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78 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 50, NO. 1, JANUARY/FEBRUARY 2014

Degradation Mechanisms of High-Power LEDs forLighting Applications: An OverviewMatteo Meneghini, Senior Member, IEEE, Matteo Dal Lago, Nicola Trivellin,

Gaudenzio Meneghesso, Fellow, IEEE, and Enrico Zanoni, Fellow, IEEE

Abstract—This paper reports on the degradation mechanismsthat limit the reliability of high-power light-emitting diodes(LEDs) for lighting applications. The study is based on the ex-perimental characterization of state-of-the-art LEDs fabricatedby leading manufacturers. We demonstrate that, despite highpotential reliability, high-power LEDs may suffer from a numberof degradation mechanisms that affect the stability of the bluesemiconductor LED chip and of the phosphor layer used for thegeneration of white light. More specifically, we describe the follow-ing relevant mechanisms: 1) the optical degradation of LEDs, dueto an increase in the nonradiative recombination rate, which canbe correlated to modifications in the forward-bias current–voltagecharacteristics; 2) the variation in forward voltage, due to the in-crease in series resistance; 3) the optical degradation of phosphorlayers used for blue-to-white light conversion; and 4) the failureof LEDs submitted to “hot plugging,” which is the direct connec-tion of an LED chain to an energized power supply, due to thegeneration of high current spikes. Results provide an overview onthe failure mechanisms that limit the reliability of state-of-the-artLEDs and on the role of current and temperature in determiningthe failure of the devices.

Index Terms—Degradation, light-emitting diode (LED), light-ing, reliability.

I. INTRODUCTION

OVER the last few years, light-emitting diodes (LEDs)based on gallium nitride (GaN) have demonstrated to

be excellent devices for the realization of high-efficiency lightsources. The GaN material system (and, more specifically, theInGaN and AlGaN alloys) can be used for the fabricationof LEDs emitting in the blue, green, and ultraviolet spectralregions. Due to the research of many industries and academiclaboratories and to improvement in technology, the externalquantum efficiency of monochromatic GaN-based LEDs has

Manuscript received March 4, 2013; revised April 23, 2013; acceptedApril 23, 2013. Date of publication June 21, 2013; date of current versionJanuary 16, 2014. Paper 2012-ILDC-714.R1, presented at the 2012 Interna-tional Symposium on the Science and Technology of Lighting, Troy, NY, USA,June 24–29, and approved for publication in the IEEE TRANSACTIONS ON

INDUSTRY APPLICATIONS by the Industrial Lighting and Display Committeeof the IEEE Industry Applications Society. This work was supported in part bythe University of Padova (Progetto Giovani Ricercatori), in part by the Cassadi Risparmio di Padova e Rovigo (CARIPARO) Foundation, and in part by theItalian Ministry of Research through the PRIN program.

The authors are with the Department of Information Engineering, Univer-sity of Padova, 35131 Padua, Italy (e-mail: [email protected];[email protected]; [email protected]; [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIA.2013.2268049

Fig. 1. (a) Schematic of an LED with chip-level conversion. (b) Schematic ofan LED light source based on the RP approach.

increased from 25% (in early 2000) to over 70% for state-of-the-art devices [1].

The most convenient method to generate white light startingfrom a monochromatic LED is to use phosphor conversion, i.e.,blue LEDs (with a peak wavelength in the range between 450and 470 nm) are covered with a phosphor layer (usually yttriumaluminum garnets doped with rare earths), which converts theshort-wavelength radiation emitted by the LEDs into a broadyellow-green spectrum. The sum of the blue peak emitted by thedevices and of the luminescence of the phosphors is perceivedas white light; the chromatic properties (color rendering indexand correlated color temperature) of the white-light source canbe tuned by varying the composition of the phosphors. Forinstance, phosphors based on aluminates can be used to achievegreen emission, whereas phosphors based on nitrides generatea luminescence signal in the red spectral region. In an LED-based light source, phosphors can be directly deposited on theLED chip [chip-level conversion; see Fig. 1(a)] or placed somecentimeters away from the blue LEDs [remote phosphor (RP)approach; see Fig. 1(b)]. The latter approach significantly in-creases the dimensions of the light source but has the importantadvantage of minimizing the self-heating of the phosphors, thusimproving the reliability of the systems.

