Even though light-emitting diodes (LEDs) may have a very long life, poorly designed LED lighting systems can experience a short life. Because heat at the p-n-junction is one of the main factors that affect the life of the LED, by knowing the relationship between life and heat, LED system manufacturers can design and build long-lasting systems. In this study, several white LEDs from the same manufacturer were subjected to life tests at different ambient temperatures. The exponential decay of light output as a function of time provided a convenient method to rapidly estimate life by data extrapolation. The life of these LEDs decreases in an exponential manner with increasing temperature. In a second experiment, several high-power white LEDs from different manufacturers were life-tested under similar conditions. Results show that the different products have significantly different life values.

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Life of LED-based white light sources

Light emitting diodes reliability review

This paper proposes a concept of electrolytic capacitor-less light-emitting diode (LED) driver, which converts the commercial ac voltage to a pulsating current with twice the line frequency driving high-brightness LEDs. As no electrolytic capacitor is used, this driver possesses the unique advantage of long lifetime to match with that of LEDs. A method of injecting the third and fifth harmonics into the input current to reduce the peak-to-average ratio of the output current is also proposed. While ensuring that the input power factor is higher than 0.9 to meet regulation standards such as ENERGY STAR, the proposed method allows the peak-to-average ratio of the output current to be reduced to 1.34 theoretically, which is beneficial for the safe operation of the LEDs. As an example, a flyback-based electrolytic capacitor-less LED driver is proposed, and its operation is analyzed. In order to inject the third and fifth harmonics into the input current, the function of the duty cycle in a half-line cycle is derived. It is then simplified to a fitting function, which can be easily implemented with the input voltage sensing. A 25 V, 0.35 A output prototype is built and tested in the laboratory, and the experimental results are presented to verify the effectiveness of the electrolytic capacitor-less LED driver and its control method.

A Method of Reducing the Peak-to-Average Ratio of LED Current for Electrolytic Capacitor-Less AC–DC Drivers

We review the degradation mechanisms that limit the reliability of GaN-based light-emitting diodes (LEDs). We propose a set of specific experiments, which is aimed at separately analyzing the degradation of the properties of the active layer, of the ohmic contacts and of the package/phosphor system. In particular, we show the following: 1) low-current density stress can determine the degradation of the active layer of the devices, implying modifications of the charge/deep level distribution with subsequent increase of the nonradiative recombination components; 2) high-temperature storage can significantly affect the properties of the ohmic contacts and semiconductor layer at the p-side of the devices, thus determining emission crowding and subsequent optical power decrease; and 3) high-temperature stress can significantly limit the optical properties of the package of high-power LEDs for lighting applications.

A Review on the Reliability of GaN-Based LEDs

New developments in theoretical studies of defects and impurities in III-Nitrides as pertinent to compensation and recombination in these materials are discussed. New results on experimental studies on defect states of Si, O, Mg, C, Fe in GaN, InGaN, and AlGaN are surveyed. Deep electron and hole traps data reported for GaN and AlGaN are critically assessed. The role of deep defects in trapping in AlGaN/GaN, InAlN/GaN structures and transistors and in degradation of transistor parameters during electrical stress tests and after irradiation is discussed. The recent data on deep traps influence on luminescent efficiency and degradation of characteristics of III-Nitride light emitting devices and laser diodes are reviewed.

Deep traps in GaN-based structures as affecting the performance of GaN devices

This paper presents failure modes observed in long-term aging of high-brightness GaN/InGaN LEDs. The blue LEDs submitted to DC aging test present large decrease of emitted optical power and increase of diode reverse leakage current. Increase of parasitic series resistance, suggesting contact degradation, has also been found in stressed sample, together with apparent carrier density increases and reduction of the junction depletion width. Furthermore stressed LEDs present modification of a specific trap property: trap activation energy decreases from 340 meV in the virgin sample down to 75 meV in the stressed sample. Generation of non-radiative recombination centers seems to be one of the dominant failure mechanisms responsible for the observed electrical and optical LED degradations.

Failure Modes and Mechanisms of DC-Aged GaN LEDs

The optical properties of blue InGaN-based light-emitting diodes aged at high current levels have been studied by electroluminescence and cathodoluminescence. The onset of a broad optical band peaked at about 3.1 eV in devices aged without a heat sink (junction temperature higher than 300 °C) has been correlated to an electrothermal threshold effect. The band is attributed to the dissociation of Mg–H complexes inside the p-type layers and to the consequent formation of Mg-related metastable complexes acting as acceptors. Subsequent electron-beam irradiation determines the almost complete quenching of the band.

Optical evidence of an electrothermal degradation of InGaN-based light-emitting diodes during electrical stress

Reliability of visible GaN LEDs in plastic package

Gallium-nitride-based light-emitting diode (LED) accelerated life tests were carried out over devices adopting two different packaging schemes (i.e., with plastic transparent encapsulation or with pure metallic package). Data analyses were done using a Weibull-based statistical description with the aim of estimating the effect of high current on device performance. A consistent statistical model was found with the capability to estimate the mean time to failure (MTTF) of devices during DC current stress and the accelerating factors of high current stresses.

Analysis of DC current accelerated life tests of GaN LEDs using a Weibull-based statistical model

This work presents the results of an extensive DC current aging and failure analysis carried out on blue InGaN/GaN LEDs which identify failure mechanisms related to package degradation, changes in effective doping profile, and generation of deep levels. DLTS and photocurrent spectra indicate the creation of extended defects in devices aged at very high current density.

Simone Levada

Papers

Failure Modes and Mechanisms of DC-Aged GaN LEDs

Optical evidence of an electrothermal degradation of InGaN-based light-emitting diodes during electrical stress

Reliability of visible GaN LEDs in plastic package

Analysis of DC current accelerated life tests of GaN LEDs using a Weibull-based statistical model

Degradation mechanisms of GaN-based LEDs after accelerated DC current aging