Quantum efficiency investigation at high current density of Ultra-High-Brightness LEDs

This study discusses the optical characteristics of Ultra-High-Brightness LEDs (UHB-LEDs) at high current density. The wafer bonding technique was applied in this research to enhance the light extraction efficiency of AlGaInP UHB-LEDs. This technique can replace the GaAs substrate with transparent substrates for achieving the high quantum efficiency. However, the structure that designed for light extraction performed excellent quantum efficiency at low current density but performed badly at high current density. The quantum efficiency droop dramatically as current density increase. Factors of efficiency droop included epitaxy design, process design, package and system thermal design, and the relations among them. In particular, heat in chip level deeply affected the quantum efficiency and highly related to all factors. Therefore, how to separate thermal effect from all factors is important for designer to optimize the structure. In this research, a transient thermal management system to minimum the heat in the chip was demonstrated. The optical and electrical characteristics was measured with a extremely short pulse time and different duty cycle at first, and then curve fitting the result to find out the optical performance without heat under certain current density. A wavelength shift method is performed to validate the thermal management system and the concept of optical performance with no heat accumulation. The experimental data shows a good agreement with the wavelength shift method. After that, the performance ratio of different current density without heat effect also showed. Comparing it to the performance ratio with TO-18 package, one can clearly define how much loss generated from thermal effect. It reveals that LED with TO-18 package has 10% loss from heat accumulation under 1A/mm², furthermore, when it is driven on higher density, the extremely heat accumulates and lead the intensity ratio to saturate, and the difference are more than 70%. Eventually, the quantum efficiency influenced by heat under high current density may be a substantial issue in further study for lighting applications.