What does the ‘flat spot’ in an LED indicate and why is it there?

The “flat spot” in an LED, often referred to as a “droop” or “efficiency droop,” is a phenomenon where the efficiency of the LED decreases as the current flowing through it increases. In other words, as the LED is driven with higher currents, its efficiency in converting electrical power into light diminishes. This characteristic is more pronounced in high-power LEDs commonly used in lighting applications.

One primary reason for the flat spot in LED efficiency is the Auger recombination mechanism. At higher current levels, excess charge carriers (electrons and holes) within the LED semiconductor material can experience Auger recombination, a process where one electron transfers its energy to another, creating a free electron and a more energetic hole. This excess energy is often released as heat rather than light, leading to a reduction in the overall efficiency of the LED.

Another contributing factor to efficiency droop is the increased temperature of the LED at higher currents. LEDs are sensitive to temperature changes, and as the operating temperature rises due to increased current, the internal quantum efficiency of the semiconductor material decreases. This decrease in quantum efficiency contributes to the reduction in light output efficiency observed in the flat spot.

Researchers and engineers continue to explore ways to mitigate efficiency droop in LEDs. Strategies include improving the thermal management of LED devices, developing more efficient semiconductor materials, and optimizing the design of LED structures. Despite the challenges posed by efficiency droop, LEDs remain a highly efficient and versatile lighting technology, with ongoing research aimed at further enhancing their performance at higher current levels.