industry LEDs
1mm2
die of
latest Osram ThinFilm generation. Surface image (left) and illumination pattern (right)
Thanks to all these enhancements in device design, the latest generation of ThinFilm LEDs set a new performance benchmark. Our 1mm2
die, which was
mounted into a laboratory package and optimised for efficient out-coupling, produces high levels of efficacy and wall plug efficiency (WPE) over a wide current range (see Figure 4).
Both these characteristics peak at a drive current of 50 mA, hitting 201 lm/W and 61 percent and falling to 168 lm/W and 52 percent at 350 mA. The reason behind these declines in efficacy and WPE is revealed by the orange curve, the plot of external quantum efficiency. This has a very broad maximum, with values of 58 percent at 50 mA and 59 percent at 350 mA.
Since the external quantum efficiency is essentially constant, declines in efficacy and WPE must be attributed to increases in operating voltage, due to ohmic resistances within the die.
Record-breaking efficacies were produced with a red die with a dominant wavelength of 609 nm at room temperature. Realising similarly high efficacies at longer
wavelengths is even more challenging, due to declines in eye sensitivity. But, counter-balancing this, high-wall- plug-efficiency values are easier to realize with longer wavelengths.
We have also produced 1mm2 die housed in identical
laboratory package that have a 645nm dominant wavelength and a peak emission at 660nm (see Figure 5). WPE exceeds 70 percent between 5 mA and 60 mA, and falls to 59 percent at 350 mA. Output power rises linearly with current to reach about 437 mW at 350 mA, using a forward voltage of 2.1 V. Due to reduced eye sensitivity, the lumen output at this drive current is just 21 lm.
The improvements resulting from out latest ThinFilm technology can be implemented with a very broad process window. This allows for high yield, which in turn lowers overall cost. We have already applied this new technology to chip sizes of varying lengths: 250 µm, 300 µm, 500 µm, 750 µm and 1 mm.
Later this year, 150µm and a 2mm2 our portfolio.
die will be added to
An already-widespread application is the combination of a highly efficient – but aesthetically unpleasant – cold-white LED with an amber or red one, because this lowers the colour temperature to a more visually pleasant and high-quality white. This modification is something of a win-win, because it increases the overall efficiency and the colour-rendering index
24
www.compoundsemiconductor.net March 2012
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