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ductor crystals such as Silicide Carbide. About ten years later in 1962, Nick Holon-yak developed the first light emitting diode for General Electric. The LED remained barely recognized in the following years despite rapid technologi-cal development and was mainly used for status indication in electronic devices. Hard-ly anybody expected the LED to become a major competitor to incandescent and fluo-rescent lamps. This article will go into detail on develop-ment in recent years. However, we first need to understand the principle of illumination in light emitting diodes.

The Light Emitting Diode A success story by Cornelius Neumann

Legislation, Standards & Technology

In the early 20th Century, the time in which the world's capitals were first enlight-ened by these novel incandescent lamps, the foundation for an illumination source was built – about 100 years later – it is being celebrated as a major source of innovation: the LED. In the year 1907 H.J. Round reported in the Electrical World Vol. 49 about a ›curious phenomenon‹ with the material Carborun-dum, today known as Silicide Carbide (Figure 1). When applying an electrical voltage, Round detected a visible ›glow‹ in various colours on certain crystals. He suspected a thermo-electric effect similar to the one found in incandescent light bulbs. Just how incorrect he was only came to light in 1951 with the development of the transistor, which boosted the semiconductor physics' progress. By then physicists were able to explain light generation in semicon-

Figure 1: The note by H.J Round in the Electrical World Vol 49, 1907

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White light by LEDs

White light can be generated in various ways, all depending on the principle of addi-tive colour mixing (Figure 3):

RGB mixing The light of a red, a green and a blue LED is mixed – similar to the principle of colour television. Changing the intensity of each primary colour enables generation of various colours within the primary colour range, in-cluding the colour white. This procedure, however constitutes two major drawbacks to usability. First of all the three light sources are separated so that the light needs to be mixed subsequently. This is done either by diffuse reflection materials or by optical fibres in which the light is mixed by multiple reflections on the reflective sur-face of the fibre. Furthermore, the individual LED chips react differently on applied volt-age and heating. This would mean continu-ous readjustment of the individual LEDs dur-ing operation in order to provide stable coloured light generation. For various appli-cations a high technical effort would be re-quired using this method.

A new type of illumination

Basically the LED is an electric diode. A pos-itively doped semiconductor (with ›positive spots‹ in the valence band) and a negatively doped semiconductor (with electrons in the valence band) are firmly bonded to one an-other. The applied voltage lowers the poten-tial difference between the respective con-duction bands and valence bands of the material, thus generating an electric current flowing through the diode. The diode blocks the reverse direction when the positive pole is connected to the n-doped material and the negative pole to the p-doped material. As opposed to a normal diode, which is supposed to distribute electrical current as lossless as possible, the light emitting diode highly depends on such efficiency losses (Figure 2). The internal loss processes required for an LED can be described as a fusion of charge carriers within the LED chip. An elec-tron from the conduction band combines with a ›positive hole‹ in the valence band. The energy released in this process is emit-ted in the form of electro-magnetic radiation. When the energy gap E between both bands is large enough, the emitted radiation is visi-ble to the human eye: the diode emits visible light. The current in the diode continuously de-livers new charge carriers, keeping the pro-cess running. A different doping with impurities chang-es the energetic gap of the bands and thus the wavelength of the emitted light. Defects in the semiconductor's structure lead to non-visible emissions, reducing the efficiency and durability of LEDs, since such defects tend to advance into the active LED layer.

The energetic gap between conduction and valence band is clearly defined. There-fore the LED's light – though not monochro-matic like laser light – has a full width at half maximum of 30 to 40nm and is therefore sin-gle coloured. The LED therefore always emits coloured light!LED colours are highly saturated and are very close to pure spectral colours.This property obviously propelled colour ap-plication types such as red and yellow signal lights in automobiles. Two LED materials are currently being used for various colour ranges. For red and yellow colours Aluminium Indium Gallium Phosphide (short: AlInGaP), for green to blue colours (up to ultraviolet) Indium Gallium Ni-tride (short: InGaN) is being used. However, if LEDs always provide saturat-ed coloured light, how can we generate un-saturated white light using LEDs? Otherwise this efficient illumination source would only be suited to a limited application range.

