LEDs technology
SemiLEDs: Vertical architecture boosts LED performance
Combining a metallic foundation with a vertical current path creates an LED that prevents current crowding, realizes excellent thermal management, and delivers the high efficacies and long lifetimes needed for general illumination, says SemiLEDs’ Trung Doan.
G
aN-based LEDs are already serving many applications. They are illuminating mobile
phone keypads, backlighting LCD displays, generating camera flash, and providing the red and green components in full-color outdoor displays. With recent breakthroughs in efficiency and cost, high-brightness, high-power versions of this device are starting to gain traction in general lighting.
The general lighting market promises to be very lucrative, but success in this sector requires low-cost manufacture of LEDs with excellent thermal management, efficacy, and reliability. Managing the heat generated by the LED is critical, because increases in junction temperature shorten device lifetime and cut efficacy – a 20 degree C rise in junction temperature drives down output power by at least 5 percent. An efficacy greater than 100 lm/W is also needed if LED products are to offer a viable alternative to incumbent lighting technologies. And these solid-state devices must also deliver a reliability of 20,000 hours under continuous operation and have a cost of ownerships that exceeds 200 lumens per dollar.
At SemiLEDs we have developed an LED that can meet all these criteria. Our firm, which is headquartered in the US and has chip fabrication facilities in Hsinchu Science Park, Taiwan, released an I-core LED in December 2009 that is capable of very high levels of performance, thanks to its vertical device architecture and metal alloy base.
The I-core shares the vertical LED (VLED) structure of all our products. It consists of: a mirror, directly deposited on copper alloy that acts as an anode and reflector; a 0.2 µm thick p-GaN/p-AlGaN layer; an InGaN/GaN multiple quantum well active region; and a 4 µm thick n-GaN layer (see Figure 1). The n-surface is patterned to enhance the light extraction. One benefit of this novel architecture is that it avoids the current crowding issues that plague conventional LEDs, because current can pass vertically from the anode to the cathode. Additional strengths are that the photons generated in the active layer can escape without passing through any semi-transparent conductive
April/May 2010
www.compoundsemiconductor.net 39
contact layer, and the extraction efficiency is improved with the mirror, which is highly reflective at visible wavelengths. Thanks to this design, our LEDs trap far less light than typical, large LEDs on sapphire.
Copper alloy vs sapphire
Our VLEDs have excellent thermal management that stems from the copper alloy foundation. The thermal conductivity of this metal, 400W/mK, is far higher than that of any substrate currently used for LEDs – it is more than ten times that of sapphire - and this gives our devices superior heat dissipation through the chip and out of the heat sink. This leads to a lower junction temperature and ultimately a higher-efficacy, longer-lasting device.
Conventional LEDs are also hampered by the location of the n- and p-type electrode pads. Both these pads are located on the same side as sapphire, an electrical insulator. Device processing involves the removal of p- GaN and the active region to expose the n-GaN layer on which the n-pad is deposited. The downside of all this processing is that it cuts the total emission area of the device.
Figure 1: SemiLEDs’ devices employ a vertical conduction geometry and feature a metal alloy foundation. The metal alloy is highly reflective, and can boost the light extraction efficiency of the LED
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