>> Note that LED lifetime degrades with both increasing current AND >> increasing temperature essentially independently. ie a LED conducting = >> more > current will have a shorter life, even if the die temperature is not = > increased. > (this radical heresy is based on LumiLeds app notes) > Hi Russell, > What is the root cause of the degradation in the case of high current > pulses? e.g. higher peak power dissipation from I*I*R causing die > heating or something to do with phosphor degradation due to the high > peak outputs? I have read technical papers thereon but it's not the sort of stuff that = sticks well in my brain these days - ie all about fancy qm effects, current = densities, dopant effects, current interactions with lattice defects (seems = to be popular), effects on diffusion (perhaps) et al. My brain seems rather = to want to remember stuff related to practical aspects of what environmenta= l = and operational effects are liable to impact performance. I can't say I = blame it :-). Some of the refences below may apply. Some don't :-). But all give a feel for how many less thannormally intuitive things are at = work in LEDs. Gargoyle knows much much more. Russell ____________________ 2007 Miran B=FCrmen, Franjo Pernus and Bostjan Likar University of Ljubljana, Faculty of Electrical Engineering, Laboratory of = Imaging Technologies, Trzaska 25, 1000 Ljubljana, Slovenia E-mail: Miran.Burmen@fe.uni-lj.si, Franjo.Pernus@fe.uni-lj.si and = Bostjan.Likar@fe.uni-lj.si Abstract. The intensity and colour (spectral) degradation of light-emitting = diodes (LEDs) can significantly affect the reliability of demanding lightin= g = products for which a certain level of intensity and colour homogeneity is = required. Intensity degradation is sometimes characterized by manufacturers = in a form of a mean half-lifetime, i.e. the time in which the initial = intensity of LEDs decreases to 50%. However, additional information on = spectral degradation in a batch of LEDs is required for designing and/or = maintaining demanding state-of-the-art lighting products. For this purpose, = a current-accelerated prediction of spectral degradation of white GaN-based = LEDs is presented. The method is based on periodical automated spectral = acquisitions of LEDs driven by above nominal currents. The obtained = information on current-accelerated spectral degradation is then used for a = model-based prediction of spectral degradation of LEDs driven by an = arbitrary current. The proposed method was tested on 640 ultra-bright white = GaN-based LEDs (from 5 to 20 cd) from six manufacturers. For this purpose, = the predicted and measured spectral degradations at the application-specifi= c = nominal currents were compared. The method has proved useful for predicting = spectral degradation of the eight tested batches by the exponential lifetim= e = prediction model. The achieved up to sixfold acceleration and the mean = absolute lifetime prediction error of 15% indicate that the proposed method = is feasible for an accelerated estimation of spectral degradation of white = GaN-based LEDs. As such, the method may be a valuable tool for designing an= d = maintaining lighting products in numerous existing and emerging = applications. http://www.iop.org/EJ/abstract/0957-0233/18/1/029 ___________________ Jianzheng Hu, Lianqiao Yang, Lan Kim and Moo Whan Shin Department of Materials Science and Engineering, Myong Ji University, 38-2 = Nam-Dong, Yongin, Kyunggi 449-728, Korea E-mail: mwshin@mju.ac.kr Abstract. Degrading characteristics of InGaN/GaN-based blue light-emitting = diodes (LEDs) in the optical increase stage (stage I) were investigated. Th= e = LEDs were under an ageing with dc current of different levels. Different = ageing phenomena were observed for both the electrical and the optical = characteristics. The tunnelling components of the forward current were foun= d = to change with different trends for the samples aged under current stresses = of 100 mA and 1000 mA, respectively. The former decreased monotonically, = while the latter decreased in the early period, and then kept increasing. = Different ageing rates but the same trends were observed for the optical = degradation of the samples aged under different stresses. The ageing factor= s = induced by the dopant activation and the change of defects competed with = each other in the ageing process and were proposed to be responsible for th= e = ageing characteristics. http://www.iop.org/EJ/abstract/0268-1242/22/12/001 _____________________ 2005 We present a study of reliability of AlGaN-based 280 nm deep ultraviolet = light-emitting diodes on sapphire substrate grown by migration-enhanced = metal-organic chemical vapor deposition. Two modes of optical power = degradation were observed: catastrophic and gradual. The catastrophic = degradation is believed to be due to metal alloying at macroscopic defects = in the top p layers of the light-emitting diode structure. For the gradual = power degradation, two time constants were determined, which were = temperature and bias dependent. For the temperature-dependent part, the = values of the activation energies and room-temperature degradation rates at = dc currents of 10 and 20 mA were determined to be 0.23 and 0.27 eV and = 1.31=D710-3 and 5.93=D710-3 h-1, respectively. =A92006 American Institut= e of = Physics http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=3Dnormal&id=3DAP= PLAB000088000012121106000001&idtype=3Dcvips&gifs=3Dyes&ref=3Dno _________________ 2005 We have studied the degradation mechanism of GaN base blue light emitting = diodes by the current-voltage and light-current characteristics driving hig= h = currents through the diodes for various times. This was a result of an = increased non-radiative recombination in the active region. The aging = effects were not observed at the high bias region due to the saturation of = non-radiative recombination centres. The mechanism of the increased = non-radiative recombination centres may be related with the generation of = defects in the active region due to the high current flow through quantum = well structure and the increase of light emitting diode chip temperature. http://www.sciencedirect.com/science?_ob=3DArticleURL&_udi=3DB6TW0-4FG89B8-= 4&_user=3D10&_rdoc=3D1&_fmt=3D&_orig=3Dsearch&_sort=3Dd&_docanchor=3D&view= =3Dc&_searchStrId=3D1141360974&_rerunOrigin=3Dgoogle&_acct=3DC000050221&_ve= rsion=3D1&_urlVersion=3D0&_userid=3D10&md5=3Dddb33dc4f644e398acd2be4c2bd6dc= f9 ____________________ INTERNAL MECHANICAL STRESSES AND CONDUCTIVITY MECHANISMS OF = p-n-InAsPSb/InAs-HETEROSTRUCTURES Influence of internal mechanical stresses = (IMS) and dislocations on conductivity mechanisms and reliability of light = emitting diode (LED) and laser InAsPSb/InAs double heterostructures has bee= n = investigated. It was shown that the presence of lattice mismatch of layers = at p-n-heteroboundaries changes LED conductivity mechanisms the same way as = longterm working challenges by current. It was determined that LED = degradation has barrier character and is followed by appearance and growth = of tunnel current components with power dependence ofcurrent from voltage. = Reliability and external quantum efficiency decrease with growth of lattice = mismatch. http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=3D12311&DID=3D234638&action= =3Ddetail _____________________ 1978 Abstract A new degradation property of N-doped GaP light emitting diodes = has been found. Diodes Zn-diffused through a thin layer of SiO2 exhibit a = complete saturation of the degradation process at 70 to 80 % of the initial = light output. After saturation of the degradation process the emission = spectrum is broadened on the low energy side. This broadening is identified = as due to the formation of additional nearest neighbour N-pairs during = degradation. Diodes diffused without a SiO2 cover show neither a saturation = of the degradation process nor a change of the emission spectrum. Deep leve= l = transient spectroscopy revealed different defect levels for the two types o= f = diodes, therefore different degradation properties might be expected. http://www.springerlink.com/content/q8572005n1506615/ ________________________ -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist