Chapter 2 High-Efficiency AlGalnP Light-Emitting Diodes

Sixty Thousand Hour Light Output Reliability of AlGaInP Light Emitting Diodes

“…The electron concentration increases with increasing current density, however, so the quadratic term increases faster than the linear An term. For some intermediate current density, the SRH and radiative recombination rates will be equal , at which point (7) becomes (9) where and is the value of at which . Equation (9) therefore quantifies the highly intuitive notion that the current density required to saturate these defects (and give dominant radiative recombination) increases as the defect concentration increases, and this observation has important consequences on the variable test current reliability studies discussed in Section III.…”

“…Ten lamps from each wafer were then soldered onto a set of five different LED circuit boards, and each of these five nominally identical circuit boards was then stressed for 1000 hours at a different fixed stress current value, 10, 20, 30, 50, or 70 mA, in a thermally controlled oven at an ambient temperature of 55 C. These LEDs were periodically taken off of stress and the LED light output was tested at a series of different test currents, , for each circuit board. Of particular interest in this study is the effect of low test currents, , on LED reliability [6]- [9]. Additional details regarding these reliability tests are included in Table I and Section III.…”

“…A series of 56 lamps was then built from each wafer, with 28 lamps being built from the standard size dice and the other 28 lamps being built from the small dice taken from adjacent rows and columns relative to the standard dice. The resulting lamps were then stressed at two different stress currents, 30 mA and 50 mA, giving four different combinations of die size and stress current as described in Table II [9]. Similar to the first set of lamps above, this second set of lamps was periodically taken off of stress and tested, however this second set of lamps was tested at a fixed test current, .…”

“…In some LEDs, both the positive and negative degradation modes in item ii appear to be active, especially for short stress times and low current densities, and competition between these two degradation modes can obscure the long-term monotonic degradation trends in item ii for short stress times [6]- [9]. These short-term reliability trends typically saturate after several hundred to several thousand hours of stress, leading to the longterm monotonic trends in item ii that continue to dominate LED degradation for stress times at least as long as 60 000 hours, as TABLE I STRESS AND TEST CONDITIONS USED IN THE VARIABLE TEST CURRENT RELIABILITY STUDY TABLE II STRESS AND TEST CONDITIONS USED FOR THE FIXED TEST CURRENT RELIABILITY STUDY demonstrated in Sections IV-VI using data from the LEDs in Table II.…”

“…We will thus also briefly discuss the reliability of some of these newer generation LEDs in this paper. These newer generation LEDs consist of Luxeon packages containing truncated inverted pyramid chips with multi-well active regions that were transferred to transparent GaP substrates using wafer bonding technology, while the first two sets of LEDs described above consist of standard 5-mm lamps containing standard cubic chips with double heterostructure active regions that remain on their absorbing GaAs growth substrates [9]- [13]. Before presenting degradation data on these LEDs, the relationship between LED light output and LED degradation is discussed in Section II.…”