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The Bright Present and Brighter Future of LED Technology Photo: clalighting.com.au Light-emitting diodes (LEDs) are fast becoming the devices of choice for all artificial lighting applications. Their many benefits, such as high efficiency, small size, long life and wide availability have made them very popular over recent years. T he first LEDs appeared during the 1960s but attracted little attention until the 1970s when they gradually became more widespread as signalling devices in electronic equipment. In this role they supplanted the previously ubiquitous miniature filament lamps. During the 1980s work was undertaken to increase the brightness of LEDs and by the 1990s illumination-class LEDs had appeared on the market. This marked the advent of a rapid upswing in LED consumption as they began to be used for mainstream lighting applications. With steady increase in the brightness level of individual LEDs and multiple LED modules, applications for LEDs have proliferated. This has gone to such an extent by Dr Faiz that these devices are now found in 14  Silicon Chip everything from domestic light bulbs and torches to high power luminaires for architectural lighting and even street lighting. Manufacturers are now combining energy-efficient LEDs with solar cells to produce lighting systems that are especially popular in developing countries. Light-emitting diodes generate light in a fundamentally different way than sources such as incandescent lamps or gas discharge tubes. LEDs utilise semiconductor materials to generate light with a small range of wavelengths. In these materials, bands of very closely-spaced energy levels separated by energy gaps can be used to create visible light photons. Electrical charge carriers of opposite signs (electrons and holes) are injected Rahman * into a LED from an external circuit. Insiliconchip.com.au Extra close-up of a 1W balanced white LED from Electrospell. side the diode these carriers recombine when electrons fall down the energy gap from the upper energy band (conduction band) to the lower energy band (valence band). This leads to an effective reduction in the electrons’ energy, with this energy being released by the recombining electronhole pairs. In ordinary silicon diodes, this energy appears as heat whereas in light-emitting diodes the energy comes out as light photons. Coloured LEDs The colour of emitted light depends on the energy gap that separates the conduction and valence bands. The wider the band gap the larger the photon energy, ie, the shorter the wavelength of emitted light. The band gap, in turn, depends on the material the LED Fig.1: electron microscope view of cadmium sulphide quantum dots inside pits on a GaN LED chip. siliconchip.com.au January 2013  15 Fig.2a (above) shows the spectrum of a flat-white LED, while Fig2b (right) shows the spectrum of a LED that mimics the light from a tungsten-halogen lamp. is made from. A wide range of semiconductor materials such as gallium arsenide (GaAs), gallium indium arsenide phosphide (GaInAsP) and gallium nitride (GaN) are used to make LEDs that emit light from infrared to the ultraviolet. The light coming out of single chip LEDs covers a narrow band of colours and is thus of a more-or-less single colour. This light can be modified in colour by using wavelength conversion materials. Several approaches are possible but two are most prevalent. Quantum dots and phosphors Materials called quantum dots, consisting of extremely small particles similar in size to viruses, can be used to convert light of one colour into another. For this purpose, quantum dots are typically made from materials such as cadmium sulphide or cadmium selenide and consist of spherical particles, a few tens of nanometres across. These particles behave somewhat like very large atoms in that they can absorb light at one wavelength and emit it, a short time later, at another wavelength. The emitted light is almost always of a longer wavelength than the absorbed light; ie, the light is ‘red shifted’ so the emitted photons have less energy than the absorbed photons. The difference in energy is simply converted into heat. Quantum dots can be used to produce sharp colours with a narrow distribution of wavelengths. A remarkable feature of quantum dots lies in the ability to make quantum dots emitting light at any desired wavelength simply by changing their size. Blue LEDs can excite red or green emitting quantum dots to produce highly saturated colour sources. The efficiency of this process can be further enhanced by making microscopic pits on the surface of LED chips and filling them up with quantum dot material, as seen in Fig.1. This configuration increases the absorption of light by quantum dots and leads to a marked increase in the brightness of such LEDs. While quantum dot LEDs are speciality devices, a different type of colour conversion material is widely used to make white LEDs. No LED chips by themselves emit white light. In order to obtain white emission