We report the effect of substrate thickness on the optical and thermal characteristics of InGaN/GaN light emitting diodes (LEDs), operating at l $ 450 nm, with different mesa sizes. For various mesa sizes with different substrate thicknesses, the junction temperature (T j ) is measured as a function of injection current by the forward voltage method and the characteristic temperature is also investigated by measuring the temperaturedependent electroluminescence spectrum. Based on the experimentally determined heat source density, the junction temperature, heat flux, and thermal resistance (R th ) are calculated theoretically by using a three-dimensional anisotropic heat dissipation model. The use of a thin substrate thickness of 150 mm effectively improves the heat extraction capability due to the shorter heat transfer path length. For 150 mm (443 mm) thick substrate, the T j value of LEDs is experimentally estimated as 307.7 K (322.6 K), 311.6 K (329.5 K), and 323 K (355.9 K) under an injection current of 80 mA for 450 Â 450, 400 Â 400, and 300 Â 300 mm 2 , respectively. At a high injection current of 400 mA, the T j of 450 Â 450 mm 2 LED with 150 mm thick substrate is reduced by $25.7% compared to that of LED with 443 mm substrate thickness, indicating a value of T j ¼ 384.5 K. The R th value is decreased for larger mesa size and thinner substrate thickness, resulting in R th < 30 K/W for 450 Â 450 mm 2 LED with 150 mm thick substrate. The theoretically calculated R th values indicate a good agreement with the experimentally measured data.

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