The Effect of Package Structure on the Light Extraction Efficiency of Near-Ultraviolet LED

Abstract

The near-ultraviolet high-power LED, with five package structures, is designed and fabricated. The efficiency of electric conversion to light (EECL) of various package structures is measured, and the EECL is used to characterize the light extraction efficiency (LEE) of the package structure. By analyzing the Fresnel loss (FNL) and total inner reflection loss (TIRL) of light at different interfaces, the experimental results are explained qualitatively. When approximate spherical lens is used, filling silica gel between chip and lens can improve the LEE. The EECL of the device can reach 66.84%, which is higher than that of bare chip by 55.10%. In planar packaging, the EECL of the device is lower than that of the bare chip, whether or not it is filled with silica gel. After filling with silica gel, a new TIRL is produced at the final interface of the device, and the EECL of the device is even lower, which is only 37.14%. The experimental results show that when the LED chip adopts graphic substrate and surface microstructure to improve the LEE, the traditional coating of silica gel (or epoxy) and other materials on the chip surface may not improve the LEE of the device. If the introduction of the silica gel layer leads to a new total inner reflection interface, it will result in a significant decrease in LEE of the device. When a layer, such as silica gel, does not result in a new total reflection interface, the FNL and TIRL of the chip surface can be effectively reduced, and the LEE of the LED device can be improved.