"U-turn" feature in the efficiency-versus-current curve of GaInN/GaN light-emitting diodes

The onset of the efficiency droop in GaInN/GaN blue light-emitting diodes (LEDs), i.e., the maximum-efficiency point, typically occurs at current densities of 1–10 A/cm2 and the efficiency decreases monotonically beyond the onset. At typical operating current densities (10–100 A/cm2), LEDs are strongly affected by the droop. At cryogenic temperatures, an increase in the efficiency, i.e., a “U-turn” feature, is found in the droop regime of the efficiency-versus-current curve. The occurrence of the U-turn feature coincides with a distinct increase in device conductivity, which is attributed to an enhancement in p-type conductivity that in turn increases the injection efficiency.