0093-9994 © 2013 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

MENEGHINI et al.: DEGRADATION MECHANISMS OF HIGH-POWER LEDs FOR LIGHTING APPLICATIONS 79

Due to improvement in the technology of blue LEDsand phosphors, white LEDs with efficiencies in excess of150 lm/W are commercially available [1], and those with ef-ficiencies of 200 lm/W are becoming feasible [2]. White LEDs,therefore, represent excellent candidates for the realization ofnext-generation light sources, which can compete with conven-tional fluorescent and incandescent lamps. (The latter has beenbanned in several countries.)

Despite the high potential of LED technology, the total LEDlighting market share was 9% in 2011 [3], mostly generatedin the architectural lighting segment. Several factors may beresponsible for the poor market penetration of LEDs, as fol-lows: 1) the relatively high cost of LED-based light sources,which can be reduced by adopting innovative technologies andsubstrates for the growth of GaN (e.g., GaN-on-silicon [4]and thin-film technology [5]); 2) several factors still limit theefficiency of LEDs, including nonradiative recombination [6]and the efficiency droop [7]; by minimizing these problems,it will be possible to further increase the efficiency of LED-based light sources; and 3) under high-stress conditions, LEDscan show significant degradation, which is activated by highcurrent densities and/or temperatures reached during operation[8]–[16]. As a consequence, the lifetime of the devices can beshorter than 50 000 h, which is usually considered as a targetfor LED technology [17].

With this paper, we analyze the main factors that limit thereliability of high-power LEDs for lighting applications. Basedon experimental results obtained on state-of-the-art devices, weprovide a detailed description of 1) the gradual degradationof the blue semiconductor chip due to increased nonradiativerecombination, 2) the degradation of electrical characteristics ofthe LEDs due to the increased series resistance or the generationof parasitic shunt paths, 3) the degradation of the phosphor layerdue to thermal effects, and 4) the effects of “hot plugging”(the connection of an LED chain to an energized power sup-ply), which may result in electrical overstress, with subsequentcatastrophic failure of the devices. The results presented withinthis paper provide a complete description of the main mech-anisms that limit the reliability of state-of-the-art LEDs andprovide information for the optimization of solid-state lightingdevices.

II. EXPERIMENTAL DETAILS

The results described in the subsequent sections have beenobtained on high-power LEDs, fabricated by three leadingmanufacturers. (The different sets of samples are referred toas groups A, B, and C hereafter.) The devices have a typicaloperating current in the range of 350–700 mA (correspondingto 35–70 A/cm2), and the characteristics are summarized inTable I. The samples were submitted to constant-current stress,with current levels in the range between 0.5 and 1.5 A. MostLED manufacturers recommend to use a maximum junctiontemperature (Tj) of 125 ◦C–150 ◦C; to slightly accelerate thedegradation kinetics, we stressed the devices close to and abovethis limit (i.e., with Tj in the range between 100 ◦C and 175 ◦C).At the different stages of the stress experiments, an electricaland optical characterization of the devices was carried out, with

TABLE ISUMMARY OF THE CHARACTERISTICS OF THE

DEVICES ANALYZED WITHIN THIS PAPER

Fig. 2. OP versus current characteristics measured on a power LED (group A)before and during stress at 1.2 A, with a junction temperature of 150 ◦C. (Inset)Schematic of nonradiative recombination (involving a defective state located inthe bandgap) and of radiative (band to band) recombination. While the firstprocess generates heat (thus decreasing device efficiency), the latter generatesa photon (represented by the wavy line). If stress induces an increase in non-radiative recombination, the efficiency of the devices significantly decreases,particularly at low (measuring) current levels.

the aim of achieving a complete description of the degradationprocess. The study of the degradation of the phosphors wascarried out on RP plates, with a circular shape, a diameter of61.5 mm, and a thickness of 2.1 mm, which were submitted tothermal stress (with temperatures in the range between 85 ◦Cand 145 ◦C), for 9000 h.

Finally, we analyzed the effects of “hot plugging,” i.e.,of the direct connection of an LED module to an energizedpower supply; more specifically, we studied the current spikesinduced by “hot plugging,” the dependence of in-rush currenton the number of LEDs in a module, and, by scanning electronmicroscopy (SEM), the origin of the failure of the LEDs.

