The LED is a light emitting diode that is used in various display and lighting applications among others. Essentially it is a PN junction semiconductor diode that emits photons when it is forward biased. This light emitting effect of the LED is called injection electroluminescence.
Injection electroluminescence or the light emitting process in a LED occurs when minority carriers recombine with carriers of the opposite type in a diode's band gap. The wavelength of the emitted light varies primarily due to the choice of semiconductor materials used as the band gap energy varies with the semiconductor.
LEDs are processed in wafer form similar to silicon integrated circuits, and broken out into dice. Chip size for visible signal LEDs generally fall in the range 0.18mm square to 0.36mm square (Figure 3). InfraRed (IR) LEDs can be larger to handle peak powers, and LEDs for lighting are larger again.
Figure 3 - GaP LED Die
The basic structure of an LED indicator - the simplest packaged LED product, consists of the die, a lead frame which houses the die, the encapsulation epoxy which protectively surrounds the die, and also disperses the light (Figure 4). The die is bonded with conductive epoxy into a recess in one half of the lead frame, called the anvil due to its shape. The recess in the anvil is shaped to throw the light radiation forward. The die's top contact is wire-bonded to the other lead frame terminal, the post.
Figure 4 - LED Indicator with Cutaway
The dispersion or radiation pattern of the light is determined by the LED lamp’s mechanical construction. A narrow radiation pattern (Figure 5) will appear very bright when viewed on-axis, but with a narrower viewing angle. The same LED die could be mounted to give a wider viewing angle, but with its on-axis intensity reduced – a tradeoff inherent in all LED indicators. High brightness LEDs with a 15° to 30° viewing angle are ideal for information panels directly in front of the subject but an automotive dashboard may require an angle of 120° or more!
Figure 5 - Narrow LED Indicator Radiation pattern
Generally, LEDs have a mean time between failures in the range of 100,000 to over 1,000,000 hours. Considering a year is 8760 or 8784 hours, this is a very long period of time. In practice, LEDs are deemed to have reached the end of their life when the light output falls to half.
Typically, when current flow within a LED junction is not uniform, small temperature differentials occur within the chip. This exerts stress on the lattice, causing minute cracks to occur. In time, this accumulation of defects reduces the photon conversion efficiency of the chip, thus reducing light output. This rate is influence by various factors including LED material, temperature, humidity, and current flow.
Essentially two technologies for generating white light from LEDs exist. One is to mount a red die, a green die, and a blue die together within a package, and mix the light outputs in correct proportions to achieve white light. The usage of 3 dice in this approach makes it costly although tricolor LEDs are popular for LCD backlights in consumer applications. The second, cheaper approach, pioneered by Nichia, involves including a phosphor with the blue LED to absorb some of the blue light and fluoresces in a second color to achieve a near-white. Some early LEDs using this technique emitted a blue shade, but recent developments have excelled as seen in full color PDAs and cell phones.
The quantum efficiency of LEDs is ever increasing with availability of all primary colors (RGB) and reliability as good as the other display technologies. Surface mount LEDs are used in backlights for smaller LCD panels, equipment panels, and indoor message boards. Outdoor message boards are also LEDs for advertising displays and traffic signs. The durability and effectiveness of LEDs makes them ideal traffic lamp replacements for incandescent lamps.
|Light Emitting Layer Timeline Comments||Timeline||Comments|
(Gallium Arsenide Phosphide)
|1960s||Original low efficiency red using liquid phase epitaxy|
|1970s||High efficiency red|
(Gallium Aluminum Arsenide)
|1980s||Single and double heterostructure processed using vapor phase epitaxy increase efficiency|
(Indium Gallium Aluminum Phosphide)
|1990s||Metal organic vapor phase epitaxy|
(Indium Gallium Nitride)
|2000s||Ultrabright green and blue|
Table 2 - LED Processes
Currently, most ultra brightness LEDs easily exceed the light output of incandescent and halogen lamps and do not need maintenance requirements of filament lamps. The challenge of LED process developers now is to build a very high brightness white LED which is economic enough to be used for domestic lighting. With the rapid advent in LED technology and innovation, anything is possible.
The LED is a 35-year old display technology that has rapidly evolved over the years. The vast improvements in its technology have stimulated various new applications for the LED.
Initial commercial research into LED technology started in the early 1960s with Bell Labs, Hewlett-Packard, IBM, Monsanto, and RCA pioneer the early work. Extensive work on gallium arsenide phosphide (GaAsP) led to the introduction of the first commercial 655nm red LEDs in 1968, by Hewlett-Packard and Monsanto. Basically, all commercial LED applications took root here.
In the early 1970s, various GaAsP-based applications were introduced. LEDs were used in portable frequency counters and as numeric displays in pocket calculators. For a short time, LEDs even appeared in digital watches although they were soon replaced by LCDs. Meanwhile, LEDs replaced incandescent and neon lamps as status indicators and became the standard numeric and alphanumeric display choice for instrumentation. LEDs also established themselves in the filed of consumer devices, instrumentation, and automotive panels.
Taiwan is the largest producer of LEDs in the world contributing up to half of the world’s demand via its 30 LED manufacturers followed by Japan and USA making up the big three. Other Led manufacturers are actually assemblers and packagers who purchase wafers or dice from foundries.
According to Insutrial Technology Information Service [ITIS, Taiwan], on the average, it is estimated that the global LED production rate is around 4 billion units per month. It has to be noted though that this figure is expected to increase by up to 12% in the next three years.
Radiometry is the measurement of radiant energy at all wavelengths both visible and invisible to the naked eye, while photometry is the measurement of apparent brightness to the human eye. The human eye views the range of light wavelengths from 380nm to 740nm as the familiar color spectrum (Figure 1).
Figure 1 - Wavelength of Color
The Commission Internationale de l' Eclairage (CIE) formalized standards for the measurement of light, and the response of the human eye or "standard observer", back in the 1930s. These standards characterized the variation in eye response over the entire visible range under a variety of lighting conditions, such as daylight and night.
The CIE also defined the primary colors (Table 1).
The human eye response peaks roughly at green 555nm, is sensitive to yellow, falls off sharply towards blue at 400nm, and also towards red at 700nm. The photopic (daylight) chromaticity diagram, which is shown in a simplified form in Figure 2, clearly indicates these facts. The curve for scotopic (night-adapted) is quite different, peaking at about 512nm.
Figure 2 - Human Eye Daylight Color Response
Radiant light intensity at all wavelengths is measured in lumens. Lumen by definition refers to 683 lumens of light as provided by 1 watt of monochromatic radiation at a wavelength of 555nm. Luminous intensity, in candelas (cd), results from the application of the CIE color response to the radiant flux and provides the measurement for the visible portion of a light source. Display intensity, therefore, is described in cd or mcd to indicate the light output useful to the observer.
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