Shiwei Feng

The thermal characterization of packaged semiconductor device

In this paper, using voltage drop of forward junction as temperature sensitive parameter (TSP) for GaAs MESFET and semiconductor laser diode, we measured temperature rise /spl Delta/T under normal operation condition. Furthermore, we composite a testing sequence in which the duration of driving pulse from several microseconds to 100 seconds. This sequence is designed actually to simulate the procedure of heat transfer from active region of device to environment. /spl Delta/T and thermal resistances R/sub th/ are measured after each testing pulse. The R/sub th/ against heating time is drawn to form heating response curve of device. Steps appear in the curve corresponding to the contribution from different component of device. By this way, we can make thermal characterization for packaged semiconductor devices easily and quickly. Some thermal quality, such as soldering quality of chip, thermal resistance of chip and package, can be characterized. We also use this method to make researches on semiconductor laser diodes LD. In their normal operation condition, LD shows different behavior from GaAs MESFET because of its light output. This also yields a new method to determine light output efficiency by thermal measurement.

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.

Thermal investigation of LED array with multiple packages based on the superposition method

In this paper, the superposition method is used to investigate the complete temperature field of a light-emitting diode (LED) packaging substrate, based on the results of transient temperature rise measurements and the thermal resistance coupling matrix. The feasibility of use of the superposition method in an LED array with multiple packages has been proved first by temperature comparisons with the simultaneous operation of an array (5i?5) of 25 high power LEDs mounted on a metal core printed circuit board (MCPCB). Compared with existing approaches, the superposition method will measure the internal temperature of chip directly, accurately and nondestructively. According to the relatively accurate and reliable self-heating and coupling temperature rise data, optimization scheme of LED lamp with multiple packages is proposed. The results show that increasing the heat source separation distance and improving the thermal conductivity of thermal interface materials will reduce the temperature rise and thermal non-uniformity.

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.

Interface states mediated reverse leakage through metal/AlxGa1−xN∕GaN Schottky diodes

Interface states modulated reverse leakage current through metal/AlxGa1−xN∕GaN diodes has been studied. Reverse leakage current and breakdown voltage have been measured over wide temperature ranges. The investigation suggests that the piezoelectric polarization and the spontaneous polarization at the AlxGa1−xN∕GaN heterostructure create polarization induced charges in AlxGa1−xN, and influence the two-dimensional electron gas at the GaN∕AlxGa1−xN interface. Both of them dictate the barrier height of the strained AlxGa1−xN Schottky contact. High density of defect states at the metal/AlxGa1−xN interface leads to high reverse leakage current via Fowler-Nordheim emission and/or Frankel-Poole emission. The most notable finding is the reduction in the leakage current almost to zero at high temperatures due to strain relaxation, reduction in defect states, and barrier height enhancement at the metal/AlxGa1−xN interface. Such finding may have significant impact on the way we design high-power microwave devices.

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.

Variation of Dominant Degradation Mechanism in AlGaN Barrier Layer With Different Voltage Stress on the Gate of AlGaN/GaN High Electron Mobility Transistors

This letter proposes that dominant mechanism that induces the change of traps and defects in AlGaN barrier layer varies with different range of voltage stress on the gate of AlGaN/GaN high electron mobility transistors. The gate-source (drain) reverse current-voltage (I-V) and capacitance-voltage (C-V) characteristics were measured after each voltage stress applied to the gate which stepped from -8 to -70 V in -1 V step. They showed similar changes in test, both decreased from -8 to -20 V and increased from -20 to -70 V. The micro-Raman spectroscopy focused on GaN layer was measured before stress, during -30 V stress, and after -70 V stress, respectively. They kept constant in the measurement. It proved that inverse piezoelectric effect had few influence in GaN layer. The increase of filled inherent traps and permanent defects in AlGaN barrier layer after different voltage stress accounted for the experimental phenomenon. The former was due to gate electron injection which played a main role in low bias stage, the latter was due to inverse piezoelectric effect which played a main role in high bias stage.

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.