Efficiency droop in GaN-based light emitting diodes (LEDs) under high injection current density perplexes the development of high-power solid-state lighting. Although the relevant study has lasted for about 10 years, its mechanism is still not thoroughly clear, and consequently its solution is also unsatisfactory up to now. Some emerging applications, e.g., high-speed visible light communication, requiring LED working under extremely high current density, makes the influence of efficiency droop become more serious. This paper reviews the experimental measurements on LED to explain the origins of droop in recent years, especially some new results reported after 2013. Particularly, the carrier lifetime of LED is analyzed intensively and its effects on LED droop behaviors are uncovered. Finally, possible solutions to overcome LED droop are discussed.

Efficiency droop is currently one of the most popular research problems for GaN-based light-emitting diodes (LEDs). In this work, a differential carrier lifetime measurement system is optimized to accurately determine carrier lifetimes (τ) of blue and green LEDs under different injection current (I). By fitting the τ-I curves and the efficiency droop curves of the LEDs according to the ABC carrier rate equation model, the impact of Auger recombination and carrier leakage on efficiency droop can be characterized simultaneously. For the samples used in this work, it is found that the experimental τ-I curves cannot be described by Auger recombination alone. Instead, satisfactory fitting results are obtained by taking both carrier leakage and carriers delocalization into account, which implies carrier leakage plays a more significant role in efficiency droop at high injection level.

On-chip frequency upconversion of a near-infrared (NIR) Kerr comb in a χ⁽²⁾ and χ⁽³⁾ system provides a convenient route to extending the comb spectra into the visible band. Yet to date, only limited visible or near-visible comb lines have been obtained using this scheme. In this work, we demonstrate the generation of multiple near-visible comb lines based on spectral translation from a broadband NIR Kerr comb. This physical process is implemented in an aluminum nitride (AlN)-on-sapphire microring, where we achieve a wideband frequency upconversion by incorporating the phase-mismatched fundamental and first-order near-visible modes. Upon tuning the pump into the resonance with sufficient power, we attain a broadband NIR Kerr comb and 153 corresponding near-visible comb lines in 720–840 nm with a reasonable efficiency over 4.1 × 10⁻⁵%. The wideband frequency upconversion can be adapted to on-chip frequency stabilization of self-referenced microcombs, as required for precision optical clocks and frequency metrology.

InGaN/GaN multilayer quantum dot (QD) structure is a potential type of active regions for yellow-green light-emitting diodes (LEDs). The surface morphologies and crystalline quality of GaN barriers are critical to the uniformity of InGaN QD layers. While GaN barriers were grown in multi-QD layers, we used improved growth parameters by increasing the growth temperature and switching the carrier gas from N2 to H2 in the metal organic vapor phase epitaxy. As a result, a 10-layer InGaN/GaN QD LED is demonstrated successfully. The transmission electron microscopy image shows the uniform multilayer InGaN QDs clearly. As the injection current increases from 5 to 50 mA, the electroluminescence peak wavelength shifts from 574 to 537 nm.