How to give full play to the advantages of high temperature resistance of silicon carbide?

[Introduction]With the development of silicon carbide (SiC) technology, devices are becoming more and more mature and commercialized. The unique high temperature resistance of its materials is accelerating the junction temperature from 150 ° C to 175 ° C. Some companies say that it has now begun Develop silicon carbide devices with a junction temperature of 200°C.

With the development of silicon carbide (SiC) technology, devices are becoming more and more mature and commercialized. The unique high temperature resistance of its materials is accelerating the junction temperature from 150 ° C to 175 ° C. Some companies say that they have begun to develop 200 ° C. junction temperature of SiC devices. Although silicon carbide is very resistant to high temperature, high temperature has a great impact on the performance, failure rate, and life of the device after all. With this question, the reporter interviewed Mr. Lu Tao, product line manager of Onsemi’s main drive power module. ON semiconductor has just completed the acquisition of silicon carbide producer GT Advanced Technologies in November 2021. This acquisition will undoubtedly enhance ON Semiconductor’s silicon carbide supply capacity and will also promote related research and development.

Lu Tao, product line manager of ON Semiconductor’s main drive power module

Challenges of Increasing Junction Temperature of Silicon Carbide Devices

How does the junction temperature of SiC devices change from 175℃ to 200℃? In this process, what challenges and difficulties need to be solved?

Lu Tao believes that as a unipolar wide-bandgap device, the SiC chip itself will gradually change its static and dynamic characteristics between 175°C and 200°C. Silicon carbide chips can easily operate in this higher temperature range. The challenge lies more in the packaging of SiC chips. Semiconductor encapsulation uses plastic encapsulated epoxy and/or silicone gel and is rated for temperatures up to 175°C. When operating temperatures exceed 175°C, these compounds tend to enter a transition state where their inherent properties begin to collapse and release unnecessarily high concentrations of ionic charges that begin to penetrate the surface of the chip, degrading performance.

Under extreme conditions, irreversible visible plastic deformation occurs. Another area of ​​concern is the alloy solder used within the package. Most semiconductor grade solder alloys have a melting point just above 200°C, and operating temperatures very close to the melting point of the alloy can exponentially accelerate semiconductor package wear.

In conclusion, silicon carbide chips can operate at higher temperatures, and the package housing needs to be developed with special materials to handle high temperatures, such as using sintering and high temperature packaging, to improve thermal cycling and power cycling efficiency.

He pointed out that in addition to the device itself, the thermal management system also needs to be optimized. In electric vehicle drivetrains, which typically use liquid cooling, the entire system requires engineering optimization to prevent thermal runaway. The thermal management complexity of the system is increasing, but currently this is seen as merely a required system-level optimization, with no fundamental blocking point.

What is the purpose of raising the junction temperature?

Is it necessary to increase the junction temperature in SiC applications? How does ON Semiconductor plan? When is it expected to launch a high junction temperature product?

Lu Tao said that letting the rated temperature of the silicon carbide solution exceed 175 °C is an important differentiating factor. This increases the Safe Operating Area (SOA) for SiC products. On the other hand, packages with high temperature ratings are still a long way from being realized, mainly due to the lack of available general market materials.

For inverter applications in electric motor drives, the silicon carbide MOSFETs operate at around 125°C for most of the drive cycle, he said. In some special cases, such as uphill or peak acceleration in electric vehicle operation, the SiC MOSFET will operate at peak power, on average 1.5 to 2 times its rated operating conditions. Operating junction temperatures above 175°C for SiC solutions will help give system designers more flexibility in choosing the most cost-effective solution for application needs.

ON Semiconductor is actively working on a silicon carbide solution that enables short-term operation above 175°C within about 5% to 10% of its operating life. This reduces the complexity of lengthy package development while meeting the needs of the application. ON Semiconductor plans to release the product in the second half of 2022, and the basic material development is underway, and the exact timetable will be announced at a later date.

He also emphasized that since the ambient operating temperature of most commonly used power switches is between 25°C and 100°C, from a technical point of view, operating at 200°C does not fundamentally enable SiC to enter new topologies. But operating at 200°C enables SiC switches to operate at higher power densities, making SiC solutions more cost-effective than their silicon-based alternatives.

future development expectations

Finally, Lu Tao said that the continuous operation of the silicon carbide solution at 200 ℃ is part of the long-term roadmap. The main SiC challenge that needs to be overcome now involves improving economies of scale. This has exponentially increased the demand for SiC given the strong market drivers brought about by the electrification of automotive functions and clean energy. The first step is to verticalize the supply chain. This will ensure a stable supply of SiC and adequate quality control throughout the value chain, from substrate to packaged finished product.

With reasonable economies of scale, the next challenge is how to improve product yield. Given the inherent properties of SiC, which has a much higher defect density compared to silicon, the manufacturing and development community will be challenged to improve the process, thereby reducing scrapping costs.

Once product yields mature, leveraging larger wafer diameters (8 inches) will play an important role in improving capital efficiency and paving the way for more advanced SiC technologies.

(Source: Power System Design)

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