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Browsing by Author "Jiang, Li"

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    Electrical and Thermal Characterizations of IGBT Module with Pressure-Free Large-Area Sintered Joints
    Jiang, Li (Virginia Tech, 2013-10-17)
    Silver sintering technology has received considerable attention in recent years because it has the potential to be a suitable interconnection material for high-temperature power electronic packaging, such as high melting temperature, high electrical/thermal conductivity, and excellent mechanical reliability. It should be noted, however, that pressure (usually between three to five MPa) was added during the sintering stage for attaching power chips with area larger than 100 mm2. This extra pressure increased the complexity of the sintering process. The maximum chip size processed by pressure-free sintering, in the published resources, was 6 x 6 mm2. One objective of this work was to achieve chip-attachment with area of 13.5 x 13.5 mm2 (a chip size of one kind of commercial IGBT) by pressure-free sintering of nano-silver paste. Another objective was to fabricate high-power (1200 V and 150 A) multi-chip module by pressure-free sintering. In each module (half-bridge), two IGBT dies (13.5 x 13.5 mm2) and two diode dies (10 x 10 mm2) were attached to a DBC substrate. Modules with solder joints (SN100C) and pressure-sintered silver joints were also fabricated as the control group. The peak temperature in the process of of pressure-free sintering of silver was around 260oC, whereas 270oC for vacuum reflowing of solder, and 280oC under three MPa for pressure-sintering of silver. The process for wire bonding, lead-frame attachment, and thermocouple attachment are also recorded. Modules with the above three kinds of joints were first characterized by electrical methods. All of them could block 1200 V DC voltage after packaging, which is the voltage rating of bare dies. Modules were also tested up to the rated current (150 A) and half of the rated voltage (600 V), which were the test conditions in the datasheet for commercial modules with the same voltage and current ratings. I-V characteristics of packaged devices were similar (on-resistance less than 0.5 mohm). All switching waveforms at transient stage (both turn-on and turn-off) were clean. Six switching parameters (turn-on delay, rise time, turn-off delay, fall time, turn-on loss, and turn-off loss) were measured, which were also similar (<9%) among different kinds of modules. The results from electrical characterizations showed that both static characterizations and double-pulse test cannot be used for evaluating the differences among chip-attach layers. All modules were also characterized by their thermal performances. Transient thermal impedances were measured by gate-emitter signals. Two setups for thermal impedance measurement were used. In one setup, the bottoms of modules were left in the air, and in the other setup, bottoms of modules were attached to a chiller (liquid cooling and temperature controlled at 25oC) with thermal grease. Thermal impedances of three kinds of modules still increased after 40 seconds for the testing without chiller, since the thermal resistance of heat convection from bottom copper to the air was included , which was much larger than the sum of the previous layers (from IGBT junction, through the chip-attach layer, to the bottom of DBC substrate). In contrast, thermal impedances became almost stable (less than 3%) after 15 seconds for all modules when the chiller was used. Among these three kinds of modules, the module with pressure sintered joints had the lowest thermal impedance and the thermal resistance (tested with the chiller) around 0.609oK/W, In contrast, the thermal resistance was around 964oK /W for the soldered module, and 2.30oK /W for pressure-free sintered module. In summary, pressure-free large-area sintered joints were achieved and passed the fabrication process for IGBT half-bridge module with wiring bonding. Packaged devices with these kinds of joints were verified with good electrical performance. However, thermal performances of pressure-free joints were worse than solder joints and pressure-sintered joints.
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    Output power of a quantum dot laser: Effects of excited states
    Wu, Yuchang; Jiang, Li; Asryan, Levon V. (American Institute of Physics, 2015-11-14)
    A theory of operating characteristics of quantum dot (QD) lasers is discussed in the presence of excited states in QDs. We consider three possible situations for lasing: (i) ground-state lasing only; (ii) ground-state lasing at first and then the onset of also excited-state lasing with increasing injection current; (iii) excited-state lasing only. The following characteristics are studied: occupancies of the ground-state and excited-state in QDs, free carrier density in the optical confinement layer, threshold currents for ground- and excited-state lasing, densities of photons emitted via ground- and excited-state stimulated transitions, output power, internal and external differential quantum efficiencies. Under the conditions of ground-state lasing only, the output power saturates with injection current. Under the conditions of both ground- and excited-state lasing, the output power of ground-state lasing remains pinned above the excited-state lasing threshold while the power of excited-state lasing increases. There is a kink in the light-current curve at the excited-state lasing threshold. The case of excited-state lasing only is qualitatively similar to that for single-state QDs—the role of ground-state transitions is simply reduced to increasing the threshold current.
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    Resistance Control MPPT for Smart Converter PV System
    Jiang, Li (Virginia Tech, 2012-04-19)
    DC nano-grid system shows promising prospect and enjoys some advantages over AC micro-grid system. It enables easier integration of multiple renewable energy sources with multiple loads. Photovoltaic (PV) is essentially a typical renewable source that serves as main power source in DC nano-grid system. Traditional PV system includes centralized PV system, string PV system and micro-converter PV system. More recently, smart converter PV system has been introduced and shown great improvement in aspects of power generation achieved by distributed Maximum Power Point Tracking (MPPT). It is also advantageous over micro-converter PV system due to lower cost and flexibility. Detailed case study demonstrates that power generation efficiency can be easily compromised because of mismatch between different panels in centralized and string PV systems. In smart converter PV system, this problem can be solved due to distributed MPPT for each individual panel. The smart converter system has a very wide voltage range within which all panels can generate maximum power. The location and the width of this range are subject to change under different mismatch conditions. A second stage converter is needed to locate the array MPPT range. However, there is instability problem when doing second stage MPPT with traditional methods. Modified methods based on conductance control and resistance control are analyzed and compared. Both methods can solve the MPPT instability problem. However, in terms of steady state performance, resistance control MPPT is more promising in terms of higher utilization ratio and faster tracking speed. It is because both methods are of inherited variable operating point step size with constant conductance or resistance perturbation step size. However, the operating point change decreases with resistance perturbation but increases with conductance perturbation otherwise. Therefore, resistance control MPPT is chosen as a good candidate. Both simulation and experimental results verifies the concept.
