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| A publication of the National Electronics Manufacturing Center of Excellence | February 2004 |
 Future high power applications will demand that power devices possess high blocking voltages, switching frequencies, increased efficiency, and extended reliability. Silicon technology however is approaching its theoretical limits. Silicon carbide (SiC), as one of the most mature wide band gap (WBG) semiconductors, is a natural choice for high power applications based on its notable material properties. Because of its larger band gap, around 3 eV compared to Si (Silicon) at 1.1 eV, the leakage currents in SiC are much lower than in Si. The high intrinsic temperature (above 800°C) of SiC offers excellent thermal stability. The high breakdown field of SiC and saturated electron velocity are key properties that make SiC ideal for high power operations. SiC devices have the benefit of high blocking voltages in the diodes and transistors that could help to avoid serial stacking and associated packaging difficulties at higher voltage levels. One of the possible benefits of a higher switching frequency in pulse-width-modulated rectifiers and inverters would be a substantial reduction in the areas occupied by passive circuit components, as well as reductions in the complexity of the circuits. Currently, SiC, IGBTs (Insulated Gate Bi-Polar Transistors), GTOs (Gate Turn-Off Thyristors), MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), Schoktty diodes, and JFETs (Junction Field Effect Transistors) are commercially available examples. Packaging is a high priority when attempting to optimize the performance of SiC die for high temperature operations. Aspects of SiC devices such as high power density, increased junction temperature and high frequency create considerable challenges for packaging and thermal management. A validated packaging technology for temperatures above 250°C is currently not available commercially. When packaging and implementing SiC devices in high power electronic systems, proper thermal management is critical. When thermal and package designs are implemented correctly, the component life can be extended well beyond specifications. Removal of waste heat from any given operating component is essential to the effort of establishing long-term reliability for a given device, and also for other components in close proximity on the same print circuit board. If not dealt with, excess heat can cause catastrophic failures of semiconductor junctions and passive components. Today's advanced packaging technologies like stacked die chips, multi-chip modules, and system-on-a-chip require that thermal management systems be designed to handle maximum power dissipation, power density, and power hot spots at both semiconductor and module levels. The ultimate goal is to produce a minimal temperature differential between the component surface and the heat spreader surface and further dissipate the heat out of the system.  Packaging materials, reliable interconnection approaches and cooling techniques need to be further modeled and evaluated. Information gained from packaging Si and GaAs (Gallium Arsenide) semiconductors has shown that the coefficient of thermal expansion (CTE) and thermal conductivity of materials are critical when the package is designed and processed. Some packaging and semiconductor materials’ CTE and thermal conductivity parameters are listed in Table 2-1. A close CTE match between the die and substrate, close CTE match between the device and the circuit board, lower thermal resistance at each heat transfer stage and a high heat dissipation rate from the junction to the ambient environment are some of the important characteristics that contribute to the high reliability of more ideal systems.The influences of many packaging materials on the overall thermal resistance of Si and GaAs devices have been evaluated extensively by the EMPF. Taking advantage of these results by incorporating suitable packaging techniques for WBG technology will not only speed up the package development but also make the WBG technology more affordable for commercial and military applications.  References: 1) J. Hudgins, et al "An assessment of wide band gap semiconductors for power devices". IEEE transactions on power electronics. May 2003 |
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