III. RESULTS

A. Gradual Degradation of LEDs: Role of theSemiconductor Chip

Here, we analyze the gradual degradation of high-powerLEDs, induced by operation and high temperature/current lev-els. In Fig. 2, we report the optical power (OP) versus current(L− I) characteristics of a power LED submitted to stress at1.2 A, with a junction temperature of 150 ◦C. These stressconditions are very close to the limits specified by the manu-facturer. Stress induced a significant decrease in the OP of the


Fig. 3. Variation in OP measured (on one of the analyzed devices, group A)during stress at 1.2 A, with a junction temperature of 150 ◦C. The differentcurves report the OP decrease measured at different (measuring) current levelsduring the same stress experiment.

devices (−14% after 1000 h). Fig. 3 provides a description ofthe degradation kinetics, for the same LED (stressed at 1.2 A,Tj = 150 ◦C); the various curves describe the variation in OPmeasured at several current levels during the stress experiment.Results indicate that the OP decrease is more prominent at lowmeasuring current levels; this behavior suggests that degrada-tion is due to an increase in the nonradiative recombination ratewithin the active region of the devices [18], possibly due tothe generation of lattice defects [19] (see the inset in Fig. 2for a schematic representation of radiative and nonradiativerecombination). According to this hypothesis, at low measuringcurrent levels (e.g., at 0.05 A), the (few) carriers injected intothe junction can be captured by nonradiative centers, and thismay result in a strong decrease in the efficiency of the devices.On the other hand, at higher (measuring) current levels, thenonradiative recombination centers are saturated by the largeamount of free carriers [18], and for this reason, they have alower impact in limiting the efficiency of the LEDs. Previousreports (see, for instance, [19] and references therein) studiedthe nature of the lattice defects responsible for the OP decreaseby using spectroscopic techniques, such as deep-level transientspectroscopy and photocurrent spectroscopy. Preliminary re-sults in [19] indicate that the optical degradation of LEDs maybe related to the increase in the concentration of a deep levelthat is commonly referred to as “E2” [20]–[23], which has anactivation energy in the range of 0.5–0.6 eV, depending on theproperties of the analyzed devices. This defective level can berelated to nitrogen antisite (NGa) defects [20], to impurities(such as carbon [21]), to the residual Mg concentration in GaN[22], or to the presence of gallium vacancies [23]. During stress,the concentration of defects may increase as a consequence ofthe injection of high densities of carriers, through a subthresh-old defect generation process, as described in [8].

In many cases, optical degradation was found to be signifi-cantly correlated with modifications in the electrical character-istics of the devices. In Fig. 4, we report the current–voltagecharacteristics measured before and during a constant-currentstress experiment. Results indicate that stress may induce an in-crease in current in the low-forward-bias region, well correlated

Fig. 4. Current–voltage characteristics measured on a power LED (group A)before and during stress at 1.2 A, with a junction temperature of 150 ◦C.

Fig. 5. OP decrease and increase in forward current measured during stress ofone of the analyzed devices (group A).

to the OP decrease (see Fig. 5). These important modificationsmay be due either to an increase in the concentration of point(nonradiative) defects within the active region of the devices,as described in [18], and/or to the generation of parasitic shuntpaths in parallel to the junction. Both mechanisms can modifythe amount of carriers that are effectively captured by thequantum well region, thus being responsible for the measureddecrease in OP.

Another relevant mechanism was detected on LEDs stressedat high-junction-temperature levels (> 150 ◦C), i.e., an increasein the series resistance (see Fig. 6). This effect was typical forthe early generations of LEDs (see, for instance, [13] and [14])and is usually ascribed to the degradation of the conductivity ofthe p-type material, due to the passivation of the Mg acceptors,induced by the formation of Mg–H bonds [14]. Even if severalmanufacturers have solved this issue (e.g., LEDs in Group C inthis paper did not show any modifications in forward voltage),one of the groups of LEDs studied within this paper (Group B)was found to show a considerable increase in series resistancewhen operated at high-temperature levels (see Fig. 6). Thiseffect can be critical for lighting applications; in fact, it leads toa considerable increase in the operating voltage of the devices,which may result in a decrease in the luminous flux, when the


Fig. 6. Current–voltage characteristics measured on one of the analyzeddevices (group B) before and during stress at 0.7 A, Tj = 168 ◦C.

