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In an increasingly energy-conscious world, demand is booming for high-power applications with superior efficiency and power density. As silicon approaches its physical limits, the semiconductor industry is exploring wide-bandgap (WBG) materials, notably silicon carbide (SiC), gallium nitride (GaN) and, a bit further into the future, diamond.
Tesla kicked off the SiC power device market in 2018, when it became the first automaker to use SiC MOSFETs in its Model 3. Over the years, Tesla has been a major contributor to the growth of the SiC market. But the industry went into a panic after Tesla announced its next-gen powertrain would use a permanent-magnet motor that would reduce its SiC use by 75%.
“Silicon carbide is an amazing semiconductor, but it’s also expensive, and it’s really hard to scale. So using less of it is a big win for us,” Colin Campbell, Tesla’s vice president for powertrain engineering, said on March 1 at Tesla’s 2023 Investor Day event.
A 75% reduction may seem concerning, but that figure does not tell the whole story. Yole Intelligence indicated in mid-March that the SiC devices market is expected to grow at a CAGR of more than 30% to reach beyond $6 billion in 2027, with automotive expected to represent around 80% of the market.
“We estimate that there will be no disruptive power electronics technology coming to the market in the next few years, since the last major disruption—the emergence of SiC and GaN—is still playing out and is grabbing a share of the traditional silicon market,” Ana Villamor, team lead analyst for power electronics activities within the Power and Wireless division at Yole Intelligence, said. “Nevertheless, several technologies are being developed that will come to fruition in the long-term, such as diamond, SiC IGBTs and gallium oxide.”
SiC and GaN have a bright future, but diamond could well prove to be the ultimate WBG semiconductor material for applications in high-power electronics.
“Diamond is a perfect candidate for high-voltage operation, high-temperature applications or high-frequency switching,” said Khaled Driche, co-founder and CTO of Diamfab. “Diamond exhibits a critical electric field 30× higher than Si and 3× higher than SiC. It is also a very good heat dissipator, with a thermal conductivity 5× higher than copper.”
However, diamond is a semiconductor like no other. “What makes its strength is also an obstacle to synthesizing it, and this is where Diamfab comes in,” said Diamfab CEO and co-founder Gauthier Chicot.
Diamfab, which is based in Grenoble, France, is a spin-off of the French National Center for Scientific Research (CNRS) and is building on 30 years of research into high-quality synthetic diamond growth by the Institut Néél-CNRS wide-bandgap semiconductor team (SC2G). The Diamfab project was founded in 2016, and the startup was incorporated in 2019.
“We synthesize and dope diamond epitaxial layers with a unique control; therefore, stacks of diamond doped layers are grown to form a high-value-added wafer ready for device fabrication,” Driche said.
Among all the industrial processes required to manufacture a diamond device, the growth of the epitaxial layer is one of the most critical because most of the electrical performance depends on the quality of these active layers.
Potential applications span from electric vehicles (EVs) with diamond-based power electronics to IoT devices with 20-year battery life, healthcare devices with a range of integrated detectors and autonomous vehicles with ultra-precise quantum sensors.
Improved power efficiency
Diamond offers three key advantages over existing semiconducting materials: thermal management, cost/efficiency optimization and CO2 reduction.
The cooling system is a heavy and bulky part in all traditional power converters. Unlike most semiconductors, diamond has a decreasing resistivity with increasing temperature. Therefore, devices made from the material perform better at 150 degrees Celsius—the typical operating temperature for power devices—than at room temperature. Whereas considerable effort must be expended to cool Si or SiC devices exposed to high operating temperatures, it is possible to simply let the diamond find a stable state during operation, Driche said.
Diamond is also a good heat dissipator. With fewer losses to dissipate, better heat dissipation capability and the ability to operate at high temperature, converters made from active devices in diamond can be 5× lighter and smaller than Si-based solutions and 3× lighter and smaller than those based on SiC, Driche added.
When designing devices and converters, a tradeoff must be found between a system’s energy efficiency and its cost, size and weight. Diamond is no exception to the tradeoff, but Diamfab is convinced diamond can bring value on key parameters for more power-efficient e-mobility.
If the focus is on reducing device cost, it is possible to design a diamond die that is 30% less expensive than a SiC die, because a diamond die consumes up to 50× less diamond area than the equivalent SiC die for the same electrical performance and efficiency, but with better thermal management.
