- A New Material for Red LEDs
Yellow, amber and red InGaNP LEDs offer brightness,improved color and light-output stability with changing temperatures.
Elliot Hicks, Quanlight
The general illumination market has seen the rapid evolution of white and blue LEDs, whereas yellow-, amber- and red-spectrum LEDs have lagged behind because they are plagued by issues of color stability and power efficiency. Traditionally, red LEDs have been composed of AlInGaP grown on a GaAs substrate. For high-brightness applications, the epilayer typically is transferred to a translucent GaP or mirrored substrate, allowing a larger portion of the light generated to be extracted.
In research conducted at the University of California, San Diego, InGaNP has shown promise for yellow, amber and red LEDs. The material has a larger-band offset than AlInGaP, and some successful initial tests have shown that InGaNP has the potential to make a brighter LED with a higher current density tolerance.
Quanlight is working with the university’s technology, which involves growing a dilute nitride material directly onto a GaP substrate, eliminating the epilayer transfer step and reducing the amount of material required. Because equipment for producing InGaNP and AlInGaP LEDs is almost identical, this simplification has the potential to lower production costs.
Compared with AlInGaP, InGaNP has a bandgap change with temperatures two times lower and band offsets that are two to three times greater; both factors contribute to improved thermal capabilities and to the brightness of the LED. Color stability with temperature was measured on a development LED grown by metallorganic chemical vapor deposition. The measured shift of 3 nm over a 100 °C change in temperature is only one-fifth the shift of an AlInGaP LED (Figure 1).
Figure 1. Quanlight’s experimental data is shown plotted against published data from conventional AlInGaP chips. Tests measured the peak wavelength emitted while the LED was externally heated from 25 to 125 °C.
The improved brightness compared with that of AlInGaP LEDs is largely from the better performance at higher temperatures. Conventional LEDs are efficient at low drive currents and temperatures, but as the temperature increases, all LEDs suffer from reduced efficiency and light output.
In a test similar to the one for color shift, the light output from LEDs was measured as the external temperature was raised from 25 to 150 °C (Figure 2). At low temperatures, absolute brightness of InGaNP and AlInGaP LEDs is expected to be comparable, but at 150 °C, the InGaNP LED was nearly two times brighter than the AlInGaP.
Figure 2. At 150 °C, the InGaNP LED emitted 48 percent of its original light output, whereas output from an AllnGaP LED was reduced to 25 percent. Results are reported as a percentage of relative light output.
Another important characteristic of the InGaNP LED is that it can tolerate high current densities because of its improved carrier confinement in the active region. The saturation current density, the point at which additional current no longer increases light output, is two to three times higher for InGaNP LEDs than for AlInGaP LEDs.
This novel material system is expected to produce efficient LEDs with outputs from 585 to 660 nm — the yellow through deep-red spectrum. Red LEDs should be available for commercial applications in mid-2008, with amber and yellow to follow. Significant performance advantages will be most prominent in applications requiring high-power or stable color output. Backlighting units for LCD televisions, light engines for projectors, outdoor displays and other red-green-blue color-mixing applications will benefit from reduced wavelength shifts, allowing for simplified control mechanisms. Potential markets include transportation, hazard, theatrical and architectural lighting.
Figure 3. Saturation current densities as high as 9 A/mm2 have been achieved in developmental tests.
For high-power applications driven primarily by cost considerations — such as traffic lights, school bus signal lamps and automotive brake lights — InGaNP LEDs will allow lamp designers to reduce costs by using smaller chips driven at a higher current, or fewer large LEDs in an array. Furthermore, with increased efficiency at higher temperatures, a more compact or heat-intensive enclosure may be used.
Dilute nitrides used for other devices — lasers, for example — have encountered problems with reliability associated with the high concentration of indium. InGaNP LEDs, however, use much less indium than do dilute nitride lasers. Long-term reliability testing for InGaNP LEDs has been initiated, and Quanlight will make results available before commercial launch.
Light output of the LED is a function of its material properties and of its structure. Although the material can produce a brighter LED, the structure also must be optimized for the light to be extracted from the die.
Meet the author
Elliot Hicks is director of operations at Quanlight in San Diego: e-mail: firstname.lastname@example.org.
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