Current spreading in a high-power flip-chip light-emitting diode (LED) and its effect on the chip thermal resistance has been studied both theoretically and experimentally. Thermal resistances of various LED units have been determined by measuring the forward voltage relaxation under pulsed current excitation of the LED at varied duty cycle. The total thermal resistance of the chip is found to rise by ∼20% while the LED operating current increasing from zero to 1 A. The current density distribution in the LED active region predicted by coupled simulations of the current spreading and heat transfer agrees well with the measured near-field distribution of the light emission intensity. The observed rise in the thermal resistance is attributed to current crowding producing lateral non-uniformity in the temperature distribution inside the LED chip.
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