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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.


GaN-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.   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   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 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 2: One of the weaknesses of conventional LEDs on sapphire is their lateral conduction path between the n- and p-type electrodes. This increase the resistance in the device, thereby increasing the LED’s operating voltage and reducing its efficiency   A further weakness of the conventional device architecture is associated with its routing of the current through the chip (see Figure 2). As current passes from the anode to the cathode it spreads laterally along the n- GaN layer. This causes current crowding underneath the n-pad, which increases serial dynamic resistance and operating voltage, and ultimately impairs LED efficacy. Conventional GaN-on sapphire LEDs can also be hampered by a lack of current spreading in the p-GaN layer, due to its low conductivity.   Figure 3: One of the strengths of SemiLEDs device architecture is the vertical conduction path that abolishes current crowding   Weaknesses associated with GaN-on-sapphire LEDs, such as current crowding issues and the low thermal conductivity of the substrate (which is typically 100 μm thick) force conventional LEDs to operate under low current density conditions in order to prevent thermal problems. This hampers the application of conventional GaN LEDs on sapphire, especially high-power versions used for required for solid-state lighting applications. Current crowding is not an issue in our VLEDs, because the current travels vertically from the bottom anode to the top cathode (see Figure 3). This path slashes the serial dynamic resistance compared to conventional LEDs on sapphire. In addition, n-GaN has much higher conductivity than its p-type cousin, and this enables sufficient current spreading – there is no need to resort to a semitransparent conductive layer. Avoiding the use of this semi-transparent layer reduces light absorption in the device, thereby boosting LED output, and the excellent current spreading in the n-type layer opens the door to scaling chip dimensions without impacting efficiency. LEDs with a vertical architecture can generate heat in the active region and at the metal contact, and dissipate this energy through the p-type GaN interface. However, we have found that heat extraction can be even faster if the thin p-type GaN (~0.2 μm) layer is directly laid on the layers of a high thermal conductivity metal alloy. Thanks to faster heat dissipation, it is possible to realize the high drive currents needed for LEDs targeting solid-state lighting applications. Electrode design

The efficacy of any LED depends on its electrode arrangement, because this affects how light can escape from the top surface of the n-GaN layer. In our devices, we employ an n-electrode pattern that takes into account the current spreading of the n-GaN layer. We have found that each n-electrode line can spread the current over 250 μm. The electrode design for our I-core LED differs from the first-generation devices, and improves light extraction and brightness. Thanks to higher conductivity in the n-GaN layer, the total area covered by the electrode can be reduced, leading to less light being blocked by this structure. On top of this increase in brightness, the latest design offers a high degree of compatibility with single and double wire bonds.   Fig.4 Even at very high drive currents, the operating voltage of SemiLEDs I-core LED is well below 4V   Figure 5: SemiLEDs latest LEDs are more robust than previous products. Output fell by less than 5 percent during a 2,000 hour burn in test at junction temperature of 115±10 degrees C   The efficacy benchmark for LED targeting conventional lighting is 100 lm/W, and our I-Core device is more than capable of this – it can deliver 120 lm/W at a drive current of 350 mA, which corresponds to a forward voltage is only 3.0V. This performance is realized by housing the device in a white LED package. When designing our I-core LED, we did more than just focus on optimizing the brightness of the chip – we also strived to improve reliability. Assessments were made by placing the chip in a SMD package with silicone capsulation, and testing this entity in a closed space with a stable ambient temperature. The device was mounted on a PCB and driven at 350 mA, and we calculated that its junction temperature was 115±10 degrees C. These tests showed that the light output fell by less than 5 percent, even after a burn-in test of 2,000 hours. In real life applications at room temperature, no degradation of light output has been observed so far. The excellent reliability of our I-core LED, which builds on the performance of earlier products, is also evident from the results of our thermal shock tests. These tests involved assessments of packaged LEDs held at temperatures ranging from –40 degrees C (15min) to 125 degrees C (15min). After 500 cycles of high and low temperature aging, our packaged I-core LEDs showed: no obvious degradation of the light output power; a typical forward voltage shift of less than 1 percent for a 350 mA drive current; and no leakage current at -5 V.   Figure 6: The reliability of packaged VLED chips in light output power after 3 times reflow   Our packaged I-core LEDs are capable of operating in harsh environments, such as desert conditions or on the outside of an airplane. Figure 6 shows the reliability of packaged I-core chips and light output power after subjected to three times reflow cycle testing, a process that involves heating the device from ambient to around 250 degrees C, and then cooling it at up to 6 degrres C per second. The light output remains steady at 100 percent throughout these cycles. Figure 7 shows that the packaged I-core LEDs show no operation voltage failures after three times reflow cycle testing.   Figure 7: The reliability of packaged I-core LEDs in operation voltage at 350mA after 3 times reflow   When we designed our I-core LED, we addressed all the major issues that are currently facing high power LEDs. Our latest product is not only the most reliable device that we have ever made – it is also significantly brighter. This is partly the result of a robust design that delivers far high light extraction, and also a consequence of a superior epitaxial structure that delivers a higher internal efficiency. Another key strength of the I-core is the patented, proprietary MvpLED technology on which it is based, which features a vertical structure with a copper alloy base that provides multiple benefits over conventional LEDs on sapphire. Thanks to all these strengths, we believe that the I-core is well-positioned to be at the forefront of the solid-state lighting revolution.   SemiLEDs has improved the electrode geometry of its LEDs. The electrodes in the previous generation of devices (left) covered a higher proportion of the chip area than the I-Core LED (right), and this led to higher light absorption



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