Silicon carbide delivers big improvements in power electronics

By Maurizio Di Paolo Emilio, contributing editor

The widespread adoption of wide-bandgap (WBG) semiconductor technologies continues to grow in the power electronics industry. Silicon carbide (SiC) and gallium nitride (GaN) semiconductor materials show superior properties, allowing for potential operation of power devices at high voltages but especially at high temperatures and switching frequency compared to conventional silicon technology. Designers of power electronics systems are working to take full advantage of GaN and SiC devices.

SiC is being adopted in several applications, particularly e-mobility, to meet the energy and cost challenges in the development of high-efficiency and high-power devices. Silicon has been used as the key semiconductor material for a majority of electronics applications, but now, it is considered inefficient compared to SiC.

SiC, consisting of pure silicon and carbon, has three primary advantages over silicon: a higher critical avalanche breakdown field, higher thermal conductivity, and wider bandgap. SiC has a wide bandgap of three electron volts (eV) and can withstand a voltage gradient over 8× greater than silicon without undergoing an avalanche breakdown. The wider bandgap leads to lower leakage current at high temperatures, thus resulting in good efficiency. The higher thermal conductivity corresponds to higher current density.

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The higher electric field strength of the SiC substrate permits the use of thinner base structures. This makes it possible to achieve one-tenth of the thickness compared to epitaxial silicon layers. Furthermore, the doping concentrations of SiC are 2× higher than their silicon counterparts. Therefore, the surface resistance of the component is lowered and conduction losses are significantly reduced.

SiC technology is now widely recognized as a reliable alternative to silicon. Many manufacturers of power modules and power inverters have laid the foundations in their roadmaps for future products. This WBG technology offers unprecedented energy efficiency by drastically reducing both switching and conduction losses under specific loads while also offering improved thermal management.

In power electronics systems, thermal design plays a crucial role in ensuring high energy density and shrinking circuit size. In these applications, SiC is an ideal semiconductor material because its thermal conductivity is almost 3× higher than silicon semiconductors.

SiC technology is suitable for higher-power projects such as motors, electric drives, and inverters. Electric drive manufacturers are developing new drive circuits to meet the demand for higher switching frequencies in converters and are reducing electromagnetic interference (EMI) by adopting more sophisticated topologies.

SiC devices require fewer external components with more reliable system layouts and lower costs for manufacturers. The higher efficiency, smaller form factor, and lower weight of SiC enable smart design with reduced cooling requirements.

Applications
Recently, several car makers have developed new propulsion concepts that have enabled them to bring the first hybrid and electric vehicle models to the market. In these vehicles, there are new components and systems such as frequency converters to power the engine (up to 300 kW), on-board battery chargers from 3.6 W to 22 kW, induction chargers (wireless charging) from 3.6 kW to 22 kW, DC/DC converters up to 5 kW, and inverters for auxiliary loads such as air conditioning and power steering.

The new high-voltage batteries represent one of the major obstacles to the adoption of hybrid and electric vehicles. With SiC, car manufacturers can shrink the size of the battery while reducing the total cost of an electric vehicle.

In addition, thanks to the thermal performance of SiC, manufacturers can also reduce the cost of cooling powertrain components. This has a positive impact on the weight and cost of electric vehicles.

The on-board chargers contain various power conversion elements such as diodes and MOSFETs. The goal is to integrate them all by miniaturizing the power electronics through the use of small passive components. This is possible if the semiconductors used can be controlled in the same circuit with a high switching frequency. However, the thermal property of silicon limits the high switching frequency solutions. SiC MOSFETs, on the other hand, offer an ideal solution for this type of application (Fig. 1 ).

Fig. 1: 3-kW EV on-board charger (Image: GaN Systems Inc.)

Long-term reliability is already a hallmark of SiC MOSFETs. The next step for power semiconductor manufacturers is to address the development of multi-chip power modules, or hybrid modules, which integrate a conventional silicon transistor and a SiC diode on the same physical device. These modules can operate at higher temperatures by providing a high breakdown voltage. They promise high-efficiency operation and a further downsizing of the equipment.

At current market prices, SiC MOSFETs offer system-level benefits over silicon IGBTs, and we expect SiC MOSFET pricing to continue to decrease as 150-mm wafer-based manufacturing is widely adopted. Some manufacturers are already moving to 200-mm (8-inch) wafers. As wafer sizes increase, the cost per die decreases, but the yield may also decrease. Therefore, processes must be continually improved (Fig. 2 ).

Fig. 2: Silicon carbide products target applications that deliver improvements in efficiency, reliability, and thermal management. (Image: Littelfuse Inc.)

The biggest challenge is the widespread adoption of SiC devices due to higher manufacturing process cost and a lack of volume production. Mass production of SiC devices imposes challenges that require a robust and well-thought-out infrastructure and manufacturing processes. This includes wafer testing, which requires the test of smaller devices that work at higher current and voltage ranges.

Once these challenges are resolved, OEM designers will ramp up the adoption of SiC devices to leverage the technology’s electrical characteristics that enable a significant reduction in system costs and an increase in overall efficiency. Electric vehicles with on-board charging units and power inverters are prime candidates for SiC semiconductor technology.

This article was originally published at sister publication Power Electronics News.