LEDs can have a near-continuous spectrum (Figs. 13 and 14). By applying different phosphors, white light within the color temperature range of 2,700–10,000 K can be produced. Often, the higher-color-temperature versions have only moderate color rendering (Ra between 50 and 75). In the lower-color-temperature versions, LEDs are available with good (Ra larger than 80) to excellent color rendering (Ra larger than 90 or even 95).
Light-Emitting Diode, LED, Fig. 13

The relative spectral power distribution of a white-phosphor LED with color temperature Tk = 4,000 K and color rendering Ra = 70

Light-Emitting Diode, LED, Fig. 14

The relative spectral power distribution of a white-phosphor LED with color temperature Tk = 2,750 K and color rendering Ra = 85

Beam Control

For lighting designers, one of the most interesting properties of an LED is its small light-emitting surface. This allows the creation of very accurately defined beams. As an illustration of this, Fig. 15 shows a near-parallel light beam made with an LED-line luminaire that is impossible to create with conventional light sources. The other side of the coin is that small light-emitting surfaces often need professional screening in order to limit excessive glare. Multi-LED luminaires have also multi-light beams that may cause multiple shadows because a lighted object is illuminated from many slightly different directions. With RGB color mixing, this may lead to disturbing, multicolored shadows.
Light-Emitting Diode, LED, Fig. 15

Near-parallel light beam with an LED-line luminaire [1]

Lifetime

In the case of high-performance LEDs, it takes a very long time before they actually fail – usually considerably more than 50,000 h. Before that time, however, their lumen depreciation is so great that the LED is no longer giving sufficient light for most applications. Therefore, for LED lifetime specifications, the length of time that it takes to reach a certain percentage of its initial light value is used. Based on a depreciation value of 70 %, lifetime values of between 35,000 and 50,000 h are common for high-performance LEDs. LED bulbs, with their limited space for handling heat, have a lifetime of some 25,000–35,000 h (25–35 times longer than an incandescent lamp).

Lumen Depreciation

The electric current passing through the chip’s junction, and the heat generated in it, degrades the chip material and is so responsible for light depreciation. Decoloration of the housing and yellowing of the primary lens may be further reasons for lumen depreciation. In white-phosphor LEDs, chemical degradation of the phosphor material also causes lumen depreciation. As mentioned above in the section on LED lifetime, for high-performance LEDs 30 % lumen depreciation is reached at around 35,000–50,000 h.

Run-Up and Reignition

LEDs give their full light output immediately after switch on and after reignition.

Dimming

LEDs can be dimmed by simple pulse-width modulation down to 5 % of full light. Not all retrofit LED lamps can be dimmed on normal, commercially available dimmers. Special retrofit LED lamps that are designed to be dimmed on such dimmers are so specified on their packaging.

Ambient-Temperature Sensitivity

As has already been mentioned several times, limitation of the junction temperature of the LED chip is essential for the proper functioning of LEDs in terms of lumen output, lumen efficacy, lamp life, and even color properties. In high-temperature environments the products perform worse, while at low temperature they perform better. The actual influence of the junction temperature is different for the different types of LEDs. In extreme temperature environments, therefore, relevant information for a particular product has to be obtained from the manufacturer.

Mains Voltage Variations

LED drivers are designed to drive LEDS on constant current. In this way the influence of mains voltage variations is not an issue.

UV and IR Component

LEDs radiate only visible radiation. There is no ultraviolet or infrared radiation.

Product Range

LED products are available as:
  • Single LEDs (Fig. 16).

  • Multiple LEDs on flat or three-dimensional PCB boards (Fig. 17).

  • LED modules (or LED engines) with secondary optics and with or without built-in driver that can be used in the same way as lamps. Interfaces are standardized for interchangeability (Fig. 18).

  • Multiple LEDs on strings (Fig. 19).

  • Retrofit LED lamps with built-in driver and conventional design (Fig. 20); new type of designs are introduced for retrofit LED lamps as well (Fig. 21).
    Light-Emitting Diode, LED, Fig. 16

    Single LEDs

    Light-Emitting Diode, LED, Fig. 17

    PCB-mounted LEDs

    Light-Emitting Diode, LED, Fig. 18

    LED engines (bottom: for use in road lighting)

    Light-Emitting Diode, LED, Fig. 19

    Foldable LED string

    Light-Emitting Diode, LED, Fig. 20

    Retrofit LED lamps for incandescent, halogen, and fluorescent lamps, respectively

    Light-Emitting Diode, LED, Fig. 21

    Designer type of retrofit LED lamps

Cross-References

References

  1. 1.
    Van Bommel, W.J.M., Rouhana, A.: Lighting Hardware: Lamps, Gear, Luminaires, Controls. Course Book. Philips Lighting, Eindhoven (2012)Google Scholar
  2. 2.
    Schubert, E.F.: Light Emitting Diodes, 2nd edn. Cambridge University Press, Cambridge, UK (2006)CrossRefGoogle Scholar
  3. 3.
    Mottier, P.: Led for Lighting Applications. Wiley, Hoboken, NJ (2010)Google Scholar
  4. 4.
    DiLaura, D.L., Houser, K., Mistrick, R., Steffy, G.: IES Handbook. 10th edn. Illuminating Engineering Society of North America, IESNA, New York (2011)Google Scholar

Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.NuenenThe Netherlands