voltage drop of the diodes exceeds the maximum output voltageof the driver used to bias the LEDs. To understand this problem,we can consider a simple example as follows: LED modules areusually constituted by the series connection of several LEDs, bi-ased by a constant-current power supply (typical specificationsof the power supply are I = 350 mA and a maximum voltagethat is equal to 48 V). In many cases, the number of LEDs in amodule is calculated (starting from the forward voltage of eachdiode) with the aim of minimizing the difference between thevoltage drop on the module and the maximum voltage of thepower supply (e.g., if each LED has a voltage drop of 3.1 V,a module constituted by the series of 15 LEDs has a voltagedrop of 46.5 V, which is very close to the 48-V limit of thepower supply). If, during ageing, the voltage drop of each LEDincreases to more than 0.1 V, the voltage required to bias theseries of 15 LEDs becomes greater than 48 V, which is themaximum operating voltage of the driver. As a consequence,the driver will no longer be able to inject the nominal currentof 350 mA to the LED module and will set the output to itsmaximum voltage of 48 V, thus reducing the current (and theOP) of the LEDs. For the realization of LED modules with highreliability, it is therefore very important to choose LEDs thatdo not show any degradation in the electrical parameters overageing time. As suggested in Fig. 6, the increase in forwardvoltage starts occurring during the initial 100 h of operation athigh temperature (Tj > 160 ◦C); a short-term stress test at hightemperatures should therefore be sufficient to verify the stabilityof the series resistance of a given set of LEDs.

B. Degradation of Phosphor Layers

In the previous section, we have described the degradationof the blue semiconductor chip, which is the core of an LED-based lamp. Another important component of a white LEDis the phosphor layer used to convert the blue light into awhite spectrum, which is suitable for lighting application. Thephosphor material is usually dispersed into a matrix (whichcan be based on silicon) and directly deposited on the chipor (for RP systems) on large-area polycarbonate plates. Thehigh temperatures reached by the LED chip during operationand the considerable conversion losses may lead to a significant

Fig. 7. Degradation of the luminous flux emitted by RP plates submitted tostress at several temperature levels (solid lines). The RP plates were stressed inthermal chambers; for the measurements, they were removed from the thermalchambers and placed on a blue LED-based light source, with a power of 3 Wand a wavelength of 451 nm, which was never stressed. The dashed linerepresents the degradation kinetics of polycarbonate plates (with no phosphorson top) stressed at 145 ◦C.

heating of the phosphor layer; phosphors directly deposited onthe LED chips may reach temperatures close to the junctiontemperature of the devices (i.e., in the range of 100 ◦C–120 ◦C),whereas RP plates, which are placed some centimeters awayfrom the blue LEDs, have smaller self-heating. (According to[24], temperatures can be in excess of 60 ◦C, with an irradiationaround 350 mW/cm2.) Here, we describe our latest results onthe degradation of phosphor layers for lighting applications; thestudy was carried out by submitting phosphor plates to thermalstress tests, with temperatures in the range between 85 ◦C and145 ◦C and a total stress duration of 9000 h. The analyzedplates consist of a polycarbonate substrate, which is coveredby a thin phosphor layer. During the stress tests, at specific timeintervals, the phosphor plates were removed from the thermalchambers and submitted to optical characterization by usinga reference (blue) LED-based light source, which was neverstressed.

In Fig. 7, we report the degradation kinetics measured duringthe thermal storage experiment; thermal stress was found toinduce a nearly exponential decrease in the luminous efficiencyof the phosphor plates, and the degradation kinetics were foundto be significantly accelerated at high-temperature levels. Theactivation energy of the degradation process was extrapolatedby plotting the Arrhenius plots for the time-to-failure (TTF)at 90% (referred to as TTF90%), which is the time necessaryto reach a 10% decrease in the efficiency of the RP plates.TTF80% and TTF70% were also defined according to the samecriteria. Results indicate that TTF90% is thermally activated,with an activation energy of 1.23 eV. It is worth noticing thatthe maximum operating temperature recommended by the man-ufacturers for these phosphor plates is 95 ◦C; we have stressedthe plates with temperatures in the range of 85 ◦C–145 ◦C,and results in Fig. 8 demonstrate that the Arrhenius relationholds in the whole analyzed temperature range. This result in-dicates that, for these samples, it is correct to use the Arrheniusplot obtained at temperatures greater than 95 ◦C for the ex-trapolation of the TTF in the nominal operating temperature


Fig. 8. Arrhenius plot of the TTF (TTF90%,TTF80%,TTF70%) for phosphorplates stressed at several temperature levels. The activation energy for thevarious TTF is reported in the figure. Solid curves represent linear fit of theexperimental data.

