Making Silicon Carbide Schottky Diodes and MOSFETs Mainstream Demands New Approaches to Wafer Fabrication and Converter Design

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1 Making Silicon Carbide Schottky Diodes and MOSFETs Mainstream Demands New Approaches to Wafer Fabrication and Converter Design by Corey Deyalsingh, Littelfuse and Sujit Banerjee, Monolith Semiconductor If an emerging semiconductor technology is to transition successfully into a mainstream one, two factors are essential: (1) the technology must offer excellent reliability and parametric stability, and (2) there must be sufficient continuing demand for the devices it enables to make full-scale production economically practicable. Silicon carbide (SiC) is a rapidly emerging semiconductor material that enables power devices to operate at higher switching frequencies with lower losses and temperatures versus conventional silicon. It allows inverters and other energy conversion systems to be built with significantly improved power density, energy efficiency and cost. Global energy consumption is growing dramatically; in fact, the U.S. Energy Information Administration s International Energy Outlook has projected a 48% increase by This growing demand highlights the importance of developing clean (non-fossil-fuel) technology and boosting energy efficiency. SiC represents one important way to advance the clean/green energy revolution. At the same time, the need for grid energy storage to balance fluctuations in energy generated by green technologies like wind and solar power is also growing and SiC technology is poised to make that possible cheaply, compactly, and efficiently through the use of batteries and other highvoltage power electronics like switches, inverters and controllers. As the demand for hybrid and electric vehicles rises, energy-efficient power conversion electronics are increasingly critical to the transportation market to extend range. SiC devices allow designing smaller, lighter, more rugged and more efficient onboard power conversion systems for traction inverters, DC-DC converters, onboard charging, etc. Silicon carbide will also enable more practical electric vehicles and other transportation systems through the use of improved SiC-based power electronics systems. Currently, up to 3 percent of all U.S. electricity generated goes to powering data centers. As more information is stored in the cloud, data centers will consume even more energy. SiC technology will be vital to increasing power density while increasing efficiency and reducing operating costs.

2 Today, however, relatively few SiC discrete devices (at least, SiC MOSFETs) are currently commercially available; fewer suppliers have stock available for purchase, which is limiting their widespread adoption and slowing their entrance into the mainstream. To encourage SiC adoption, suppliers must do more to improve confidence in the availability of qualified SiC devices. They must also be willing to work in cooperation with smaller, emerging customers to help them apply the emerging technology to their needs. In particular, suppliers need to take into account the needs of customers who don t represent an enormous volume of business immediately but offer the potential for future growth. Investing in the Future of SiC Starting in December 2015, Littelfuse, Inc. began a strategic initiative to help bring SiC devices into the mainstream by investing in and partnering with Monolith Semiconductor, Inc., a technology-focused company developing silicon carbide technology. Littelfuse recently invested another $15 million investment in Monolith and now holds a majority ownership position. Littelfuse has also committed to add to its investment once Monolith achieves certain milestones. Monolith leverages a fabless model to manufacture SiC devices in a high-volume, automotivequalified, 150mm CMOS fab. Existing fab processes and tools are reused wherever possible and SiC wafers are run in parallel with existing silicon CMOS production. This manufacturing approach makes it possible to reduce manufacturing costs at lower volumes to enter the SiC market at a price competitive with major SiC device suppliers. Avoiding the enormous cost of building and operating a fab is central to being agile and maintaining a low-cost structure. This also accelerates time to market, enables faster ramp-up and improves device quality. The Littelfuse-Monolith SiC devices leverage the processes and quality systems employed for high-volume automotive products. In automotive-qualified fabs, quality is ensured through stringent Statistical Process Control (SPC) monitoring of the fabrication process, materials inspections, Wafer Level Reliability (WLR) testing, reliability monitoring, and strict change control management. Littelfuse- Monolith products can match the performance of industry leaders in SiC. The SiC MOSFETs provide 5-10 reduction in terms of switching losses compared to silicon IGBTs. In a recent power converter demonstration, these 1200V MOSFETs had the highest efficiency, comparable to the best-performing SiC MOSFET available on the open market. The combination of Monolith expertise in SiC technology and Littelfuse expertise in supply chain management ensures superior customer support. Customers can be confident of the best application

3 support as Monolith and Littelfuse provide the tools needed to enable accelerated design of our SiC devices into their products. SiC Schottky Diodes Silicon carbide has a high thermal conductivity and temperature has little influence on its switching and thermal characteristics. Over the last two decades, SiC Schottky diodes have become available with increasingly higher voltage ratings. SiC Schottky diodes have ~40 lower reverse leakage current than PN silicon Schottky diodes. LSIC2SD120 Series 1200 V SiC Schottky Diodes (Figure 1) are the Littelfuse- Monolith partnership s first products to reach the marketplace. These diodes are designed to reduce switching losses dramatically and enable substantial increases in system efficiency and robustness. Figure 1. LSIC2SD120 Series 1200 V SiC Schottky Diodes, the first of a full portfolio of SiC devices slated for introduction by Littelfuse-Monolith, are currently available with current ratings from 5A to 10 A in TO-220-2L or TO-252-2L packages. These diodes provide best-in-class capacitive stored charge and negligible reverse recovery to ensure low switching losses in high-frequency power switching and reduced stress on the opposing switch. Ultra-low forward voltage drop allows for reduced conduction losses. A maximum junction temperature of 175 C provides for a larger design margin and relaxed thermal management requirements. Their merged pn-schottky (MPS) architecture enhances surge capability and ensures extremely low leakage. Applications include boost diodes in power factor correction, switch-mode power supplies, uninterruptible power supplies, solar inverters, industrial motor drives, EV charging and power conversion. SiC MOSFETs

