10 Nov 2006
Knowing the exact requirements of an application and understanding performance characteristics will help you to avoid common pitfalls when purchasing high-brightness LEDs. Mike Godwin and Gunnar Klick look at the technology and its inherent trade-offs.
There has been an avalanche of applications for LEDs over the last 30 years. Starting as status indicators or backlights for switches on consumer products such as telephones and computers, LEDs are now commonplace in traffic signal installations and interior and exterior automotive lighting.
LED technology is advancing rapidly to meet the demand for a new class of high-brightness, ultra-compact emitters that are suitable for emerging applications such as LCD backlighting, projection, full-colour displays, general lighting and automotive forward lighting. These advanced devices will also find applications in consumer products such as camera phones and compact projectors.
The output of a high-brightness LED (HBLED) is measured in lumens – a unit of luminous flux that describes how much light falls on a surface. In the last 10 years, the possible flux per device from a single white LED has risen from 0.5 lm in 1996 to 400 lm today, with the cost per lumen shrinking from €3 to approximately €0.1 over the same period.
The internal quantum efficiency of white LEDs has also improved from 10 to 60% over the past decade, representing a significant increase in the device's efficiency at different wavelengths. The introduction of thin-film technologies and enhanced reflector structures has also increased the light-extraction efficiency from 25 to 75%.
In addition to brightness, HBLEDs are commonly characterized by their performance based on chip size in various surface-mount-technology packages. Chip sizes can range from sub-300 μm up to 1 mm and larger. Advances in scalable chip technologies mean that smaller scale chips with improved efficiency can be used to maintain a higher brightness without added costs. The table "Characteristics of an HBLED in the high-end range" shows the typical characteristics of an HBLED in the high-end range in a single-chip configuration.
HBLEDs are also characterized by lifetime, which is typically up to 50,000 h with 50% degradation at rated conditions. Silicone encapsulation also significantly improves the life and quality of the white light emitted by an HBLED.
An epitaxial wafer is the foundation of any HBLED. Epitaxy (crystal growth using bandgap engineering techniques) is used to create an active p–n junction on the substrate. The epitaxial growth determines the device's internal quantum efficiency, its emission spectrum, degradation and light guiding. The substrate is carefully selected so that its properties, such as lattice constant, closely match those of the epitaxial layers of the device.
The LED's epitaxial layer is grown on the substrate using a processing technique such as MOVPE (metal organic vapour phase epitaxy), then the wafer is bonded to a carrier to conduct current and dissipate heat. Further chip processing includes metallization, lithography, optical optimization and thermal management. The wafer is then separated into individual chips using sawing, dicing and/or cutting.
The LED's emission spectrum depends on the bandgap energy of the compounds forming the p–n junction. Material systems that can be processed by MOVPE include indium gallium aluminium nitride (UV: blue, 460 nm; verde/cyan, 505 nm; and green, 528 nm) and indium gallium aluminium phosphide (IR: red, 625 nm; amber, 617 nm; yellow, 590 nm; green, 570 nm; and pure green, 560 nm). The concentration of dopants in these two material systems controls the bandgap energy and LED emission spectrum. White LEDs use a blue (455–470 nm) LED and convert its output with a phosphor to deliver coordinated colour temperatures for cool white of 5600 K and warm white of 2800 K.
The application will have a lot to do with the specified HBLED device. Basic considerations include desired brightness, colour mixing, uniformity, mechanical dimensions, driving of the LEDs, thermal management, lifetime and environmental requirements.
LCD backlighting and camera flash
Consider a backlight for a 32 inch diagonal TFT LCD with the following requirements: the front-of-screen brightness needs to be in the 500 cd/m2 range; a power consumption of less than 150 W; a back light unit thickness of less than 50 mm; a colour gamut better than 100% NTSC; uniformity greater than 85%; and the operational lifetime of the backlight exceeding 50,000 h. These specific parameters restrict the type of HBLED suitable for this application.
An HBLED for backlighting may include, at 500 mA, a typical intensity of 62 lm (red), 70 lm (green) and 8 lm (blue) with respective wavelengths of 625, 528 and 455 nm. Benefits to the LCD could include a larger colour gamut with RGB exceeding the standard 105% NTSC; a long lifetime due to encapsulated silicone with an operating temperature range of –40 to 85 °C; and low-profile side emissions.
An appropriate HBLED for a large-screen LCD would be, for example, OSRAM Opto Semiconductor's Golden DRAGON (figure 2). This device features light-mixing optics that deflect the emitted light in such a manner that the point light source is transformed into a flat, homogeneous distribution of light. Direct backlight designs with high-power LEDs and wide radiating top-emitter lenses provide significantly high optical system efficiencies and are ideal for this type of application.
Another growing market for HBLEDs is for camera-flash applications in mobile phones. By using the increased luminance of an LED light source, built-in lenses and controlled colour emissions, the amount of useable light for a camera is increased, which greatly improves the quality of the photographs taken.
Generic HBLEDs emit about as much light from the side as they do through the top surface – essentially only 50% of the light is emitted in the desired direction. In contrast, a thin-film HBLED can emit as much as 97% of its output via the top of the LED, making it much more efficient. Lenses designed specifically to utilize this feature of the new die technology can direct the most light to the targeted viewing field of the camera, resulting in only a minor decrease in brightness in the boundary regions.
Choosing an LED
When choosing an LED, be aware of the trade-offs. Keep in mind that as the current increases the lumens increase (which is good as it means more brightness) but the lumens-per-watt (LPW) decreases (which is bad, as the device becomes less efficient at higher currents and temperatures). On the other hand, reducing the current will increase the LPW but lower the light output. This is not a direct linear relationship so finding the optimum levels can be difficult. Beware of lumens per watt claims. All claims should be validated as the actual operating range of usable lumens shrinks as the drive current increases.
An LED's efficiency increases as operating temperature decreases, so make a careful note of the test-condition temperature and test duration for the LED being considered. A low junction temperature increases efficiency, which is a function of drive current, forward voltage and the thermal resistance of the package. All of these factors need to be optimized.
A higher light output may require a higher current, which means more power, and more heat to dissipate. Since efficiency decreases as the p–n junction temperature increases, thermal management is essential to achieve reliable and optimal performance. Make sure that your LED package and lighting system is optimized to remove heat efficiently.
Certain colours are more efficient than others due to bandgap engineering limitations. Orange is a more efficient colour than yellow, for example, because of this physical limitation. Certain cool whites with lower coordinated colour temperatures will have higher efficiencies, whereas red phosphor conversion can reduce the efficiency of a warm white by 15–20%.
The lifetime of a white LED depends on the operating temperature and application. For example, plastics and epoxy resins degrade with dosages of blue light at high intensities. Deeper blues and UV emitters will degrade these materials more rapidly.
The optical design is critical in any lighting system and should strive for the highest efficiency to reduce the LED's light-output requirements and, therefore, the driver and heat-sink requirements. In many cases, additional optics must be attached to the LED and added to the system to deliver the light where it is needed.
Generally speaking, it is prudent to work with a vendor who understands HBLEDs. Keep an open dialogue with your LED vendor to make sure that your requirements are understood and that all factors affecting performance, manufacturability and cost are taken into account.
In the end, one fact is clear: the new breed of HBLEDs makes it possible to design a new range of products that are smaller, brighter and more energy efficient. These new high-performance HBLEDs are based on new, highly efficient chip technologies in combination with thermally optimized device packages.
• This article originally appeared in the November 2006 issue of Optics & Laser Europe magazine.