LED manufacturers use the same approach as is used with fluorescent lights. The blue light emitted by a GaN LED is passed through a coating of a suitable phosphor material which converts some of the blue radiation to yellow light. Typical phosphors consist of crystalline oxides or sulphides doped with rare-earth elements, such as cerium, europium or gadolinium. The combination of yellow and blue light appears white to our eyes. It is easy to guess from this description that ordinary white LEDs produce a very poor quality of white light which is severely deficient in red and green. In recent years, manufacturers have introduced better phosphors that generate warm white light that has a distinctive yellowish tint. Most LED manufacturers now offer warm white LEDs which are used for making LED light bulbs and other luminaires. Balanced white LEDs For even better performance, companies such as Citizen, Fig.3: spectrafill broadband red, green and blue LEDs. The red and green LEDs use special phosphors whereas the blue LED uses a ‘stressed’ chip. 16  Silicon Chip Fig.4: spectra from red, green and blue Spectrafill LEDs. siliconchip.com.au B0 B = B0Cos Θ Θ Fig.5: Lambertian light intensity distribution pattern. The angular emission from most LEDs takes this form. Electrospell and Bridgelux now offer full-spectrum white LEDs that feature a balanced white spectrum where all colours are present in roughly equal proportions. Fig.2a shows the spectrum of a flat-white LED from Electrospell whereas Fig.2b shows the spectrum of a LED that emits a close approximation to light from tungsten halogen lamps. Innovations in phosphors and other optical materials have made such high performance LEDs possible. Flat spectrum white LEDs are rapidly penetrating high colour fidelity lighting markets. Museums, art galleries and retail outlets are increasingly turning towards lamps MR16 7W LED GU5.3 siliconchip.com.au Fig.6: a narrow emission angle LED. This device emits light in a forward cone which is only 5° wide. based on such colour-rich LEDs to display their exhibits in full splendour. Innovative phosphor technology has also enabled a new generation of wide spectrum primary colour LEDs. These so-called ‘broadband’ LEDs emit light with a much wider spectrum than ordinary LEDs. Whereas usual colour LEDs display a spectrum which is typically 30nm wide, their broadband counterparts emit light with 60 to 90nm wide spectra. By combining PAR 38 19W E27 Outdoor LED Classic A 10W E27/B22 January 2013  17 Fig.7a (left): electron microscope view of the patterned surface of a photonic crystal LED. Fig.7b (right): the pattern of light emitted by a photonic crystal LED when observed from very close to the surface of the chip. broadband red, green and blue LEDs it is possible to make colour-tuneable white light luminaires. By controlling the intensities of the red, green and blue channels with pulse width modulation (PWM) waveforms from a microcontroller it is possible to generate millions of distinct hues. The wide spectra from individual LEDs enable subtle variations of colour shades in illuminated objects to be readily distinguished. The exceptionally high colour rendering capability of wide spectrum LEDs is creating new lighting markets. Spectrafill LEDs Fig.3 shows the spectra of red, green and blue Spectrafill LEDs from Electrospell. This LED family is aptly named as the LEDs each fill up their assigned slots in the red, green and blue regions of the visible spectrum. Broadband LEDs are also being used for indoor plant growth and for various skin therapy applications. Most LEDs on the market emit light in a characteristic fan-like pattern seen in Fig.4 where the light appears brightest when seen head on. As one moves away from the vertical the light intensity falls as the cosine of the angle away from the vertical. This is known as the Lambertian intensity distribution. Typical commercial LEDs emit most of their light in a 120° wide Lambertian fan. By changing the chip geometry and the way it is mounted in the LED package it is possible to reduce the emission angle to as small as 15°. By combining surface texturing with internal reflective optics LEDs with emission angles as small as 5° can be obtained. Fig.5 shows a 5° narrow emission angle green LED. Such LEDs are useful for applications such as back illumination of instrument clusters. Even more interesting angular emission profiles can be obtained by etching the top surface of LED chips with shallow depressions in various regular patterns. Dimples arranged in square or hexagonal patterns, called photonic crystals, are often employed for this purpose. Additionally, the relief causes light to come out of the LED chip with higher efficiency, making the device appear considerably brighter. Textured surface photonic crystal LEDs emit light in a collimated beam. Fig.6(a) shows the surface of a photonic crystal LED whereas Fig.6(b) shows the near-field pattern 18  Silicon Chip of light emitted by this device. The separate emissions combine into a well-collimated