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MicroMagazine.com

The Emergence of Solid-State Lighting

Rapid Advances and a Multitude of Applications Promise a Large and Growing Market

by Bijan Moslehi

Bijan Moslehi, PhD, is chief technology officer and senior vice president, semiconductor technology research, for The Noblemen Group, a boutique investment banking, strategic advisory, and business development firm. Moslehi has 20 years' experience working in the semiconductor and semiconductor equipment industries. He can be reached at bmoslehi@noblemengroup.com.

Nearly 130 years after Thomas Edison invented the first practical incandescent light bulb, lighting—one of the last remaining analog functions—is in the process of being converted to solid-state technology. Following decades of intense research and development efforts, widespread solid-state lighting applications based on light-emitting diodes (LEDs) are finally coming to fruition.

Electroluminescence from an LED’s forward-biased p-n junction results in the emission of incoherent light with a narrow spectrum. Over the past decade, rapid advances in the science and technology of high-brightness LEDs (HB-LEDs) have resulted in products that cover virtually all colors across the visible spectrum (from red to blue), white-color light, and ultraviolet (UV).

Research efforts have been led by the United States and Japan, while companies such as Lumileds, Cree, GELcore, Osram, Nichia, and Toyoda Gosei are conducting ongoing work. The current $4 billion packaged HB-LED market is expected to grow steadily so that by 2010 it will exceed the $7 billion to $8 billion mark, corresponding to an average annual growth rate of around 15%. Approximately one-fourth of this market is devoted to the sale of LED chips and epitaxial wafers.

Asia Pacific countries (particularly Taiwan, South Korea, and China) produce more than half of the global LED supplies. Collectively, they have emerged as the world’s largest producers of HB-LEDs. While they primarily offer low- and medium-performance products, they are actively in the process of entering the high-performance market.

Solid-state lighting has many advantages over traditional lighting. It consumes approximately 5 to 10 times less power than equivalent incandescent lamps, generates low levels of heat, is highly efficient, and has an operational lifetime about 50 times greater than that of incandescent bulbs. It is reliable, shock and vibration resistant, rugged, and durable. With a response rate that is approximately 1,000,000 times faster than that of incandescent light, it offers high color efficiency, directional capabilities, color control (without filters), and intensity control. Environmentally friendly, it comes in innovative shapes and forms, has small form factors, and does not emit UV radiation.

HB-LEDs with luminous efficacy in the range of 20–40 lumens per watt (LPW) are primarily used to manufacture low-power colored lighting and various bright monochromatic light sources. Such applications include keypad backlighting; traffic signals; automotive lighting (e.g., dashboard and instrument-panel lights, taillights, auxiliary lights, and daytime running lights); architectural accent lighting; and large outdoor video displays and signs. This first wave of applications accounts for close to 70% of the current market.

Recently, higher-performance low- to medium-power blue- and white-light HB-LEDs (with luminous efficacy of ~40 to 60 LPW) have been used in demanding applications such as storefront lighting, LCD backlighting for cellphones and PDAs, flashlights, digital camera flashes, instant-on and battery-operated image projectors, copiers, printers, scanners, and CD readers.

In the next five years, these and a few other new applications will drive a second wave of HB-LED adoption. Automotive headlights, for example, which require a minimum of ~50 to 70 LPW for lead implementation and ~100 LPW for large-scale market adoption, represent a growing area. Backlighting for larger LCD computer monitors and flat-panel TVs is another application on the horizon. This shift to replace cold-cathode fluorescent lamps with HB-LEDs will improve the range of color reproducibility significantly, so that LCDs will be capable of meeting much more advanced standards, such as Adobe RGB. Leading Asian TV manufacturers such as Sony and Samsung have already introduced large-screen TV sets based on this approach. All of these high-growth segments account for around 25% of the present market.

Accounting for only about 5% of the market, solid-state lighting in the interior, industrial, and events segments has remained largely untapped. While it is anticipated that this will change over the next 5 to 10 years, leading to a third market wave, HB-LEDs must overcome major cost and performance challenges before they can displace conventional lighting in the huge general illumination market.

