Life Test And Reliability Of GaN Based Light Emitting Diodes

Due to the advances in material science, manufacturing processes, the gallium nitride-based Light Emitting Diodes'(LEDs) output power has been very high. Following the advent of blue and green gallium nitride-based LEDs, there has been a rapid growth in the use of LEDs in a multitude of signaling, display, and illumination applications. The long life and high reliability of LEDs becomes more and more important. The dissertation describes the research of reliability.There are two research fields in this dissertation: Life Time Test and Failure Analysis. The detail content are:Firstly: Introduction of LED. The development of Semiconductor Materials,including the development of semiconductor material and LED, and the progress of gallium nitride(GaN) based LED; Introduction of GaN based LED consist of structure and principle of lighting; The progress of reliability research, and so on.Secondly: Reliability Theory. Theory and concept of Life Test and Failure Analysis are specified.Thirdly: GaN based LEDs heat features analysis. The junction temperature under working current and working condition's temperature is introduced. The affect of junction temperature rise is specified. Thermal characteristics of GaN based LED such as temperature coefficient and heat resistance are introduced and measured.Fourthly: Life Test. Life time of GaN based LEDs is obtained by current stresses and high temperatures accelerate life test (ALT). Life time of GaN based green and blue LED are 32015 hours and 87425 hours get from the current stress test and 30022 hours and Ea =0.4723 of blue LED get from temperature stress test .Fifthly: Failure Analysis. Failure modes are concluded through the long term of accelerate life test (ALT) of GaN based LEDs. Failure mechanisms are discussed.The degradation of encapsulation material is the main reason of the degradation of LED lamps which works at high p-n junction temperature, esp. higher than 125℃。Mechanical stress caused by differences in the thermal coefficient of expansion within the packaged lamp, exposure to high internal temperatures beyond the maximum ratings or repeated thermal cycling can potentially cause different types of catastrophic failures. Electric over stress and thermoelectric damage and metal migration are all degradation reasons. Defect is observed that it is still the main reason that will cause the degradation of GaN based LEDs'properties through analyzing the changes of electric feature.