beam that is suitable for use in projectors and for back-lighting of LCD televisions. GaN LEDs GaN LEDs form the basis of not only blue and UV light emitters but also of all phosphor-based LEDs. As these LEDs power all LED bulbs and TV back-lights, there is much research directed at improving them further and reducing their prices. The most prominent development in this direction is the emergence of silicon-based GaN LEDs. Conventional GaN LEDs are made by depositing the active device material on a sapphire substrate. This is now a well-established process but the LED chips made in this way are both expensive (because of the high cost of sapphire) and poor at getting rid of the heat produced as the LED operates (because the heat has to pass through nearly half a millimetre of sapphire which is a poor heat conductor). The next generation of GaN LEDs will be made by a very different process using silicon wafers in place of sapphire as the substrate material. This reduces the cost of LED chips because, thanks to the silicon chip industry, silicon wafers are much cheaper than sapphire wafers. Silicon wafers are also available in sizes larger than 12 inches in diameter. In contrast, commercial GaN-onsapphire LEDs are made on 4-inch diameter wafers. The larger wafer diameter will mean many more LED chips can be obtained from each processed wafer, again contributing to a reduction in LED costs. GaN-on-Si LEDs will also perform better than sapphirebased LEDs because silicon has a higher thermal conductivity and thus LEDs will be able to run cooler, producing more light and achieving longer lifetimes. LEDs made on silicon substrates should be widely available within two years once their specialised fabs come on line. Zinc Oxide LEDs Even more exciting LEDs are currently under development in various academic and industrial labs around the world. A significant amount of effort is being directed to develop LEDs from zinc oxide (ZnO). This material can produce cheap and highly efficient siliconchip.com.au LEDs that emit blue and UV radiation. There have been persistent problems in creating high quality p-type zinc oxide which has so far held back the realization of a commercial ZnO LED. With continuing research, however, it is possible that one day we will see these devices becoming as commonplace as GaN LEDs are today. Yet another interesting breed of LEDs under development is based on very thin filaments of semiconductor materials called quantum wires. Created by carefully etching long strings of the base semiconductor material and then topping them with suitable electrode materials, quantum wire LEDs are substantially more efficient than LEDs made from bulk material. Furthermore, there is evidence that quantum wire LEDs might enable electrical tuning of the colour emitted by a single LED chip, without the use of any colour conversion material. Such a device will be a true breakthrough in LED technology and thus several large LED companies, such as Philips and Osram are working on this technology. Massive industry LEDs are now a major industry worth several tens of billions of dollars a year and growing at an astounding 25% per annum. As their prices fall further and as new types of devices come on the market their usage will grow even more. Eventually, all light bulbs will be replaced by LED-based lamps for better energy efficiency and longer lifetimes. This transition has already started and will only be spurred on by ongoing developments in universities SC and companies around the world. Bigger - Brighter - Wider Angle Outdoor LED Displays Here are two economical, high performance, JUMBO displays for wide angled outdoor applications such as race timing, lap counting and sports scoreboards Featuring state-of-the-art Fully super-bright elliptical LED Assembled technology, the NEW D8-HB 300mm and 400mm 7 Segment Displays are visible over long distances and at an incredible 75 degrees either side of normal.(actually 300mm 400mm more than 150o in total) Other features include: Black Background for higher contrast On Board Segment Drivers On-Board Serial Interface User-accessible segment connections for custom interfaces Compatible Modules are available for Counting, deMultiplexing, BCD to 7 Segment Decoding and Driving For further details and to buy on-line see us at: www.kitstop.com.au P.O. Box 5422 Clayton Vic.3168 Tel:0432 502 755 * Dr Faiz Rahman is from Electrospell Ltd, Glasgow, UK LED Lighting Specialist Huge range of stocked LED replacement lamps Visit our showrooms Ceiling Lights Ultra Low Profile GLS Lamps Halogen Lamps Brisbane 24-26 Campbell St. Bowen Hills QLD 4006 Ph: (07) 3252 7466 Fax: (07) 3252 2862 Southport Unit 11 The Brickworks Centre Warehouse Rd, Southport QLD 4215 Ph: (07) 5531 2599 Fax: (07) 5571 0543 Miniature Lamps Flexible LED Tape Hi Bays Fluoro Tubes Floodlights Automotive / Bi-pin Boat / Caravan Lights Shoplights Sydney 185 Parramatta Rd Homebush NSW 2140 Ph: (02) 9704 9000 Fax: (02) 9746 1197 Est.1978 prime-electronics.com.au siliconchip.com.au January 2013  19