interview epistar
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How do sales of LEDs for general lighting and backlighting compare?
In 2011, general lighting contributed roughly 25-30 percent of revenue. We are focusing on increasing this at more than 5 percent per year. LED backlighting for LCD displays is around 50 to 60 percent of revenue. This includes small, medium and large-sized backlights – for example, mobile phones, tablets, TVs and monitors.
Are you seeing saturation in the global market for LED display backlighting ?
No. The emerging markets are still developing, such as those in India and Africa. Even mainland China does not have that high a penetration rate. Although the quantity of LEDs used in one TV will decrease and the average selling price (ASP) will go down, the total quantity will increase. So it will be a slowly growing market.
Is Epistar still able to operate profitably in the display business, given the declines in average selling price?
This is hard to say, but we think the answer is yes. Our scale of economy helps us to keep a profit, but it will be a decreasing proportion [of overall profit]. We have not only the display business – we have general lighting, general indicators, and so on.
You have a strong history in the manufacture of red and yellow LEDs. Are you the world’s leading manufacturer, in terms of volume?
Yes! We estimate that we have more than 50 percent market share. In terms of the performance of these red and yellow LEDs, which we call four-element-material LEDs, we are number one.
Our red and yellow LEDs are used in outdoor and indoor RGB displays, which differ from LCD displays. This is a major application. Other applications are automotive interior and exterior lighting, and indicators for lighting and consumer electronics.
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You also use red LEDs with blue ones to create white light sources. How easy is it to deliver good colour mixing with that approach?
In many LED packaging houses, they can easily put a phosphor on a blue LED. If you put another red chip beside a blue chip and cover them with a phosphor, the phosphor behaves like a diffuser. That means that the colour mixing
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will be easier than a pure RGB combination, which does not use a phosphor.
To make a high-performance white-light source you need good red and blue LEDs. How good are your blue and red LEDs, and what technologies do they employ to deliver those levels of performance?
For our red LEDs, the wavelength is around 611 nm. We can deliver about 125 lm/W in mass production with the best bonding and light extraction technologies.
We have different designs. One is the traditional low-voltage LED, and its size is not that large – around 14 mil by 14 mil (350 µm by 350 µm). These can be driven from 20 mA to 40 mA. Our other design is the high-voltage type. This is also 20 mA, but can operate up to 20 V.
Our blue LED can deliver up to 150 lm/W at 5700 K in mass production, by using current spreading and internal quantum efficiency optimization. This LED is also available as a low-voltage and a high-voltage structure.
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Are there significant advantages in using a red LED, rather than a red phosphor, for making white LEDs?
The red LED can significantly boost the CRI (colour rendering index) and the lumen-per-Watt in the region of 2700 K to 3000 K. If you use just a phosphor with a blue chip, efficiency is good at around 5700 K to 4000 K. But below 4000 K, efficiency drops very fast. We have compared these two types of LEDs. Without the red LED, performance drops by 30 percent.
March 2012
www.compoundsemiconductor.net 39
Testing the reliability of Epistar’s red and yellow LEDs
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