range (e.g., at 85 ◦C). TTF80% and TTF70% were also foundto follow an Arrhenius relation on temperature, with activationenergy values of 1.33 and 1.37 eV, respectively. On the basis ofthe experimental results summarized in Fig. 8, we extrapolatedthe following values for the TTF of the analyzed phosphorplates at the temperature of 85 ◦C: TTF90%(85

◦C) = 8600 h,TTF80%(85

◦C) = 28 500 h, TTF70%(85◦C) = 54 500 h. In

many cases, the expected TTF70% of an LED-based light sourceis 50 000 h; the results of our investigation demonstrate that iftemperature is kept below the maximum levels recommendedby the manufacturers, phosphors can guarantee reliable opera-tion for more than 50 000 h. However, one has to consider thatalso the blue LED lamp used for the excitation of the phosphorcan degrade during ageing, thus shortening the overall lifetimeof a white LED lamp.

As described above, the analyzed phosphor plates consistof a polycarbonate substrate, which is covered by a thin layerof phosphors. To understand what are the contributions ofphosphors and of polycarbonate to the degradation process, wecompared the degradation kinetics of bare polycarbonate diskswith those obtained by stressing finished plates (i.e., with thephosphor layer deposited on top of the polycarbonate disk).Results (summarized in Figs. 7 and 9) indicate that thermalstorage determines both browning of the phosphor layer and adecrease in the transparency of the polycarbonate disks. Resultsof midterm (1000 h) stress tests carried out with high-intensityillumination (350 mW/cm2, 451 nm) at 85 ◦C indicated thatprolonged exposure to high luminous intensities does not in-duce any strong darkening of the polycarbonate (not shown herefor the sake of brevity), compared with what we have foundfor high-temperature stress (see Fig. 7); this result suggeststhat, with midterm stress tests, significant degradation can beobtained only through exposure to high-temperature levels,as described above. From the comparison of the degradationkinetics of bare polycarbonate disks and of finished phosphorplates (see Fig. 7), we can conclude that most of the degradationcomes from the darkening of the polycarbonate; by choosing adifferent substrate material (such as glass), it could be possible

Fig. 9. Images of RP plates and polycarbonate disks; both unstressed andstressed plates are shown.

to significantly improve the reliability of the RP plates since inthat case, degradation would be only due to the browning of thephosphor layer.

C. Failure of LEDs Due to Hot Plugging

In the previous sections, we have described the most commonprocesses responsible for the gradual degradation of LED-basedlight sources, namely, the degradation of the optical propertiesof the blue semiconductor chip, the electrical degradation ofLEDs, and the worsening of the efficiency of phosphor layersused for the conversion of blue light into white light. Apart fromgradual degradation, LEDs can also show sudden degradation(or catastrophic failure) when they are submitted to electricaloverstress or current spikes. A quite frequent situation occurswhen LEDs are directly connected to an energized powersupply; this case is often referred to as “hot plugging” andinduces the flow of high current (or in-rush current) throughthe devices.

To understand what are the consequences of “hot plugging,”we can consider the most simple case, in which we have anLED module constituted by n LEDs connected in series and aconstant-current power supply, with a nominal output current of350 mA and a maximum voltage of 48 V (see the schematic inthe inset in Fig. 10). When the switch is open, the output of thedriver is set to the maximum voltage (48 V), since no currentcan flow through the circuit. When (at t = 0 s) the switch isclosed, the output voltage of the driver has to quickly drop tothe value required to deliver 350 mA to the LED chain. Thisdecrease is not instantaneous; as a consequence, for a shortinterval, the output current of the driver becomes significantlyhigher than the nominal current of the LEDs (current spikeor in-rush current). Quantitative results on “hot plugging” aresummarized in Fig. 10, which reports the current transientsmeasured on modules constituted by an increasing number ofLEDs submitted to “hot plugging.” The results indicate that1) immediately after the switch is closed, current increases toseveral amperes (in excess of 30 A for n = 3), and then, 2) itexponentially decreases to 350 mA (nominal output current of


Fig. 10. Current transients measured during “hot plugging” tests carried outon modules constituted by increasing numbers of LEDs. (Inset) Schematic ofthe setup used to study the effects of “hot plugging,” showing the driver, the(integrated) output capacitance of the driver, and the LED chain.

