Optical design of large-area GaN-based LEDs

The light extraction process in GaN-based light emitting diodes (LEDs) is studied in this paper. In order to increase the light extraction efficiency of large area LEDs, several novel LED geometries are discussed. The light propagation in the LEDs is simulated numerically by using the finite-difference time-domain (FDTD) method. It is shown that the following improvements in the GaN-based LEDs are very effective for increasing the light extraction: (1) To fabricate GaN micro-pyramid array on the surface of the LED, which guides the generated light to the surface; (2) To make inverted V-shaped groove formation on the GaN layer, which restricts the average length of ray path in the LEDs and refracts the waveguide-mode light to the surface; (3) To separate the LED epilayer from its substrate and then mount it on a metal mirror base, which is used to reflect the backside light to the LED surface. The FDTD simulation results show clearly that these improved geometries guide most of the internal luminescence to escape from the LED, and increase greatly the external light-extraction efficiency of GaN-based LEDs.