Time for an LED Switch!
White Light LEDs are Replacing Traditional Lighting
Light Emitting Diodes (LEDs) are semiconductor p-n junctions that emit light when forward-biased with electrical current. The peak wavelength emitted is determined by the bandgap of the semiconductor material. Since their invention in the 1960s, LEDs have been made from conventional compound semiconductors such as GaAs and AlGaxAsy. The peak wavelength of these conventional LEDs was in the infrared (GaAs) or the red (AlGaxAsy ).
However, in the 1990s, researchers from Japan demonstrated the epitaxial growth of InGaxNy based LEDs that emit blue wavelengths. These InGaxNy layers are grown as single crystal thin films on monocrystalline substrates. The most common substrates for blue LEDs are monocrystalline sapphire (Al2O3), silicon carbide (SiC) and silicon (Si). The InGaxNy monocrystalline layers are most commonly grown by Metal Organic Chemical Vapor Deposition (MOCVD). Sapphire substrates are always insulating, so LEDs on sapphire are either etched to form contact to a p-n junction, or they are flipped to bond to a PC board via flip-chip bumps. SiC and Si substrates may be doped heavily n-type, so the current in these LEDs can flow vertically through the p-n junction and down through the substrate.
Blending Three or More Monochromatic LEDs = White Light
Most TVs and computer monitors are called “RGB,” meaning that all the colors of the visible spectrum can be created by mixing various intensities of red, green and blue lights. Many large LED displays (such as in New York’s Times Square) are comprised of pixels, where each pixel is a trio of three different LEDs. Each LED in each pixel can be individually controlled for intensity. By turning on all three LEDs at the correct relative intensity, the display will emit white light. Such LED trios are used for displays and for chromaticity tunable lighting. For example, the cabin lighting for the Boeing 787 Dreamliner features programmable white lights. Their purpose is to control the chromaticity and color temperature of the lights to simulate the spectrum of sunset (time to go to sleep) and sunrise (time to wake up - we are preparing to land.)
From a Blue LED to White Light
The market for monochromatic blue LEDs is rather limited. However, using these LEDs to generate white light has opened a very large market for lighting. In 2014, worldwide annual sales of LEDs used for lighting was $5.3 billion. The most common approach to generate white light from an LED is to place a blue LED beneath a photoluminescent material that will absorb some of the blue light and re-emit it at longer wavelengths. The simplest white LED is a blue LED placed beneath a yttrium aluminum garnet (YAG) ceramic doped with a rare earth activator like europium. Figures 1 & 2 illustrate how such white light LEDs are constructed. An LED chip is typically submerged in silicone which can be loaded with the ceramic phosphor particles.
Figure 1, Left: Schematic cross-section of white light generated by a blue LED irradiating a phosphor. (Credit: Pacific Light Technologies conference presentation)
Figure 2, Right: Schematic of a white light LED in its package. The LED chip is submerged in silicone that is loaded with the phosphor, typically a dispersed ceramic powder. (Credit: Dow Corning conference presentation)
Simple two-part LEDs are used in low cost applications like inexpensive pen lights. Such LEDs have a strong bluish-tint (a high correlated color temperature CCT) that tends to cause headaches in many people after prolonged exposure. However, for general illumination, consumers have demanded a more uniform emission of light over the visible spectrum ( 400 nm < λ="">< 700="" nm)="" that="" would="" relieve="" this="" problem. ="" most="" leds="" for="" illumination="" use="" at="" least="" two="" phosphors:="" the="" standard="" yag:eu="" for="" the="" yellow,="" plus="" a="" red="" phosphor. ="" the="" combination="" of="" blue="" light="" (emitted="" from="" the="" led),="" plus="" yellow="" light="" (emitted="" by="" the="" yag:eu="" phosphor)="" plus="" red="" light="" (emitted="" by="" the="" red="" phosphor)="" can="" be="" engineered="" to="" create="" a="" white="" light="" with="" a="" cct="">< 3000k="" that="" is="" pleasing="" to="" the="" eye. ="" figure="" 3="" (following)="" illustrates="" a="" typical="" total="" emitted="" spectral="" power="" distribution="" of="" a="" white="" light="" led. ="" the="" spike="" emission="" at="" λ="455nm" is="" the="" blue="" led,="" while="" the="" broad="" emission="" at="" longer="" wavelengths="" is="" from="" the="" two="" other="">
Figure 3: White light is generated by combining the blue light (peak λ = 455nm) of the LED plus other wavelengths (colors) created by photoluminescence of the phosphor atop the LED chip. Inexpensive white LEDs use one phosphor while expensive, higher quality LEDs use two or three phosphors. (CREE conference presentation)
What Are Quantum Dots?
In addition to the ceramic phosphor powders previously discussed, other photoluminescent materials being developed are quantum dots (QDs). QDs exploit the phenomenon that the effective bandgap of semiconductors increases as the particle size drops below 0.1 micron. Therefore, dispersions of such nano-sized semiconductor particles will photoluminesce. An example of QDs are CdTe particles of 5 nm particle size. The photoluminescent properties (emitted color) of the QDs can be engineered by controlling: (1) the composition of the semiconductor, and (2) their particle size. An early product incorporating quantum dots is the “Quantum Dot Enhancement Film” backscreen light diffuser for LCD displays from 3M. QDs are blended into the plastic diffuser so that when the edges of the plastic film are “pumped” by a string of blue LEDs, the entire diffuser will re-emit white light behind the LCD for the LCD TV monitor.
Traditional Illumination Displaced by LEDs
Energy efficient white LEDs are steadily replacing incandescent and fluorescent lights for general illumination. LEDs offer many advantages over older traditional technologies, including:
• Higher light output per watt of wall-plug power consumed (luminous efficacy)
• Longer lifetime
• Instant-on and dimmable
• Ability to be used in groups of two or more LEDs in order to provide a near infinite number of hues and chromaticity
In order to promote energy efficiency, many countries (including the USA) are in the process of outlawing the manufacture of incandescent light bulbs.
White LEDs are becoming ubiquitous. Examples include:
• The price/performance ratio of white LED replacement bulbs improves annually. For example, CREE is now on its third generation of LED bulbs sold at The Home Depot.
• The 2015 Super Bowl was played at the University of Phoenix Stadium in Arizona. The entire stadium was illuminated by CREE’s white LEDs.
• Several firms offer LED replacements for tube fluorescent bulbs. These LED replacements can be configured to work with the existing fluorescent ballast, or with a new LED-specific power supply.
• Philips LumiLEDs offer the HUE line of programmable light bulbs. Each bulb contains several monochromatic LEDs whose relative intensity can be self-programmed. HUE light bulbs come with a wireless server that enables them to be controlled by the WiFi output of smartphones and tablets.
White Light LEDs and Materion
Materion’s products enable the fabrication of white light LEDs. The Advanced Material Group supplies PVD metals used to form metal contacts to LEDs, including Ti, Ag, and Au evaporation slugs. Materion also is a leading supplier of precursor salts used for the fabrication of phosphors, especially red phosphors. For more information, contact Richard Koba, Marketing Manager at Richard.Koba@materion.com.