Fig. 11. Amplitude of the current spikes measured on LED modules con-stituted by increasing number of LEDs submitted to “hot plugging” tests, asschematically described in the inset in Fig. 10. (Left) Results obtained by usingdrivers with different output capacitance values. (Right) Results obtained byusing drivers with different maximum voltage levels.

the driver); 3) the current spikes are almost completely finishedafter some milliseconds, typical time constants are in the rangeof 0.2–4 ms, depending on the number of LEDs in the seriesand on the output capacitance of the driver (see Fig. 11). A moredetailed investigation, which is carried out by using drivers withdifferent voltages and output capacitance values, indicates thatthe amplitude and time constant of the current spikes are notstrongly determined by the structure of the chips but are mostlycorrelated to the characteristics of the driver and to the numberof LEDs in series.

More specifically, as shown in Fig. 11, 1) the amplitudeof the current spikes is strongly determined by the maximumoutput voltage of the driver, and 2) the time constant of the(nearly exponential) current spikes is determined by the outputcapacitance of the driver. The “hot plugging” phenomenon canbe, therefore, modeled by considering that when the switch inFig. 10 is closed, the output voltage of the driver decreases fromits open-circuit value (Voc, e.g., 48 V) to the voltage Vmodule

required to drive the LED modules with a 350-mA constantcurrent. This induces the generation of a current spike, whoseamplitude is determined by the difference Voc−Vmodule. On the

Fig. 12. SEM micrograph of a (failed) high-power LED submitted to “hotplugging” (courtesy of M. Vanzi and G. Mura of the University of Cagliari).

other hand, the time constant of the current spikes is stronglycorrelated to the output capacitance of the drivers.

Hot plugging (and the resulting in-rush current of severalamperes) can have a destructive effect on the LEDs, due tothe high energy that is released to the devices. The effectof “hot plugging” strongly depends on the amplitude of thecurrent spikes; the typical effects of destructive spikes are wellsummarized in Fig. 12, which was obtained by SEM on anLED, after the removal of the plastic lens and the phosphorlayer. When submitted to high current spikes, LEDs can faildue to the shortening of the junction, which possibly occurs inproximity of a preexisting defective region. The high energyreleased by the current spike results in severe cracking of thesemiconductor material and in the fusion of the metal lines,which are used to distribute current on the whole device area.In addition, bonding wires may be destroyed after a strongelectrical overstress.

As it is clear from the discussion above, in-rush currentoccurs when LEDs are directly connected to an energizedpower supply; possible ways to avoid catastrophic failure ofLED modules are 1) to design the LED chains with the aimof matching the voltage of the modules to the maximum outputvoltage of the driver (thus reducing the amplitude of the currentspikes; see Fig. 10) and 2) to use overvoltage protection (orcurrent limiting) devices. The latter solution implies highercosts but (in principle) better protection toward current/voltagespikes.

IV. CONCLUSION

In summary, with this paper, we have discussed the mostcommon degradation mechanisms of high-power LEDs forlighting applications. In the first part of this paper, we havedescribed the gradual degradation of white LEDs submittedto high-current/temperature stress; results indicate that long-term stress may induce a decrease in the OP of the devices,


which can be due to the generation of nonradiative defectswithin the active region of the LEDs. After stress, the electricalcharacteristics of the devices may show both an increase in thelow-forward-bias current components (due to the generation ofpoint defects and/or to the generation of parasitic shunt paths inparallel to the junction) and (only for some series of LEDs) anincrease in the series resistance (possibly due to the passivationof acceptor doping).

Within the second part of this paper, we have described theeffects of thermal stress on the efficiency of phosphor layersused for the generation of white light; the tests have beencarried out on phosphor plates, with a polycarbonate substrate.Results indicate that the efficiency of the phosphor plates cansignificantly decrease as a consequence of thermal treatment,due both to the darkening of the phosphor material and to thedecrease in the transparency of the polycarbonate substrate.The lifetime of phosphor plates has been extrapolated from theArrhenius plots of TTF.

Finally, we have studied the mechanisms responsible forthe catastrophic failure of LEDs submitted to “hot plugging”;results, which are obtained on specific test modules, indicatethat when an LED module is directly connected to an energizedpower supply, it may suddenly fail due to a current/voltagespike. The shape of the current transients, the dependence oftheir amplitude on the number of LEDs, and the typical failuremechanisms are discussed in detail in this paper.

The results described within this paper indicate that evenif state-of-the-art LEDs have a high performance (thus beingexcellent candidates for the realization of next-generation lightsources), they can suffer from both gradual and sudden degrada-tion if the lighting systems are not properly designed. Operatingconditions, thermal dissipation, and driving waveforms mustbe, therefore, carefully optimized with the aim of limiting thefailure of the devices.

