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Using a novel experimental technique, Korean scientists provide new insight into the efficiency loss (droop) in GaN-based light emitting diodes (LEDs). This droop phenomenon puzzles countless researchers around the world for more than a decade. The scientists employed scanning thermal microscopy to measure the temperature profile across the epi-layers of an operating GaN-LED. Featuring nanoscale resolution, the measurement shows a hot spot which moves from the active layers towards the p-doped side of the LED with rising current, accompanied by increasing efficiency droop. The authors conclude that electron leakage into the p-doped side becomes the main cause of the droop at high current, while Auger recombination inside the active layers dominates only at low current.