An analytic model is developed for the droop in the efficiency-versus-current curve for light-emitting diodes (LEDs) made from semiconductors having strong asymmetry in carrier concentration and mobility. For pn-junction diodes made of such semiconductors, the high-injection condition is generalized to include mobilities. Under high-injection conditions, electron drift in the p-type layer causes a reduction in injection efficiency. The drift-induced leakage term is shown to have a 3rd and 4th power dependence on the carrier concentration in the active region; the values of the 3rd-and 4th-order coefficients are derived. The model is suited to explain experimental efficiency-versus-current curves of LEDs. V C 2012 American Institute of Physics. [http://dx.

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.

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.