Packaging designs for geometrical shaping light emitting diodes

Abstract

Over more than two decades, Gallium-nitride (GaN)-based light-emitting diodes (LEDs) have successfully penetrated many applications starting with point light sources, general indoor lightings, LED displays and backlight displays, followed by automotive. This can be attributed to the LED’s breakthroughs in energy efficiency, robustness, environmental sustainability and versatile form factor. Despite the achievements, some of their features stills lag its former counterparts, such as fluorescence lamp and incandescent bulb. While integrating multiple LED chips into old-fashioned form factors, LED luminaires inevitably exhibit non-uniform luminance output and generate visual discomfort. Many attention has been drawn to the geometry shaping of the LEDs which can effectively improve the visual comfort and enhance optical performance. Nevertheless, a conventional packaging scheme (CM-LED) for the geometry-shaped LEDs, in which a foreign substrate, such as sapphire, is mounted onto a package surface, brings about drawbacks in electrical and optical properties. Therefore, the thesis demonstrates two different packaging schemes for geometry-shaped LED devices and investigates their characteristics. In addition to it, to explore functionalities of the geometry-shaped LED, miniaturized LED tubes equipped with 12 stripe-shaped LEDs are fabricated and studied. First, InGaN/GaN stripe-shaped LEDs in flip-chip packaging (FC-LED) and vertically-mounted packaging (VM-LED) were demonstrated. Compared with the CM-LED, the two packaging schemes enhanced light extraction efficiency, and emission profile in a suitable way for general lighting applications. The FC-LED improves luminous output by ~15% at high current operation without efficiency droop. On the other hand, the VM-LED achieved ~45 % higher light extraction efficiency by enlarging illumination surface, which can be a promising alternative to the current packaging scheme. Comparisons, through simulation modeling and experiment result, justify the application of the different packaging schemes for the stripe-shaped LEDs. Lastly, a LED tube equipped with the stripe-shaped LEDs is demonstrated, and compared with a typical LED tube in which square-shaped LEDs are integrated. While the square-shaped LED tube only can produce uniform light output by using an external optical diffuser, the stripe-shaped LED tube achieves uniform luminous output by arranging them in a line. When it comes to luminous intensity of the uniform luminaires, the stripe-shaped LED tube exhibits fivefold increase in output intensity than that of the square-shaped LED tube with the optical diffuser. Moreover, the unified glare rating (UGR) which quantify the degree of discomfort glare is measured to further compare discomfort glare that each luminaire causes. The stripe-shaped LED tube ameliorates the UGR value to a desirable level for office lighting application. As a result, the LED tube with stripe-shaped LEDs not only improve optical properties without compromising LED’s advantages, but it also enhances visual comfort by accomplishing the uniformity in luminous output.