M. Dal Lago

Degradation Mechanisms of High-Power LEDs for Lighting Applications: An Overview

This paper reports on the degradation mechanisms that limit the reliability of high-power light-emitting diodes (LEDs) for lighting applications. The study is based on the experimental characterization of state-of-the-art LEDs fabricated by leading manufacturers. We demonstrate that, despite high potential reliability, high-power LEDs may suffer from a number of degradation mechanisms that affect the stability of the blue semiconductor LED chip and of the phosphor layer used for the generation of white light. More specifically, we describe the following relevant mechanisms: 1) the optical degradation of LEDs, due to an increase in the nonradiative recombination rate, which can be correlated to modifications in the forward-bias current-voltage characteristics; 2) the variation in forward voltage, due to the increase in series resistance; 3) the optical degradation of phosphor layers used for blue-to-white light conversion; and 4) the failure of LEDs submitted to “hot plugging,” which is the direct connection of an LED chain to an energized power supply, due to the generation of high current spikes. Results provide an overview on the failure mechanisms that limit the reliability of state-of-the-art LEDs and on the role of current and temperature in determining the failure of the devices.

Failure causes and mechanisms of retrofit LED lamps

Abstract This paper describes one of the first studies of the degradation of retrofit light bulbs based on white GaN light emitting diodes. The results indicate that the lifetime of LED lamps depends mostly on the stability of the driver and optical elements, rather than on the degradation of the LED chips, that have a stable output over stress time. By comparing lamps from four different manufacturers stressed at room and high temperature, we found that (i) long-term stress causes a change of the chromatic properties of the lamps, which is ascribed to the degradation of the phosphors or to the inner LED reflector; (ii) during aging the LED driver may degrade gradually and/or catastrophically, causing a reduction of the output optical power, or a complete failure; (iii) proper thermal management and heat dissipation reduce the degradation rate; (iv) spectral transmissivity measurements and visual inspection reveal the degradation of the diffusive optical elements, which is induced by the short wavelength side of the LED emission spectrum.

Reliability issues in GaN-based light-emitting diodes: Effect of dc and PWM stress

Abstract This paper describes an extensive analysis of the degradation of high-power white LEDs, submitted to dc and pulsed stress. By means of combined electrical and optical characterization we provide experimental evidence for the following: (i) dc stress can induce a significant decrease in the luminous flux of the devices; (ii) degradation rate has a linear dependence on the stress current level, thus indicating that current is a major driving force for degradation; (iii) optical degradation is significantly correlated to the increase in the defect-related current components, and to the red-shift of the emission wavelength of the main blue peak. On the basis of these results, degradation is ascribed to the increase in the defectiveness of the active region, with subsequent generation of defective or shunt paths; (iv) PWM stress determines a stronger degradation with respect to dc stress, for devices aged under similar conditions. The different mechanisms that may contribute to increasing the degradation rate during PWM stress with respect to dc case are discussed in the paper.

“Hot-plugging” of LED modules: Electrical characterization and device degradation

Abstract GaN-based light-emitting diodes have recently demonstrated to be almost ideal devices for the realization of next generation light sources, thanks to their high performance and long expected lifetime. For a massive market penetration of these devices, their reliability must be significantly improved: if on one hand the physical mechanisms responsible for gradual degradation have been extensively described, on the other hand the origin of catastrophic failures still has to be investigated in detail. One of the most critical situations occurs when an LED module is directly connected to an energized power supply: this event (usually referred to as “hot-plugging”) can generate current spikes up to several tens of amperes, that can potentially destroy or damage the LEDs. The aim of this work is (i) to analyze, for the first time, the nature of the current spikes generated during hot-plugging, (ii) to understand the failure mechanisms of LEDs submitted to high current spikes, and (iii) to present a simplified model to explain the hot plugging phenomenon. The study is based on transient electrical measurements, carried out on several LED modules (fabricated by different manufacturers), connected to three different power supplies. Results reveal that the amplitude and the time constants of the current spikes are directly determined by the number of LEDs connected in series and by the output capacitance of the current driver, and provide information on the gradual or catastrophic failure of LEDs submitted to current spikes.

A study on the reverse-bias and ESD instabilities of InGaN-based green LEDs

Over the last years, important efforts have been done in order to understand the degradation mechanisms of GaN-based LEDs submitted to forward-bias stress tests. On the other hand, only little work has been done to understand the degradation of LEDs submitted to reverse-bias stress. However, this topic is of high interest, since (i) the reverse-bias robustness of the LEDs is strongly correlated to their stability under Electrostatic Discharge (ESD) events and (ii) the analysis of the reverse-bias degradation can provide important information on the role of high electric fields and reverse current in limiting the reliability of the LEDs. Therefore the aim of this paper is to describe a detailed investigation on the reverse-bias degradation of GaN-based LEDs. The results described in this paper indicate that: (i) under reverse bias, LEDs can show a weak luminescence signal, due to the recombination of carriers injected in the quantum-wells; (ii) reverse-bias stress can induce the degradation of the electrical characteristics of the LEDs (increase in reverse-current, decrease in breakdown voltage), due to the generation of point defects in proximity of pre-existing defective regions. (iii) Furthermore, our tests indicate that the defective regions responsible for reverse-current conduction can constitute weak points with respect to ESD events: ESD failures are determined by the shortening of the junction in proximity of one of the defective sites responsible for reverse-current conduction.

Analysis of the mechanisms limiting the reliability of retrofit LED lamps

This paper describes one of the first studies of the degradation of retrofit light bulbs based on white GaN light emitting diodes. The results indicate that the lifetime of LED lamps depends mostly on the stability of the driver and optical elements, rather than on the degradation of the LED chips, that have a stable output over stress time. By comparing lamps from four different manufacturers stressed at room and high temperature, we found that (i) long-term stress causes a change of the chromatic properties of the lamps, which is ascribed to the degradation of the phosphors or to the inner LED reflector; (ii) during ageing the LED driver may degrade gradually and/or catastrophically, causing a reduction of the output optical power, or a complete failure; (iii) proper thermal management and heat dissipation reduce the degradation rate; (iv) spectral transmissivity measurements and visual inspection reveal the degradation of the diffusive optical elements, which is induced by the short wavelength side of the LED emission spectrum.

Innovative methodology for testing the reliability of LED based systems

With this work we propose an innovative method for the analysis of the reliability of LED and Laser devices and systems. The basic idea of the proposed method is the separation of the different degradation forces that lead to the decrease of LED performances during ageing. By using a specific reliability analysis procedure it is possible to separately evaluate the effects of the single accelerating factors: temperature, current intensity, applied signal waveform, voltage overstress, optical and mechanical solicitation. To individually determine the degradation kinetics it is fundamental to separate the effects of temperature and current. For these reasons we carried out iso-currents reliability tests, where several devices have been stressed with the same current at different junction temperatures, and iso-thermal stresses, where junction temperature is instead constant for different applied currents. The result of the analysis will be a multivariable law that relates the several degradation parameters in the form of degradation kinetics. This will allow the estimation of the devices lifetime for a very wide operating conditions region. During degradation an extensive set of measurements have been carried out at fixed steps in the form of photometric, optical, electrical, capacitive, mechanical and thermal characterization. The combination of these results allows the understanding of what degradation mechanisms are taking place and therefore it is a fundamental tool to improve system reliability. Degradation has also been studied by analyzing catastrophic damages by means of failure analysis; the failure investigation is useful for the catastrophic damage: melting of bonding wire, contacts evaporation, facet melting (for laser diodes).