312 Electrical-Efficiency Analysis of GaN-Based Light-Emitting Diodes with Interdigitated-Mesa Geometry YI-SHENG TING, 1 CHII-CHANG CHEN, 2,4 JINN-KONG SHEU, 1 GOU-CHUNG CHI, 1 and JUNG-TSUNG HSU 3 1.—Department of Physics, National Central University, 320 Jung-Li, Taiwan. 2.—Institute of Optical Sciences, National Central University. 3.—Opto-Electronics and Systems Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 310, Taiwan. 4.—E-mail: trich@ios.ncu.edu.tw We propose a novel method to analyze the current-voltage (I-V) characteristics of GaN-based light-emitting diodes (LEDs) with different p-type electrode- mesa geometries. The electrical efficiency is analyzed by calculating the elec- tric field under the quasi-coplanar electrodes of GaN-based LEDs. The experi- mental results for GaN-based LEDs of chip sizes of 350  350 m 2 and 1,000  1,000 m 2 with interdigitated fingers are compared. A good agreement is ob- tained between the experimental and theoretical electrical efficiency of the GaN LEDs with a chip size of 1,000  1,000 m 2 . The current-crowding effect is an- alyzed by measuring the electroluminescence spectra of the devices. The result indicates that the current-crowding effect is largely reduced by increasing the number of interdigitated fingers. The electrical efficiency of a LED with a chip size of 1,000  1,000 m 2 can be also enhanced by increasing the number of interdigitated fingers, showing the advantages of GaN LED with interdigi- tated-mesa geometries. Key words: GaN-based LEDs, integrated-mesa geometry, efficiency Journal of ELECTRONIC MATERIALS, Vol. 32, No. 5, 2003 Special Issue Paper (Received October 2, 2002; accepted December 3, 2002) INTRODUCTION The GaN and its alloys with InN and AlN have at- tracted considerable interest because of their suit- ability for blue, green, and ultraviolet light emitters, such as light emitting diodes (LEDs) and laser diodes. The epitaxial layers of the conventional GaN-based LED are often grown on the insulating substrates (sapphire) and are etched to form square- shaped mesa patterns. The p- and n-type electrodes of the conventional GaN LED are quasi coplanar. The maximum electric field under the coplanar elec- trodes occurs at the edge of the electrodes. 1 This leads to a significant current crowding under the p-type electrodes near the n-type electrodes. There- fore, different designs of GaN LEDs have been pro- posed to increase the uniformity of the current spreading. 2–6 Jin et al. have proposed the microdisk LED. 3,4 Lumileds Lighting has proposed the high- power AlGaInN flip-chip LEDs. 5,6 The similar struc- tures with interdigitated-mesa geometry have also been studied by Guo et al. 7 Although the light emitting area (p-n junction area) is reduced when the p-type electrodes are sep- arated into a interdigitated or microdisk pattern, the reported results still showed that the quantum efficiency of a GaN LED with interdigitated-mesa geometry and the microdisk LED is higher than the conventional square-shaped mesa structure. This is not only due to the fact that the larger mesa side- wall results in a better light extraction but also due to the reduction of the current spreading effect. Because the refractive index of the GaN 8 (2.48) is much higher than sapphire 9 (1.77), the GaN-based epitaxial layers can be seen as a planar waveguide. The light extraction of the GaN LED depends on the loss of the planar waveguide that takes place at the etched-mesa sidewall between the p- and n-type electrodes. The overlap of the optical fields in the etched and nonetched waveguides, which has been intensively used in the research of integrated op- tics, 10 can be performed to evaluate the loss of the light propagating in the planar waveguide. With this model, recently, we reported the optical im- provement of the extraction efficiency for a GaN LED with interdigitated-mesa geometry compared