ACKNOWLEDGMENT

The authors would like to thank Prof. M. Vanzi andDr. G. Mura (both with the University of Cagliari, Cagliari,Italy) for the fruitful discussion on the degradation modes ofLEDs and for SEM investigation of failure and A. Compagnin(University of Padova, Padova, Italy) for the characterization ofaged LEDs.

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[14] M. Meneghini, L. Rigutti, L. Trevisanello, A. Cavallini, G. Meneghesso,and E. Zanoni, “A model for the thermal degradation of metal/(p-GaN)interface in GaN-based light emitting diodes,” J. Appl. Phys., vol. 103,no. 6, pp. 063703-1–063703-7, Mar. 2008.

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Matteo Meneghini (S’06–M’08–SM’13) receivedthe Laurea degree (summa cum laude) in electronicsengineering, with a thesis on the electrical and opti-cal characterization of gallium nitride light-emittingdiodes (LEDs), and the Ph.D. degree in the optimiza-tion of GaN-based LED and laser structures fromthe University of Padova, Padova, Italy, in 2004 and2008, respectively.

He is currently a Research Fellow with the De-partment of Information Engineering, University ofPadova. He has been involved in several national

and European projects on GaN LEDs and high-electron mobility transistors(HEMTs). He is currently engaged in the electrooptical characterization andmodeling of the performance and reliability of InGaN LEDs and lasers and inthe characterization of power HEMTs and E-mode transistors. He has publishedmore than 190 journal and conference proceedings papers and presented severalinvited presentations. His current research interests include the characterization,reliability, and simulation of compound semiconductor devices.

Dr. Meneghini is a Reviewer of the Journal of Applied Physics, Semiconduc-tor Science and Technology (Institute of Physics), Microelectronics Reliability(Elsevier), the IEEE TRANSACTIONS ON ELECTRON DEVICES, and IEEEELECTRON DEVICES LETTERS. He was a recipient of the Carlo Offelli Awardfor Best Young Researcher from the Department of Information Engineering(DEI), University of Padova, in 2008. He was also a recipient of Best PaperAwards at the European Symposium on Reliability of Electron Devices, FailurePhysics and Analysis (ESREF), in 2009 and 2012, and at the InternationalWorkshop on Nitrides, in 2012. He is a member of the Solid State DeviceResearch Conference (ESSDERC) 2013 Technical Program Committee and ofthe technical subcommittees of the ESREF.

Matteo Dal Lago received the degree in electronicsengineering in 2009 from the University of Padova,Italy, where he is currently working toward the Ph.D.degree in information engineering.

Since 2009, he has been with the MicroelectronicsResearch Group, Department of Information Engi-neering (DEI), University of Padova, working onthe characterization and reliability studies on powerlight-emitting diodes (LEDs) for general lighting.His research work is focused on the analysis ofthe degradation mechanisms of LED-based light

sources related to environmental conditions, driving techniques, and packagingmaterials.

Nicola Trivellin was born in Padua, Italy, in 1983.He received the Laurea degree in electronics en-gineering from the University of Padova, Padua,in 2007, with a thesis on the characterization andreliability of short-wavelength GaN optoelectronicdevices. He is currently working toward the Ph.D.degree in electronic and telecommunication engi-neering at the University of Padova.

His current research interests include the char-acterization of GaN heterostructures and the studyof degradation mechanisms induced by thermal and

electrical stress on GaN light-emitting diodes and laser diodes.

Gaudenzio Meneghesso (S’95–M’97–SM’07–F’13) received the B.S. degree in electronicengineering, working on the failure mechanisminduced by hot electrons in metal–semiconductorfield-effect transistors and high-electron mobilitytransistors, and the Ph.D. degree in electrical andtelecommunication engineering from the Universityof Padova, Padova, Italy, in 1992 and 1997,respectively.