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    Theoretical study of performance characteristics of semiconductor quantum dot lasers
    Jiang, Li (Virginia Tech, 2008-09-02)
    The effect of different factors on the operating characteristics of a semiconductor quantum dot (QD) laser is studied. Specifically, the following topics are included in the dissertation: 1) Effect of carrier-density-dependent internal loss in the optical confinement layer (OCL) on the characteristic temperature. Internal optical loss in a QD laser couples the confined-carrier level occupancy in QDs to the free-carrier density in the OCL. Due to this coupling, which is controlled by the threshold condition, the free-carrier density is increased and more temperature-sensitive, and also the confined-carrier level occupancy becomes temperature-dependent. As a result, the characteristic temperature of a laser is considerably reduced. Carrier-density-dependent internal loss also sets an upper limit for operating temperatures of a QD laser and constrains the shallowest potential well depth and the smallest tolerable size of a QD at which the lasing can be attained. The dependences of the characteristic temperature, maximum operating temperature, and shallowest potential well depth on the parameters of the structure are obtained. At the maximum operating temperature or when any parameter of the structure is equal to its critical tolerable value, the characteristic temperature reduces to zero. 2) Effect of excited-states in QDs on the light-current characteristic (LCC). The carrier capture from the three-dimensional reservoir (optical confinement layer – OCL) into the QD ground-state and escape from the ground-state to the OCL are assumed to occur via the QD excited-state. Such a two-step capture places a fundamental limitation on ground-state lasing—the output power saturates at high injection currents. The saturation power is controlled by the transition time between the excited- and ground-state in a QD. The longest, cut-off transition time exists, beyond which no ground-state lasing is possible. The following characteristics are analyzed versus the injection current density and the transition time: occupancies of the ground- and excited-state, free carrier density in the OCL, threshold current density, number of stimulated photons emitted, output power, internal and external differential quantum efficiencies. 3) Effect of longitudinal spatial hole burning (SHB) and multimode lasing on the LCC. The number of modes is shown to remain limited with increasing injection current. The maximum number of modes that can oscillate in a QD laser is analytically estimated. While this number increases with increasing surface density of QDs or cavity length, it remains limited (first increases and then decreases) with increasing scatter in the QD-size. The critical tolerable values of the structure parameters are derived beyond which higher-order longitudinal modes can not oscillate. It is notable that, in addition to the maximum tolerable scatter, there also exists the minimum scatter in the QD-size for each higher-order mode to start lasing. The threshold currents and output powers of modes are computed numerically. The power of the main mode is reduced due to lasing of higher-order modes and spatially nonuniform carrier distribution. As a new mode turns on, kinks appear in the LCCs of existing modes. SHB reduces the total optical power of a laser and contributes to nonlinearity of the overall LCC. The effect is more significant when any of the structure parameters is close to its critical tolerable value. The LCC becomes more linear with improving QD-size uniformity or increasing surface density of QDs or cavity length.
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    Thermo-Mechanical Reliability of Sintered-Silver Joint versus Lead-Free Solder for Attaching Large-Area Devices
    Jiang, Li (Virginia Tech, 2010-12-08)
    This study mainly evaluated the thermo-mechanical reliability of lead-free packaging techniques for attaching large-area chip. With 3 MPa pressure, a low-temperature (<300oC) sintering technique enabled by a nano-scale silver paste was developed for attaching 100 mm2 silicon die. This new lead-free packaging technique for die-attachment was compared with soldering by vacuum reflow. Lead-free solder SAC305 and SN100C were selected and used in this work since they were widely used in electronic packaging industry. Inspection of as-prepared die-attachments by X-ray and optical microscopy (observation of cross-section) showed that the voids percentage in solder joint was less than 5% and no voids was observed at the scale of hundreds of micron in sintered silver joint. Then these die-attachment were thermal cycled with the temperature range from -40oC to 125oC. Deduction of curvature and residual stresses were found for both soldered and sintered die-attachment. After 800 cycles, the residual stresses in silicon-solder-copper sample already decreased to around 0. The SEM images of solder and silver joint after 800 thermal cycles showed that cracks longer than 2.5 mm already grew in both kinds of solder joint (die-attachment of Si-Solder-Copper). In contrast, no cracks or voids at the scale of hundreds of micron were defected in silver joint. Based on these observation, different mode of stress-relaxation were proposed for sintered silver and solder, respectively. While solder joint released stresses by crack growth, the silver joint relied on the deformation of porous structure, and plastic deformation may occur. The pressure-sintering process with double printing and drying was proved to be a reliable process to produce sintered - silver bonding with high strength. The reliability of silver joint was better than that of SAC305 or SN100C. Besides, the technique of measuring the curvature by laser scanning, introduced in this work, showed its significance by directly reflecting the bonding integrity of die-attachment. As a nondestructive testing technique, It was a cheaper and faster way to examine the die-attachment. Additionally, it overcame the disadvantage of X-ray Inspection: it was of the ability to differentiate between layers of die-attachment.