Figure 2: Simplified LED working principle

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Combination of UV diode and phosphoric material This conversion method works much the same way as a fluorescent light bulb. In-stead of visible light the LED generates ultra-violet light. This UV light will be absorbed by a phosphoric material coated on top of the LED and emitted as visible light. Using a combination of phosphoric materials, vari-ous spectral ranges can be emitted, includ-ing high-quality white light. Unfortunately the efficiency of diodes generating UV light is far lower than those generating visible light. Therefore this technology is currently only suitable for a low number of industrial applications.

Combination of a blue LED with phosphoric material Instead of a UV diode, a blue LED will be used and some of its light converted into green/yellow spectral range using phos-phoric materials. The combination of these colours will be observed as white light. This quite simple technology has become popular and is cur-rently state-of-the-art. The colour tempera-ture of the light can be varied between cold

The Light Emitting Diode - A success story

Figure 3: Generation of white light (from left to right) by RGB mixing (1). Conversion of UV light by phosphoric materials and partial conversion of blue light by phosphoric materials (2). Above: Operating principle, below: generated light spectrum (3), source: Zukaukas et al.

and warm white by the amount and composi-tion of the phosphoric material. With the ability to efficiently generate white light us-ing LEDs, the light emitting diode has be-come an interesting light source for general illumination purposes. Additionally the effi-ciency of the LED has increased by several orders of magnitude, exceeding the efficien-cy of an incandescent light bulb by the fac-tor 10.

LED Efficiency

Such rapid development were foreseen by the Hewlett Packard employee Haitz in the nineties. Similar to Moore's law in com-puter technology, the so called Haitz's law predicts the increase in light intensity for LED lights and the development of manufac-turing costs. He predicted that the light in-tensity of a packaged LED – probably includ-ing more than one LED chip – would multiply by a factor of 20 to 30 per decade, while manufacturing costs would drop by a factor of 10 per decade. The market development in the last years has repeatedly confirmed this prediction (Figure 4). Today LEDs with four

chips are available on the market with a size of only a few square millimetres. They are able to replace a 100 W standard incandes-cent light bulb. While cost development is driven by the increase in quantity and manufacturing inno-vation, the efficiency development mainly focuses on chip design, light extraction from the chip and heat dissipation from the LED. Since LEDs (as opposed to incandescent and fluorescent light bulbs) generate less light when warm, a high-quality LED system requires a low heat resistance in order to dissipate the heat produced by light genera-tion. A good LED requires a ›cool head‹. The efficiency increase of LEDs is reflect-ed by the decrease of heat resistance. Older models such as 3 mm LEDs had a heat resist-ance of more than 200 °C/W, while newer high-performance LEDs feature a heat re-sistance of 10 °C/W or less. At a perfor-mance loss of one Watt the chip's tempera-ture of a high-performance LED therefore only increases by about 10 °C related to the ambient temperature.However, a low LED heat resistance alone does not provide an efficient LED light. The heat resistance of the complete illumination

1 2 3

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system determines its performance.This is an obstacle in LED technology and re-quires particular attention when designing LED systems. As we will see, LED development match-es the predicted efficiency improvement.

