Thermal properties of AlGaInP/GaInP MQW red LEDs are investigated by thermal measurements and analysis for different chip sizes and substrate thicknesses. To extract the thermal resistance (R-th), junction temperature (T-j) is experimentally determined by both forward voltage and electroluminescence (EL) emission peak shift methods. For theoretical thermal analysis, thermal parameters are calculated in simulation using measured heat source densities. The T-j value increases with increasing the injection current, and it decreases as the chip size becomes larger. The use of a thin substrate improves the heat removal capability. At 450 mA, the T-j of 315 K and 342 K are measured for 500 x 500 mu m(2) LEDs with 110 mu m and 350 mu m thick substrates, respectively. For 500 x 500 mu m(2) LEDs with 110 mu m thick substrate, the R-th values of 13.99 K/W and 14.89 K/W are obtained experimentally by the forward voltage and EL emission peak shift methods, respectively. The theoretically calculated value is 13.44 K/W, indicating a good agreement with the experimental results. (C) 2010 Elsevier Ltd. All rights reserved.