Research NoteDegradation of InGaN blue light-emitting diodes under continuous and low-speed pulse operations
Introduction
Advances in light-emitting diode (LED) technology––both in its applications for display devices and the illumination source itself––have resulted in higher brightness capacity. Blue LED based on GaN has been realized; the three colors have been gathered, and white luminescence attained [1], [2]. Compared to light bulbs, LED has advantages of long life, energy efficiency, and compactness. It is expected that LED technology will replace bulbs in the near future. Field applications are various, ranging from high-speed pulsation to intermittent and continuous operations. For the design of such systems, however it will be necessary to predict the expected lifetime and responses to environmental and usage conditions. While blue LED of high brightness has been realized using a wide band-gap semiconductor, the data on its reliability are scarce [3], [4].
Herein, we report on differences in commercial InGaN blue LED degradation among several types of operations. The current accelerated test was performed for both pulsing and continuous operations, and the dependence of degradation in light output on the operating conditions were investigated, along with the estimated lifetimes.
Section snippets
Experimental detail
The sample used was a commercial blue LED of the Nichia InGaN type. The device chip was mounted on a lead frame (silver plating) and molded using epoxy resin. At the recommended DC current of 20 mA, the LED had the following characteristics: brightness, 1.4–1.7 candela; peak emission wavelength, 466–469 nm; and 30 nm spectral half-width. Continuous current tests were performed for 3100 h in a dried test chamber at 40 °C, at the following three DC current stress IF (estimated current density JF)
Results and discussion
Fig. 1 shows the degradation of the optical output P/P0 (normalized to the initial value P0) in continuous current operation and pulse operation tests. It was found that the degradation during pulse operation was small in comparison with that during continuous operation. In fact, the shorter the pulse width WP, the smaller the degradation in pulse operation. Various approximate LED degradation pattern formulas have been reported for use with particular materials and structures [4], [5], [6]. In
Conclusion
Accelerated current tests on InGaN blue LEDs were performed under continuous and low-speed pulse operations. A half-life of approximately 2–4 × 104 h was estimated under the pulse condition of 40 mA at 40 °C Lifetime depended on pulse width: the shorter the pulse width, the longer the lifetime. Lifetime during the pulse operation was estimated to be 2–4 times longer than that during the continuous operation.
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