LED applications in automobiles

The main reason for LED application in automotive technology as an energy-saving, innovative and stylish signalling light is the required colour range: red for brake lights, yellow for turn indicators and blue for bea-con lights. In the mid-nineties an additional centre brake light (Figure 5) for new cars became mandatory in Europe. These laws became effective at a time when red LEDs with a light intensity between one and three Lumen – re-quired for such a task – were available on the market, and 20 to 30 of such light sources would be sufficient to comply with the re-quirements on light distribution and minimum area. Very quickly this application gained a sig-nificant market share. The third brake

Figure 4: Haitz's law predicts the development of light intensity in Lumen per package (source: LumiLeds)

Figure 5: Typical additional first generation brake lights (source: Hella)

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The Light Emitting Diode - A success story

Figure 6: Examples of LED signalling lights and beams with innovative style. Left: Peugeot 308 CC with LED day-light headlamps, right: Ford S-Max with Edge Light, below: Audi A8 with Full-LED AFS lights (sources: Hella, netcarshow.com)

light was mandatory in the USA much sooner than in Europe, and light bulb solutions still prevail in the market that are rarely seen in Europe. Along with the increase in performance, rear lights, brake lights and turn indicators with LED technology have been introduced. The only reason for the delay in further mar-ket penetration was the associated high cost-effort for the new illumination source. Since diodes and novel optical systems such as optical fibres facilitated completely new designs, LED signalling functions became very popular among car manufacturers. When taking a closer look at lights on cars, they are often a dominant element of car de-sign, generating a unique night-time appear-ance (Figure 6). Most styling innovations are

the result of a combination of light source and novel optical system. The same applies for signalling functions within the lights and the light itself (illustration 6 below). When designing LED position and day-time lights, headlights and high beams, white diodes are being used regularly as light sources for the first time in automobiles. The first daytime light with LEDs went into series production in 2002. Audi offered the first car completely equipped with LED lights: the R8 sports car. In contrast to typical light sources such as halogen and Xenon the light intensity of a single LED is not sufficient to generate the required light distribution. Therefore several chips are combined to one light source, and illumination of the road ahead is undertaken

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by several of such multi-chip LEDs. By now LED solutions provide the same brightness as Xenon lights and they are of-fered with variable light distribution (so called adaptive front lighting systems). This variable light distribution operates without mechatronics, simply by switching on and off individual optical components depending on the driving situation. We expect the LED to be an innovation driver for various new lamp- and design- styles for automobile lights in cars' interior and exterior.

LED in general Lighting

Of course the LED has gained a foothold in other areas of lighting technology as well – in aviation or in signalling lights for road and rail traffic control. Only the last three decades showed a tremendous development in general illumination that follows the per-formance development of these light sourc-es. Manufacturers present more and more LED products at conventions and fairs. Sometimes it even seems that other light sources no longer exist, although the LED- market share still remains in the single-digit range. The controversially discussed ban of incandescent light bulbs will, among techno-logical development, soon lead to a quick in-crease in market share for the LED. The ban of incandescent light bulbs sug-gests the first important application for LED lamps, namely the replacement of incandes-cent lamps – also called ›retrofit‹ (Figure 7).LED retrofits also aim for energy efficient il-lumination. The retrofits combine light source, elec-tronic control and cooling and are the size of a typical incandescent light bulb.

Figure 7: LED retrofits in a long-term measurement stand at the Phototechnical Institute in Karlsruhe.

Figure 8: Example of a new design with LEDs in gene-ral illumination; the award-winning Circolo lamp (source: Sattler)

The variety of solutions (varying in quali-ty) all have one characteristic in common: they all need to feature the light bulb's typi-cal socket as a connector. This is called new technology with an old fingerprint. Many consider this to be a transitional solution which will retreat in about 15 years. The second type of application involves lamps specifically designed for LEDs. These will gain more influence in our every-day lives as they provide high-quality light and new ideas for our homes, offices, public spaces and other areas of life. New designs, coloured and white light, and the small size of LED lamps will provide new ideas for illumination applications. Figure 8 shows an example of a new, award-winning LED design. For general illumination technology the industry and institutes are currently survey-ing open issues in LED applications such as useful life, light quality or reflection poten-tial. The limits of LED technology are not yet fully explored. If we manage to fully explore the potential of this light source, utilise it in a sensible manner and create appealing prod-ucts, there is nothing that could stop LEDs from becoming a true success story.