Efficiency droop, also called current density droop, refers to the reduction in the efficiency of LEDs at high operating currents. Operating an LED at higher current densities is desirable because it maximizes light output per unit chip area. However, the lumen output of phosphor converted LEDs isn't always linearly proportional to the forward current. At low current densities, InGaN LEDs exhibit a high internal quantum efficiency (IQE). When the LEDs are driven above the saturation point of current densities, their IQE drops due to the increased rate of Auger recombination. Auger recombination is a non-radiative carrier recombination process which leads to generation of heat (phonon, thermal energy), instead of light (photon, radiant energy). Efficiency droop is not to be confused with thermal droop. Thermal droop is simply the reduction of the optical power when the junction temperature is increased, whereas efficiency droop is associated with the high carrier concentration in the active region of the diode.
