We model the carrier recombination mechanisms in GaInN/GaN light-emitting diodes as R=An+Bn2+Cn3+f(n), where f(n) represents carrier leakage out of the active region. The term f(n) is expanded into a power series and shown to have higher-than-third-order contributions to the recombination. The total third-order nonradiative coefficient (which may include an f(n) leakage contribution and an Auger contribution) is found to be 8×10−29 cm6 s−1. Comparison of the theoretical ABC+f(n) model with experimental data shows that a good fit requires the inclusion of the f(n) term.

The reverse leakage current of a GaInN light-emitting diode (LED) is analyzed by temperature dependent current–voltage measurements. At low temperature, the leakage current is attributed to variable-range-hopping conduction. At high temperature, the leakage current is explained by a thermally assisted multi-step tunneling model. The thermal activation energies (95–162 meV), extracted from the Arrhenius plot in the high-temperature range, indicate a thermally activated tunneling process. Additional room temperature capacitance–voltage measurements are performed to obtain information on the depletion width and doping concentration of the LED.

The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.GaN-based high-power light-emitting diodes (LEDs) have become increasingly prevalent in illumination applications such as interior/exterior lighting and automotive headlights. However, a long standing problem called "efficiency droop" has been dimming the future prospects of LEDs as the ultimate illumination sources. The efficiency droop can be categorized using two classifications: current-density droop and temperature droop. First, the conventional definition of efficiency droop describes the decrease in radiative efficiency with increasing operating current, which we call the current-density droop, J-droop, in this study. Several explanations have been proposed for the causes of the J-droop, including electron leakage due to polarization mismatch 1 and poor hole injection caused by asymmetry of carrier-transport properties, 2 delocalization of carriers, 3,4 density-activated defect recombination, 5,6 and Auger recombination. 7 These have led to possible solutions such as polarization matched multiple quantum well (MQW) structures, double heterostructure designs, and large-size (large junction area) devices for reducing the current density. Second, GaNbased LEDs also suffer from a strong decrease in radiative efficiency with increasing temperature, 8,9 which we call the temperature droop, T-droop, in this study. Figure 1 shows the external quantum efficiency (EQE) of a commercial high-power LED as a function of driving current measured at several ambient temperatures. The EQE peaks at a low forward current and then drops with increasing current, showing typical J-droop behavior. In addition, the EQE decreases significantly as the ambient temperature increases; increasing temperature from 300 to 450 K results in the reduction of the EQE by about 30% of its peak value, indicating that the T-droop can be more detrimental than the J-droop. High temperature-tolerant LEDs are becoming increasingly important in applications where a weak-temperature-dependence of the EQE is highly desirable, for example, automotive headlights for which the ambient temperatures can be as high as 90 C. Until now, however, T-droop has been less focused on than the J-droop.There has been a great deal of research to understand the mechanisms of efficiency droop caused by high current densities; however, the understanding of T-droop, originating from high temperature, is not comprehensive. One of the technology trends in current state-of-the-art LEDs is to increase the chip size, thus decreasing t...

The effect of the asymmetry in carrier concentration and mobility is studied in GaInN pn-junction light-emitting diodes (LEDs). We propose and present experimental evidence that the asymmetry in carrier concentration and mobility, and associated high-level injection phenomena, cause efficiency droop in GaInN LEDs. Low temperatures exacerbate the degree of asymmetry of the junction by reducing acceptor ionization, and shift high-injection-phenomena to lower currents. Accordingly, at temperatures near 80 K, we measure a greater droop compared to room temperature. The analysis of temperature-dependent I–V curves shows an excellent correlation between the onset of high-level injection and the onset of droop.

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Reduction in the light-output power in GaN-based light-emitting diodes (LEDs) with increasing temperature is a well-known phenomenon. In this work, temperature dependent external-quantum-efficiency versus current curves are measured, and the mechanisms of recombination are discussed. Shockley-Read-Hall recombination increases with temperature and is found to greatly reduce the light output at low current densities. However, this fails to explain the drop in light-output power at high current densities. At typical current density (35 A/cm2), as temperature increases, our results are consistent with increased Shockley-Read-Hall recombination and increased electron leakage from the active region. Both of these effects contribute to the reduction in light-output power in GaInN/GaN LEDs at high temperatures.

Optimization of solid-state lighting spectra is performed to achieve beneficial and tunable circadian effects. First, the minimum spectral circadian action factor (CAF) of 2700 K white light-emitting diodes (LEDs) is studied for applications where biologically active illumination is undesirable. It is found that white-LEDs based on (i) RGB chips, (ii) blue & red chips plus green phosphor, and (iii) blue chip plus green & red phosphors are the corresponding minimum-CAF solutions at color-rendering index (CRI) requirements of 80, 90, and 95, respectively. Second, maximum CAF tunability of LED clusters is studied for dynamic daylighting applications. A dichromatic phosphor-converted blue-centered LED, a dichromatic phosphor-converted green-centered LED, and a monochromatic red LED are grouped to obtain white spectra between 2700 K and 6500 K. A maximum CAF tunability of 3.25 times is achieved with CRI above 90 and luminous efficacy of radiation of 313 - 373 lm/W. We show that our approaches have advantages over previously reported solutions on system simplicity, minimum achievable CAF value, CAF tunability range, and light source efficacy.

At room temperature, AlGaInP pn-junction light-emitting diodes (LEDs) emitting at 630 nm do not exhibit an efficiency droop. However, upon cooling the AlGaInP LEDs to cryogenic temperatures, they show a pronounced efficiency droop. We attribute the efficiency droop in AlGaInP LEDs to electron-drift-induced reduction in injection efficiency (i.e., carrier leakage out of the active region) mediated by the asymmetry of the pn junction, specifically the disparity between electron and hole concentrations and mobilities, with the concentration disparity exacerbated at low temperatures. V