■ Sustainable Energy Era, Rapidly Rising Demand for SiC Power Semiconductors

Global demand for energy continues to increase. People’s daily consumption of massive amounts of digital content, the connection of electronic devices to the cloud, and the acceleration of vehicle electrification are all factors that are rapidly increasing energy demand.

On the one hand, there is a growing demand to reduce carbon emissions and produce and consume energy sustainably.

One way to meet these demands is to design efficient power conversion designs using silicon carbide (SiC) power semiconductors .

There are already a variety of SiC-based power semiconductors on the market. The basic advantages of SiC are that it enables higher switching frequencies, achieves high efficiency, and reduces heat generation. This leads to reduced system costs. This article explains three aspects that must be considered to achieve high-efficiency SiC designs.

■ Device Design - Pioneer in Trench SiC MOSFETs

Infineon is a pioneer in trench SiC MOSFET technology, having first introduced this technology in the 1990s. Based on accumulated know-how on the limitations and possibilities of Si-MOS technology, the trench-based device design was conceived (Figure 1). Trench SiC MOSFETs have the advantage of low on-state resistance without compromising reliability. In general, trench-based vertical power devices maximize material utilization in a small die size for a given performance. This has been proven with silicon-based power switches in the past, and is especially important with SiC devices, as SiC itself is a major cost factor.

Additionally, the unique physical properties of the SiC crystal in trench-based devices allow for lower resistance per unit area than planar devices, as long as a safe operating field is maintained by the gate oxide. Therefore, trench MOSFET designs can combine reliability with excellent performance. Additional advantages include sufficiently high threshold voltages, adjustable short-circuit robustness, excellent avalanche ratings, and good capacitance ratios, which enable low-loss, robust, and simple converter designs.

Another feature in the device design is that the intrinsic body diode is integrated into the SiC MOSFET, eliminating the need for an external high-speed freewheeling diode. Infineon's CoolSiC™ MOSFETs maximize the use of the body diode to increase robustness during hard commutation and reduce reverse recovery charge. Technical measures and quality checks ensure that the active body diode operation does not cause significant drift.

Trench-based SiC MOSFETs can use high threshold voltages and set a new standard for signal-to-noise ratio at current turn-off. This is important for implementing zero gate voltage turn-off at system level, similar to Si MOSFETs. Zero voltage turn-off simplifies gate driving as it does not require negative drive voltage. This reduces component count and complexity, leading to improved system reliability.

However, zero voltage turn-off can cause unwanted parasitic turn-on. Since the MOSFET can be represented as a capacitance divider, any change in drain-source voltage leads to a change in gate-source voltage. If the gate-source voltage is higher than the MOSFET threshold voltage, the MOSFET can be weakly turned on. This increases the turn-on losses.

CoolSiC™ MOSFETs are designed to enhance robustness against parasitic turn-on through the following measures:

- Increased threshold voltage. Infineon’s CoolSiC™ technology sets a new standard in threshold voltage.

- By optimizing the capacitance ratio, unwanted voltage coupled due to drain-source voltage changes is minimized.

Infineon's trench technology combines low on-resistance with short-circuit robustness. CoolSiC™ MOSFETs offer short-circuit withstand times of up to 3 μs, depending on the product configuration. Robust short-circuit performance is a key requirement for drive-type applications. Figure 2 shows the short-circuit behavior of a 1200 V discrete MOSFET. In some cases, additional measures may be necessary to meet the specified short-circuit withstand times. Infineon performs 100 percent production testing of all products prior to shipment with a specified short-circuit withstand time.

■ Synergy between chips and packages

Compared to silicon (Si) chips, SiC-based chips can be made smaller. Infineon has applied sophisticated interconnect technology to maximize the use of materials to mitigate the potential trade-off between thermal performance and power handling capability. CoolSiC™ chip technology thus enables increased thermal performance in discrete and module packaging.

Diffusion soldering technique is used for discrete devices. The biggest advantage of diffusion soldering is that it can greatly reduce the thermal impedance of the device. Conventional die attach technology uses a solder layer between the chip and the package, but the new technique applies a special metal layer to the back of the die to form a mechanical and thermal connection (Figure 3).

Higher power density is achieved by lowering the thermal impedance. The static value (RthJH) is improved by 25 percent, and the thermal impedance (ZthJH) is improved by about 50 percent in 10 ms.

Another advantage of diffusion soldering is that it increases reliability at the application level. Devices can operate at lower temperatures than when using standard soldering for a given total loss, which reduces defects such as bond wire liftoff and therefore achieves higher cycling reliability.

Module packaging is also different from discrete devices. The SiC chip characteristics have shown that flexible pin-based configurations, such as the EasyPACK™ platform developed by Infineon decades ago and which eliminates the baseplate, are well suited.

Infineon's EasyPACK™ modules are highly flexible in topology and internal layout and offer a highly reliable pin rivet connector system with a dense grid array, thus reducing the inductance in the gate loop and power loop. This point is important when mounting modules on multilayer PCBs (Figure 4).

The thermal performance of the module is also important. EasyPACK™ modules express this as the junction-to-heatsink thermal resistance (RthJH). Infineon recently launched an EasyPACK™ module using aluminum nitride (AlN) ceramic. This module has a 40 percent lower RthJH compared to conventional materials. This thermal advantage leads to system-level benefits.

Figure 5 shows a comparison between the existing EasyDUAL™ 11mΩ/1200V half-bridge module and AIN products. It can be seen that AIN products can reduce the junction temperature by up to 20K or alternatively increase the output current by up to 30 percent.

Lower junction temperatures not only increase efficiency but also extend life with respect to power cycling. On the other hand, higher output current enables higher output power ratings.

■ Manufacturing know-how

Infineon’s manufacturing facilities are designed for high-volume, flexible manufacturing of Si and SiC chips, as well as assembly in discrete or module packages. This has been proven through successful volume production of advanced Si device technologies such as CoolMOS™ and TRENCHSTOP™ IGBTs. Since 2001, we have been mass producing commercial SiC devices using the same equipment and quality processes used for silicon power technology.

Stable process and zero-defect strategy play a key role in successful mass production and are highly valued by customers. Infineon has key technologies such as epitaxy growth and advanced packaging in-house. Based on many years of experience in the field, it has developed robust quality assurance procedures to ensure quality not only during the production phase but also in the field. In addition, Infineon recently announced an investment of €2 billion to increase wide bandgap (WBG) production capacity in its plants in Austria and Malaysia as a major step towards making SiC mainstream.

■ Infineon provides SiC products that meet customer needs

SiC is becoming the mainstream power semiconductor material in niche technology. Infineon is achieving excellent chip performance based on its accumulated experience and deep manufacturing know-how in this market. This article explains three aspects that must be considered to achieve high-efficiency SiC design. Infineon provides SiC products that meet customer requirements for reliability, flexibility, and system performance based on device design using pioneering trench technology, synergy between chip and package, and accumulated manufacturing capabilities. This raises expectations for a bright future for SiC.

More information about Infineon's CoolSiC™ technology and CoolSiC™ products can be found here .

※ author
- Eva Gabriel, Senior Manager, Product Marketing SiC
-Dr. Zhihui Yuan, Director, Technical Marketing SiC discretes
- Andre Lenze, Senior Staff Engineer, SiC low- and medium power modules
Infineon Technologies