In 1995, he was with the University of Twente,Enschede, The Netherlands, with a Human Capital

and Mobility fellowship (within the SUSTAIN Network) working on thedynamic behavior of protection structures against electrostatic discharge (ESD).Since 2011, he has been a Full Professor with the Department of InformationEngineering, University of Padova. He is a Reviewer of several internationaljournals, including Electronics Letters, the Journal of Applied Physics, Applied

Physics Letters, Semiconductor Science and Technology (Institute of Physics),Microelectronics Reliability (Elsevier), the IEEE TRANSACTIONS ON ELEC-TRON DEVICES, and IEEE ELECTRON DEVICE LETTERS. He has publishedabout 600 technical papers, of which more than 60 are invited papers and eighthave won Best Paper Awards at the 1996, 1999, 2007, and 2009 EuropeanSymposium on Reliability of Electron Devices, Failure Physics and Analysis(ESREF) and at the 2006 Electrical Overstress/Electrostatic Discharge Sympo-sium. His research interests include electrical characterization, modeling, andreliability of microwave and optoelectronic devices on III–V and III-N; elec-trical characterization, modeling, and reliability of radio-frequency microelec-tromechanical system switches for reconfigurable antenna arrays; developmentof ESD protection structures for complementary metal–oxide–semiconductorand SmartPower integrated circuits; and the characterization and reliability oforganic semiconductor devices.

Dr. Meneghesso was the recipient of the Italian Telecom award for histhesis work in 1993. For several years, he has served on the Executive Com-mittee of the IEEE International Electron Devices Meeting as the EuropeanArrangements Chair in 2006 and 2007. He has been serving on the Techni-cal Program Committee (TPC) of the IEEE International Reliability PhysicsSymposium since 2005 and on the Management Committee since 2009. He isin the Steering Committee of several international conferences, including theEuropean Solid-State Device Research Conference, the ESREF, the Workshopon Compound Semiconductor Devices and Integrated Circuits (WOCSDICE),and the European Workshop on Heterostructure Technology (HETECH), andhas been serving on the TPC of several international conferences. Since 2007,he has been an Associate Editor of the IEEE ELECTRON DEVICES LETTERS

for the compound semiconductor devices area. He has been nominated toIEEE Fellow Class 2013, with the following citation: "for contributions to thereliability physics of compound semiconductor devices." In 2010, he joined theAdministrative Committee of the IEEE Electron Devices Society.

Enrico Zanoni (S’81–A’82–SM’93–F’09) was bornin Verona, Italy, in 1956. He received the Laureadegree in physics (cum laude) from the Universityof Modena and Reggio Emilia, Modena, Italy, in1982, after a student internship with the S. CarloFoundation, Modena.

During 1985–1988, he was an Assistant Professorwith the Faculty of Engineering, University of Bari,Bari, Italy. From 1988 to 1993, he frequently visitedthe U.S. and established research collaborations withBell Laboratories; Hughes Research Laboratories;

IBM T. J. Watson Research Center; Massachusets Institute of Technology,Cambridge, MA, USA; TRW (currently, Northrop Grumman); University ofCalifornia, Santa Barbara, CA, USA; and many other industrial and academiclaboratories. During 1996–1997, he was a Full Professor of industrial electron-ics with the University of Modena and Reggio Emilia. He is currently withthe University of Padova, Padua, Italy, where he was an Assistant Professorduring 1988–1992, an Associate Professor of electronics during 1992–1993,a Full Professor of microelectronics during 1993–1996, and has been a FullProfessor of digital electronics with the Department of Information Engineeringsince 1997. He has been a Representative of the University of Padova forthe European project MANPOWER and Manufacturable Power MonolithicMicrowave Integrated Circuits for Microwave Systems Applications, a Eu-ropean Coordinator for the subproject “Reliability” of the European projectEUREKA PROMETHEUS (automotive electronics), a Principal Investigator ofthe European project “Procedures for the early phase evaluation of reliability ofelectronic components by the development of European Committee for Elec-trotechnical Standardization (CENELEC) Electronic Components Committeerules” on qualification and reliability of integrated circuits, and a EuropeanCoordinator of the subproject “Reliability” of the European Defence Agencyproject “Key Organization for Research on Integrated Circuits in GaN Technol-ogy.” He is nationally or locally responsible for several Italian research projects,such as the Italian Space Agency, the Italian Research Ministry Projects, andthe Italian National Council of Research. He is the author or coauthor ofapproximately 450 papers in refereed international journals and conferenceproceedings, including more than 35 invited papers. His microelectronics groupis composed of five professors, three assistant professors, and, on average, 15Ph.D. students and two postdoctoral researchers. This research group publishesapproximately 80 papers each year on a wide range of research topics, includinganalog and radio-frequency signal complementary metal–oxide–semiconductordesign, biochip development, the analysis of radiation hardness, and the relia-bility of electronic devices and circuits. His research interests include micro-electronics, particularly concerning the design, characterization, reliability, andfailure analysis of electronic devices and circuits.

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