Thermal droop refers to the loss in efficiency of LEDs as they're operated at elevated temperatures. Because of temperature-dependent semiconductor properties an LED will experience an increase in electrons lost via carrier leakage and/or overshoot with increasing junction temperature. Thermal droop occur in both AlGaInP and InGaN LEDs. At their maximum rated operating temperature of 150°C the temperature related decrease of efficiency can be as high as 70% in AlGaInP LEDs, whereas InGaN LEDs can lose up to 25% of optical power. Thermal droop is not to be confused with efficiency droop which is the decrease in internal quantum efficiency (IQE) as the current density is raised.