If the focus is on efficiency, diamond can cut energy loss by a factor of three compared with SiC, with up to a 4× smaller die, allowing a direct savings in energy consumption.
If the focus is on system volume and weight, by allowing an increase in the switching frequency, diamond devices could decrease passive components’ volume by a factor of four compared with SiC-based converters. This volume reduction is added to the one allowed by a smaller heat dissipator.
Diamond in every EV
E-mobility is a priority segment for Diamfab, and it has recently filed a patent on an all-diamond capacitor for EVs. When asked for details, Driche said the idea of the all-diamond capacitor emerged when an industrial capacitor manufacturer said it was looking for a passive-component solution to protect SiC- and GaN-based active components like diodes and transistors, as the active devices were being subjected to voltage peaks that were higher than they could withstand (>1,500 V).
“One of the biggest advantages of diamond is its ability to operate at high temperatures. Therefore, the all-diamond capacitor can be placed closer to other components—something impossible with conventional capacitors—and we can reduce parasitic inductance with diamond capacitors,” Driche said. Here, diamond is used as an insulator and a conductor.
Diamfab said it expects to see diamond in all EVs within a decade, much as its Grenoble-based neighbor Soitec has driven the entrenchment of silicon-on-insulator in every smartphone.
Dual business model
In the value chain, Diamfab operates at the interface between materials and devices. It has defined a dual business model under which it will sell its technology both directly and through application-oriented strategic partnerships and alliances.
First, Diamfab expects to sell high-value-added diamond wafers and manufacturing processes for diamond components to integrated device manufacturers. Second, Diamfab intends to sell high-performance diamond devices directly to end users through a co-development approach.
“Substrate wafers are the basis we use to add our technology—namely diamond layers with conductive properties—which we call high-value-added wafers,” Chicot said. “We are bringing value to a non-electronic diamond wafer and making it ready to be processed in the foundry.”
Diamfab is currently working with partners on the development of high-performance devices, including diodes, transistors, capacitors, quantum sensors and high-energy detectors.
The prospects for diamond are promising in view of the increasing electrification of society, but many hurdles must still be overcome for the technology to become an industrial reality.
Advances in synthesis have made it possible to produce engineered synthetic diamonds with predictable properties and consistent performance. The first synthetic diamonds were produced in the 1950s, using high pressure and high temperature. In the 1980s, wafer-scale diamonds were produced using chemical vapor deposition (CVD).
“The technical progress made in recent years on the CVD synthesis technique is considerably accelerating the development of the technology, and the age of diamond has never been so close,” Chicot said. “The recent demonstration of large wafers, up to 4 inches, and the growing interest of many R&D centers and now industrial partners to develop diodes, transistors and capacitors testify to this.”
Expanding the wafer diameter from 0.5 inch to 4 inches could allow Diamfab to achieve the competitiveness required for the automotive market.
As for other roadblocks, the startup believes dislocation reduction could improve component manufacturing yields. Diamfab is also exploring several avenues to achieve a vertical component architecture in order to increase current density.
Diamfab is in talks with potential investors to achieve its technical and business objectives, the company acknowledged. A first seed round of €3 million (about $3.3 million dollars) in equity is under way to allow the installation of a pilot line plant, which would make it possible to develop reproducible products ready to be processed and sent to mass production, Chicot said.
Asked when Diamfab expects to enter mass production, the CEO said, “Today, we are able to produce high-value-added diamond wafers for public and private research laboratories. After the acquisition of our machines and our pilot site, we will be able to continue producing by increasing the volume and our turnover. We are aiming to increase our production volume over the next five to seven years, primarily by responding to the electric car market.”
Improving the energy efficiency of an EV means reducing the energy consumption, but this should not be done at the expense of an energy-intensive and polluting manufacturing process. Driche said the production of diamond wafers is “up to 20 times less CO2 emitting” than the production of SiC wafers.
“SiC requires very high temperatures, up to 2,700 degrees Celsius, for several days, while the CVD technique used to synthesize diamond wafers is done with temperatures 3 times lower,” he said. “Considering the lower [surface area requirement for] the diamond material, and related to an EV, the CO2 footprint of the semiconductor material for power components could be divided by a factor of up to 1,000 with diamond.”