Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

Efficiency droop, i.e. the loss of efficiency at high operating current, afflicts nitride-based light-emitting diodes (LEDs). The droop phenomenon is currently the subject of intense research, as it retards the advancement of solid-state lighting which is just starting to supplant fluorescent as well as incandescent lighting. Although the technical community does not yet have consented to a single cause of droop, this article provides a summary of the present state of droop research, reviews currently discussed droop mechanisms, and presents a recently developed theoretical model for the efficiency droop. In the theoretical model, carrier leakage out of the active region caused by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, is discussed in detail. The model is in agreement with the droop's key behaviors not only for GaInN LEDs but also for AlGaInP LEDs.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201200025

Efficiency droop in light‐emitting diodes: Challenges and countermeasures

Temperature dependent Cr3+ photoluminescence in garnets of the type X3Sc2Ga3O12 (X = Lu, Y, Gd, La)

Temperature dependent luminescence Cr3+doped GdAl3(BO3)4 and YAl3(BO3)4

Gallium nitride nanostructures for light-emitting diode applications

The effect of gradual indium gallium nitride (InGaN) quantum wells (QWs) on the suppression of efficiency-droop in green light-emitting diodes (LEDs) is numerically investigated. The presented scheme increases the internal quantum efficiency by 45.5% at I=20nmA and 55.7% at I=100nmA, indicating a considerable reduction of efficiency-droop. This improvement is attributable mainly to the use of the gradual InGaN QW's structure that significantly alleviates band bending in the valence band, improving the transport efficiency of injected holes above that of conventional LEDs. The radiative recombination is thus enhanced as the overlap between electron and hole wave functions is increased. Most importantly, the leakage of injected electrons to p-type region is correspondingly reduced, in turn suppressing the efficiency-droop in the LED.

Reduction in the Efficiency-Droop Effect of InGaN Green Light-Emitting Diodes Using Gradual Quantum Wells

The junction temperature of a light-emitting diode (LED) directly and greatly affects its performances. Therefore, the reliable measurement and accurate estimation of the junction temperature of an LED is extremely important. This paper proposes an approach for directly determining the dependence of junction temperature on injected currents in InGaN and AlGaInP LEDs. Various important physical parameters that affect the junction temperature of an LED are also considered.

Determination of Junction Temperature in InGaN and AlGaInP Light-Emitting Diodes

This paper describes a novel structure of an AlGaInP light-emitting diode (LED) to extract the waveguided light for high-brightness applications. Four devices are considered and compared. They are AlGaInP-sapphire LEDs with: 1) a planar Ag reflector (LED-A); 2) a patterned Ag reflector (LED-B); 3) a planar Ag reflector and surface-roughened features (LED-C); and 4) a patterned Ag reflector and surface-roughened features (LED-D). The patterned Ag reflector can be used to direct some of the waveguided light that bounces within the AlGaInP LED and sapphire substrate to the top escape cone of the LED surface. Additionally, the roughened features, which are randomly distributed on the LED surface, enable the waveguided light that is trapped inside the LED chip to be coupled efficiently into the air. As a result, the external quantum efficiencies of LED-A, LED-B, LED-C, and LED-D measured at I = 350 mA are η = 9.20%, 10.46%, 15.22%, and 16.75%, respectively.

Effect of Surface Texture and Backside Patterned Reflector on the AlGaInP Light-Emitting Diode: High Extraction of Waveguided Light

The current-voltage (I- V) characteristics of the InGaN nanorod light-emitting diode (LED) are evaluated using nanoprobes installed on a field-emission scanning electron microscope. The saturated current of the InGaN nanorod LED (IO = 2 × 10-9A) is found to be orders of magnitude higher than those obtained by downscaling a conventional InGaN LED to a chip size of 300 × 300 μm2 (IO1 = 1 × 10-25 A; IO2 = 1 × 10-14 A). This observation is explained by the fact that the nanorod LED is associated with enhanced tunneling of injected carriers and, therefore, reduction of the defect-assisted leakage current and the diffusion-recombination process that normally occurs in InGaN LEDs.

https://ir.nctu.edu.tw:443/bitstream/11536/21832/1/000293755500026.pdf

Elucidating the Physical Property of the InGaN Nanorod Light-Emitting Diode: Large Tunneling Effect

The junction temperature of light-emitting diodes (LEDs) directly and significantly affects LEDs performances. Thus, accurate measurement and precise estimation of LEDs junction temperature become extremely important. In this study, we analyze the physical foundation of temperature-dependent electrical characteristics and develop a new scheme to directly express the dependence of junction temperature on injected current for InGaN and AlGaInP LEDs. From a more general viewpoint, our scheme for the estimation of junction temperature is primarily based on the LEDs external properties, and therefore can be applied to other kinds of III–V compound-based semiconductor LEDs.

Chih-Hao Chen

Papers

Reduction in the Efficiency-Droop Effect of InGaN Green Light-Emitting Diodes Using Gradual Quantum Wells

Determination of Junction Temperature in InGaN and AlGaInP Light-Emitting Diodes

Effect of Surface Texture and Backside Patterned Reflector on the AlGaInP Light-Emitting Diode: High Extraction of Waveguided Light

Elucidating the Physical Property of the InGaN Nanorod Light-Emitting Diode: Large Tunneling Effect

Estimating the Junction Temperature of InGaN and AlGaInP Light-Emitting Diodes