Guangchen Zhang

Effect of Self-Heating on the Drain Current Transient Response in AlGaN/GaN HEMTs

The self-heating effect on the drain current transient response in AlGaN/GaN high electron mobility transistors (HEMTs) was investigated experimentally. Two steps were observed as the drain current dropped with time. The relationship between the transient temperature rise in the channel and the drop in the drain current are discussed in detail. The decrease in the transient drain current and the rise in the channel transient temperature had a complementary relationship. The decrease in the channel electron mobility, caused by self-heating, was an important factor in the drain current drop in the AlGaN/GaN HEMTs.

Thermal Fatigue Characteristics of Die Attach Materials for Packaged High-Brightness LEDs

With the increase of driving current and power density, the thermal fatigue characteristics of die attach materials have become an important issue impacting the long-term reliability of high-brightness light-emitting diodes (HB LEDs). In this paper, an accelerated power cycling method is used to predict the thermal fatigue life of die attach materials for packaged HB LEDs. The thermal fatigue life of Au80Sn20 eutectic and silver paste is investigated at driving currents of 650, 675, and 700 mA. Changes in the thermal resistance of die attach materials are monitored by the noninvasive structure function method based on the measurement of transient temperature response curves through power cycling experiments. Results of C-mode scanning acoustic microscopy and cross-sectional scanning electron microscopy indicate that delamination is the main failure mechanism of the die attach materials under thermal fatigue stress. Furthermore, the thermal fatigue life of die attach materials can be derived from the relationship between lifetime and temperature difference according to the Coffin-Manson relationship. The results suggest that Au80Sn20 eutectic possesses a longer thermal fatigue life than silver paste at the same temperature difference of thermal fatigue stress.

The study on the thermal behavior of packaged power LEDs

The reliability of power LEDs has strong dependence on successful package thermal management. In this paper, heating response curves of LEDs were recorded by the temperature sensitive parameter (TSP) method and fast-switching circuit techniques. It provided a novel method to analyze thermal resistance composition of power LED¿s packaging. Cover-and-block method was presented to provide a correction to the apparent thermal resistance of optical devices, and an electrical method to obtain radiant efficiency was also put forward.

Assessment of pulse conditions effects on reliability in GaN-based high electron mobility transistors by transient temperature measurements

The forward Schottky characteristic method, utilizing the temperature dependence of forward gate-source Schottky junction voltage, has been used to measure the transient temperature rise under DC and cycle pulse for multi-finger AlGaN/GaN high electron mobility transistor. The effect of electrical pulse on channel temperature has been studied. The transient temperature rise of channel under pulses with different duty cycles and frequencies are determined, respectively. The measurement results show that operation under high frequency and/or low duty cycle can improve the lifetime and performance reliability of AlGaN/GaN HEMT devices effectively. Furthermore, the measurement results are found to be consistent with the electro-thermal simulation results performed for the device.

Determination of Thermal Fatigue Delamination of Die Attach Materials for High-Brightness LEDs

The thermal fatigue delamination interface of die attach materials in high-brightness light-emitting diodes (HB LEDs) is determined using a noninvasive approach. Failure analysis of HB LEDs containing Au80Sn20 eutectic alloy and silver paste as die attach materials is performed by monitoring the changes in the partial thermal resistances in differential structure function curves of the HB LEDs through power cycling experiments. The results suggest that delamination of the Au80Sn20 eutectic and silver paste materials occurs at the chip-to-die attach interface and die attach-to-heat sink interface, respectively, which is consistent with cross-sectional scanning electron microscope analysis.

Determination of channel temperature of AlGaN/GaN HEMT by electrical method

In this paper, the linear relationship of forward Schottky junction voltage of AlGaN/GaN HEMT with temperature was investigated. It was used as temperature sensitive parameter to determine the channel temperature at its normal working state by fast switch circuit technique. The regress of tested data was used to reduce the scatter error too. The thermal resistance constitution was extracted from the transient heating response curves.

Thermal stability evaluation of die attach for high brightness LEDs

The long term thermal stability of the die attach is a crucial issue for high brightness light emitting diodes (HB LEDs), which affects the junction-to-case thermal resistance, luminous flux and life time seriously. In this paper, an improved power and temperature cycling method is proposed to evaluate the thermal stability of the die attach materials for HB LEDs. The structure function method is adopted to monitor the degradation of the die attach level thermal resistance during the cycling process instead of the traditional junction-to-case thermal resistance measurement, which provides more accurate, quick and intuitive results. The experimental results indicate that the forming of solder voids is the main degradation mechanism of the die attach for HB LEDs, which is also supported by the scan acoustic microscope (C-SAM) measurement. Comparing thermal stability of different die attach materials, Au/Sn eutectic soldered LED samples present better performance than Ag paste soldered samples in this experiment.

Structure optimization of heat sink for high power LED street lamp

Light emitting diode (LED) is now widely used in many fields including traffic lights, vehicle backlights and liquid crystal display (LCD) displays because of their long life, good illumination efficiency and low energy consumption. For the high power LED street lamp, a low operating temperature is essential for LED chips. However, its waste and meaningless to increase the size of heat sink for high power LED street lamp immoderately. In this paper, structure optimization of heat sink is studied on the condition that the heat sink is sufficient for cooling demand of 196W LED street lamp. 1/7, 2/7 and 3/7 length diminution of the heat sink are studied respectively. Results show that temperature of plate surface is increased 16°C with 3/7 bulk and weight reducing. And cross-sectional shape optimization of fin is also studied. The result show that the bulk and weight of fins are reduced over 50% with only 0.42°C temperature rise. The results of this work provide useful guidelines for thermal management of the heat sink without affecting the cooling demand of high power LED street lamp.

Thermal analysis of high power LED array system

Its proved that the physical meaning of the electrical average temperature rise of series LED array system tested by electrical temperature sensitive parameter (TSP) method is the arithmetic mean of the temperature rises of all sub-LEDs in the system. Based on this relationship, a novel method to evaluate the temperature distribution of series LED systems by scan measurement and recursive calculation is proposed in this paper. Transient heating response measurements of GaN based high power LED arrays are presented to provide experimental verification for this method. The results show that this method is real-time valid and package form independent. The error between the results of individual measurement and this proposed method is less than ±2°C.

Enhanced thermal measurements of high power LEDs by junction characteristic

The measurements of junction temperature rise and thermal resistance of power LEDs by junction characteristic are proposed in this paper. The measured thermal resistance is corrected by the light-absorbing method. It is also proved that the physical meaning of the electrical average temperature rise of series LED array system is the arithmetic mean of the temperature rise of all sub-LEDs in the system. Therefore, a novel method to extract the temperature distribution of series LED systems is presented.