The luminous efficacy of an advanced commercial white-light HB-LED is in the range of ~60 to 70 LPW, exceeding that of incandescent bulbs (~10 to 16 LPW) and virtually comparable with that of fluorescent or high-intensity-discharge (HID) lights (~45 to 100 LPW). Since the maximum possible luminous efficacy is 683 LPW, only about 1.5 to 2.3% of the electrical energy of an incandescent lamp is converted into visible light, while the rest is dissipated in the form of heat (through conduction and infrared radiation). That is much lower than the conversion rate of fluorescent and HID lamps (about 7 to 15%) or of current-generation white HB-LEDs (about 9 to 11%). In contrast, by 2015, about 22 to 32% of the electrical energy of HB-LEDs is expected to be converted into visible light. Heat generation and proper thermal management will be critical in high-output HB-LEDs, since high temperatures affect emission efficiency adversely and degrade performance.

LEDs have service lifetimes of more than 50,000 hours, which by 2015 could exceed 100,000 hours. In contrast, incandescent lamps, fluorescent lights, and HID lights have service lifetimes of approximately 1000, 20,000, and 24,000 hours, respectively.

The cost of HB-LEDs is high. Even at <$45 per kilolumen (klm), it is at least 10 times higher than what is considered acceptable for their adoption by the general illumination market. In comparison, incandescent lamps, fluorescent lights, compact fluorescent lamps, and HID lamps cost ~$0.30/klm, ~$0.60/klm, ~$3.50/klm, and ~$2/klm, respectively—nearly 13 to 150 times less than LEDs. However, a more-precise cost comparison based on the overall cost of the light source throughout its useful service lifetime is more favorable to LEDs. According to Department of Energy literature, when the initial cost, operating costs, light-conversion efficiency, and lifetime of the light source are factored in, LEDs cost ~$20/million lumen-hours (Mlm-hr) while incandescent lights cost $27/Mlm-hr and fluorescent lights cost $7/Mlm-hr.

According to Lumileds’ Haitz’s Law, the luminous output of LEDs has doubled every 18 to 24 months over the past 30 to 35 years, while their cost has decreased every decade by a factor of approximately 10 (20% per year). If this trend continues, the luminous efficacy of white-light HB-LEDs should exceed 200 LPW by 2010, and their cost should drop exponentially to below $20/klm. While 50 to 100 white LED modules equal the brightness of a 40-W fluorescent lamp, that number could drop to less than 15 or 20 within the next 10 years.

LEDs are fabricated using wide-band-gap compound semiconductor materials that are epitaxially grown on suitable substrates via organometallic vapor-phase epitaxy (OMVPE), metal-organic chemical vapor deposition (MOCVD), or molecular beam epitaxy. More-mature, efficient, and less-costly Group III phosphides (AlGaInP alloys), which are used for producing highly visible deep-red to yellow-green lights, are grown using OMVPE on lattice-matched gallium arsenide (GaAs) substrates. To reduce the absorption of internally reflected light, the GaAs substrate is later removed and replaced with a thermally bonded gallium phosphide substrate.

Newer and more-expensive Group III nitrides (AlGaInN or InGaN alloys) and binary Group III-V compounds such as gallium nitride, which are grown using MOCVD on thin sapphire or silicon carbide substrates, generate visible yellow-green to blue and UV light. The undesirable lattice mismatch of nitrides with these substrates, which are later removed using a lift-off process, introduces crystal dislocations and high defect densities (up to ~1 × 1010/cm2). Together with surface defects and roughness, these defects affect yields and degrade device performance. To minimize this problem, a low-temperature buffer layer can be deposited. In addition, specialized inspection tools have been introduced to improve yields.

Typically, LEDs are manufactured on 2- or 3-in. substrates. While shifting to larger substrate sizes would certainly help reduce costs, some manufacturers are hesitant to rush into such a transition because of the many operational challenges, the relative immaturity of the emerging LED-manufacturing segment, costs, and existing yield issues.

The fabrication of more-efficient HB-LEDs holds the promise of huge economic, environmental, and productivity gains. The technology will result in potential savings of tens of billions of dollars in capital investment and operating costs, decreased carbon dioxide emissions driven by energy conservation, and reduced hazardous wastes (such as mercury). The first two phases of HB-LED adoption are well under way. However, the third phase—breaking into the vast general illumination market—may prove to be the most exciting as well as the most challenging phase.


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