4 The quality of commercially available 1200 V SiC MOSFETs has improved dramatically in recent years: channel mobility has risen to suitable levels, oxide lifetimes have reached an acceptable level for most mainstream industrial designs, and threshold voltages have become increasingly stable. For example, accelerated TDDB testing performed at NIST has predicted an oxide lifetime of more than 100 years, even at junction temperatures higher than 200 C, for Monolith Semiconductor s MOS technology. In these tests, the acceleration factors of applied electric field across the oxide were taken to more than 9 MV/cm and junction temperature to 300 C; for reference, oxide electric fields as used in practice are around 4 MV/cm, and junction temperatures are typically lower than 175 C. It is also worth noting that although temperature-dependent acceleration factors are commonly seen in silicon MOS, they have not been seen by NIST for SiC MOS prior to their work with the devices from Monolith. The threshold voltage stability of these MOSFETs has also been demonstrated convincingly. High-temperature gate bias (HTGB) testing was performed at a junction temperature of 175 C and under negative (V GS = -10 V) (Figure 2a) and positive (V GS = 25 V) (Figure 2b) gate voltages.

5 Figure 2. (a) Negative, V GS = -10 V, and (b) positive, V GS = 25 V, high-temperature gate bias (HTGB) stress tests performed at 175 C on 77 devices from three different wafer lots out to 2300 hours. Negligible deviation was observed. The blocking voltage and off-state leakage of these MOSFETs has also proven to be stable over the long term based on high temperature reverse bias (HTRB) reliability testing results. With respect to device ruggedness, preliminary measurements reveal a short-circuit withstand time of at least 5 microseconds and an avalanche energy of 1 J (Figure 3).

6 Figure 3. Short-circuit testing of a 1200 V, 80 mω SiC MOSFET at a dc link of 600 V and V GS = 20 V, indicating a withstand time of at least 5 μs. Even if SiC MOSFETs remain more expensive than comparable silicon IGBTs, designers are already viewing substantial system-level price benefits of using them rather than Si IGBTs at today s price levels, perhaps even using SiC discrete devices vs. more expensive IGBT modules. SiC MOSFET pricing is likely to become increasingly competitive as economies of scale take hold with 150 mm wafers and the rest of the SiC supply chain. Extracting Maximum Performance from SiC Devices Obtaining the maximum performance from high-voltage SiC MOSFETs and diodes requires more than just plugging them into an application. For example, it requires access to a characterization board designed for use with SiC devices that has minimal parasitic inductances and supports precise measurement of voltage and current waveforms. It also calls for careful and optimized gate drive design and layout to minimize noise coupling. The Monolith Littelfuse Dynamic Characterization Toolkit (Figure 4) helps accelerate converter design by allowing designers to characterize the switching behavior of SiC devices accurately and explore optimized driving via gate driver parameter tuning.

7 Figure 4. Monolith Littelfuse Dynamic Characterization Toolkit The Littelfuse-Monolith 5 kw Evaluation Converter (Figure 5) is designed to accelerate the power converter design and layout process. It employs a modular design strategy and components with similar footprints to provide an extremely versatile and adaptable platform for converter-level testing.

8 Figure 5. The Littelfuse-Monolith 5 kw Evaluation Converter. Conclusion After decades of development effort, it s increasingly clear that SiC diodes and MOSFETs are finally coming into their own and are poised for widespread commercial success and a substantial role in the green energy movement. # # # About the Authors: Corey Deyalsingh is Director of Power Semiconductor in the Electronics Business Unit. He joined Littelfuse as a Product Engineer in 2003 via the acquisition of Teccor Electronics. Corey s current responsibilities include strategic direction and leadership for expansion of the power semiconductor portfolio including silicon carbide, IGBT, and thyristor & rectifier diode technologies. He received his BSEE from The University of Oklahoma and MBA from The University of Dallas and has been involved in the semiconductor industry for over 21 years in various roles from Applications Engineering to Operations to Marketing. Corey is based in the Dallas Forth Worth area and can be reached at cdeyalsingh@littelfuse.com.

9 Dr. Sujit Banerjee is the CEO and co-founder of Monolith Semiconductor focused on commercializing and enabling widespread adoption of Silicon Carbide power semiconductors. Prior to Monolith, he led technology development at Power Integrations, focusing on high-voltage ICs and MOSFETs. At PI, Sujit and his team developed robust HVIC processes running in CMOS foundries at high volumes that enabled PI s worldleading off-line switchers for consumer and industrial applications. Prior to PI, Sujit completed his PhD from RPI, NY in 2002 for which he demonstrated SiC high-voltage lateral MOSFETs that can enable hightemperature power ICs in SiC. Sujit has more than 25 patents for his work in power semiconductors. Sujit can be reached at sbanerjee@monolithsemi.com.