Tzung-Te Chen

Investigation of dynamic color deviation mechanisms of high power light-emitting diode

Abstract Environmental concerns have led to the popularity of solid stating lighting, in which a high quality white light source depends on the stable property of light emitting diode. This study examines a white-light high-power light-emitting diode composed of a blue chip and yellow phosphor. A white-light light-emitting diode can be divided into four parts—a blue chip, yellow phosphor, transparent silicone, and reflector. In a transient experiment, the wavelength shift of the blue chip markedly affects the conversion efficiency of yellow phosphor, causing white-light deviation, especially in the sharp variation region of absorption of yellow phosphor. A series of short-term experiments was conducted to identify the mechanisms of color deviation between yellow phosphor and transparent silicone. The robustness of commercial phosphor and silicone was much stronger than expected. In addition to a yellowed reflector and blue chip degradation, several combinations of degradation mechanisms between yellow phosphor and transparent silicone. In a long-term experiment, damaged silicon confines blue light resulting in warm white light. Two suggestions are provided to obtain white-light light-emitting diodes with high color reliability.

Evaluation of temperature distribution of LED module

Abstract For high-power light-emitting diodes, heat management and reliability are important issues. The reliability and long lifetime of solid state lighting are especially essential in the high temperature applications, such as street lighting and automotive lighting. Because of the characteristics of semiconductor, the electrical property of light-emitting diodes is varying with operating temperature. Then, the alternation of electrical property changes the heating power and operating temperature of light-emitting diodes. This is a mutual interaction between electrical property and operating temperature, until they reach the steady state. In this paper, we designed experiments and calculation to optimize the simulation of temperature distribution of light-emitting diode module. With forward voltage measurement and thermal transient testing of light-emitting diodes, we obtained the initial values of simulations. The infrared camera captured the thermal images to verify the simulation results. With this method, we can effectively evaluate the temperature distribution of light-emitting diode module.

Analysis of Thermal Resistance Characteristics of Power LED Module

Multichip LED arrays are widely used for lighting to provide high luminance. Luminous efficacy, lifetime, and color temperature are highly dependent on the temperature at p-n junction. This paper investigated the effects of distance, number of chips, and driving current on the thermal resistance of LED module. Thermal resistance dramatically increased as the distance between LED chips decreased due to significant thermal spreading impedance for heat dissipation from junction to ambient. The parallel-resistance formula substantially underestimated the junction temperature of the LED modules due to significant thermal crowding effect. Thermal boundary can also rise junction temperature as the distance to the board edge decreased in both the two-chip and four-chip modules. Infrared results showed that chip temperatures were highly consistent with thermal resistance measurements under different driving currents.

Measurement of LED chips using a large-area silicon photodiode

We propose the output power measurement of bare-wafer/chip light-emitting diodes (LEDs) using a large-area silicon (Si) photodiode with a simple structure and high accuracy relative to the conventional partial flux measurement using an integrating sphere. To obtain the optical characteristics of the LED chips measured using the two methods, three-dimensional ray-trace simulations are used to perform the measurement deviations owing to the chip position offset or tilt angle. The ray-tracing simulation results demonstrate that the deviation of light remaining in the integrating sphere is approximately 65% for the vertical LED chip and 53% for the flip-chip LED chip if the measurement distance in partial flux method is set to be 5-40 mm. By contrast, the deviation of light hitting the photodiode is only 15% for the vertical LED chip and 23% for the flip-chip LED chip if the large-area Si photodiode is used to measure the output power with the same measurement distance. As a result, the large-area Si photodiode method practically reduces the output power measurement deviations of the bare-wafer/chip LED, so that a high-accuracy measurement can be achieved in the mass production of the bare-wafer/chip LED without the complicated integrating sphere structure.

Study of Different Testing Methods Used in UV-LED Optical Measurement

In this study, three different partial radiant flux testing methods including the large-area silicon (Si) photodiode method, the assembling Si photodiodes box method, and the integrating sphere method were used to measure the optical power of UV-LED die. The optical simulations and the experiments have been performed to study the influence of the varying LED die position and tilt angle. The optical simulations based on the real measurement geometries were used to confirm the gathering hitting optical power on Si photodiode through different methods. The simulation and measurement results in this study can be used as the future reference of different partial LED flux testing methods to measure the large number of UV-LED dies.

Low-Frequency Noise Analysis of Electrostatic Discharge Tolerance of InGaN Light-Emitting Diodes

In recent years, with an extensive use of InGaN light-emitting diode (LED), how to assess the LED quality and further improve the LED reliability are very important. In this paper, the noise spectrum measurement techniques are used to assess the electrostatic discharge tolerance and quality of InGaN LED devices. Experimental results show that the noise spectrum measurement distinguishing the LED device reliability is more effective than the current-voltage curve measurement. In the evidence, emission microscope, scanning electron microscope, and transmission electron microscopy images show that the noise source and the cause of failure of the LED device are attributed by the poor quality of the SiO2 and Indium Tin Oxide (ITO) interface.

Thermal transient characteristics of flip chip high power light emitting diodes

The die-attached quality and the thermal transient characteristics of high power flip chip light emitting diodes (LEDs) are investigated using thermal transient tester. Various die-attached materials were utilized to compare the difference in their thermal resistances and long term performance. By applying accelerated aging stress, the deterioration rates at the die-attached layers were obtained. Numerical simulation provided further understanding of LED device temperature distribution and also revealed that the thermal variance at the die-attached interface can be recognized within only few milliseconds for the flip chip structure. The effects of bump arrangement and material were analyzed to achieve high temperature uniformity and low thermal resistance for high power LEDs.

Reliability of nitride LEDs

Abstract: In this chapter, we introduce the relevant standards for commonly used LED reliability tests, LED failure analysis techniques and a failure analysis flow chart for LEDs. Finally, we discuss the underlying physical mechanisms behind LED failure based on failure analysis results.

The low-frequency noise spectrum analysis of the reliability of the InGaN LED

In recent years, with extensive use of InGaN LED, estimation of LED quality and improvement of LED reliability has become very important. In this report, the noise spectrum measurement techniques were used to estimate the reliability of InGaN LED devices and compare its reliability with its ESD tolerance test result. Experimental results show that the noise spectrum measurement more effectively distinguishes the LED device reliability than that of the current voltage curve measurement. EMMI, SEM and TEM images show that noise source and cause of failure of the LED device are attributed to poor quality of the SiO2 and ITO interface.

Evaluation of optical and chromatic properties under electrical and thermal coupling in solid state lighting systems

For energy-saving, high efficiency and low pollution, the lighting of LED systems is important for the future of green energy technology industry. The solid state lighting becomes the replacement of traditional lighting, such as, light bulbs and compact fluorescent lamps. Because of the semiconductor characteristics, the luminous efficiency of LEDs is sensitive to the operating temperature. Besides increasing the luminous efficiency, effective controlling electricity and thermal characteristics in the design of LED lighting products is the key point to achieve the best results. LED modules can be combined with multi-grain process or through a combination of multiple LED chips. Accurate analysis of this LED module for the electrical, thermal characteristics and high reliability is the critical knowledge of modular design. In this report, we studied the electrical and thermal coupling phenomenon in solid state lighting systems to analyze their reliability. By experiments and simulations, we obtained the apparent variation of temperature distribution of LED system due to differences of their forward voltages and thermal resistances. These events may reduce their reliability. Besides, the evaluation of optical and chromatic properties was based on the variation of temperature distribution and current of LED system. This is the key technology to predict the optical and chromatic properties of LED system in use.