It is obvious to all that the development speed of SiC power semiconductors in recent years has almost exceeded everyone's expectations. Among them, SIC MOSFETs have received special attention due to their potential to replace existing silicon superjunction (SJ) transistors and IGBT technologies.
Industry rivals and new players are pouring in, doubling down on this emerging market.
In fact, the development of SiC MOSFETs has a long history, as early as 1987, John Palmour, one of the founders of Cree Corporation, the predecessor of Wolfspeed, a global leader in the SiC industry, applied for a structure involving the generation of MOS capacitors on SiC substrates, and this patent was later regarded as the key to the creation of SiC MOSFETs.
However, due to issues such as substrate yield and manufacturing process, it was not until around 2010 that SIC MOSFETs were commercialized.
At that time, CREE introduced the first SiC MOSFET on the market, the CMF20120D with a planar gate structure (according to other accounts, ROHM was the first to introduce the first planar SiC MOSFET in 2010). In 2015, ROHM became the first to mass-produce SiC MOSFETs with trench gate structures, which can make better use of the characteristics of SiC materials and make the process more complex.
After nearly 10 years of development, trench SiC MOSFETs are being considered as a more advantageous technical route and development direction in the technical route of SiC MOSFETs.
Planar or grooved?
In the battle over the technical roadmap of SiC MOSFETs, there have always been two different structural types: plane gate and trench gate.
Both the planar gate and the trench gate are vertically conductive MOSFETs, and the two have similarities in structure, with the source at the top and the drain at the bottom, and the difference between the two is in the gate.
Planar gate SiC MOSFET structure: The gate electrode and the source electrode are distributed in a "plane" on the same horizontal plane, and the channel is parallel to the substrate. The plane grid structure is characterized by simple process, good consistency of elements, and high avalanche energy. However, planar gate SiC MOSFET devices have a large input capacitance due to the presence of the JFET region, which increases the on-state resistance and reduces the current capability of the device.
Trench SiC MOSFET structure: The gate is located below the source to form a "trench" in the semiconductor material, and the channel and gate in the trench gate structure are perpendicular to the substrate, which is also a significant difference from the planar gate structure. Despite the complexity of the process, the unit consistency is worse than that of a planar structure. However, because the trench structure has no JFET effect, has a higher channel density, and has a higher channel mobility on the SiC crystal plane where the channel is located, it is possible to achieve a lower specific on-resistance, a higher current turn-on, and a wider switching speed.
Therefore, the new generation of SiC MOSFETs is mainly researched and adopted with this structure.
SiC power MOSFET device structure.
Relatively speaking, the complexity of the planar gate SiC MOSFET process is not so high, and the development history is relatively long, and related products at home and abroad have achieved mass production earlier, and driven by many car companies such as Tesla and BYD, the planar gate SiC MOSFET power module has entered the main drive inverter since 2018.
However, in the process of reducing the chip size and thus increasing the yield, the lateral topology of the planar gate SiC MOSFET limits how much it can ultimately be reduced.
In contrast, trench SiC MOSFET devices offer the following outstanding advantages due to their trench-gate structure
The conductive channel is changed from horizontal to vertical, which effectively saves the device area and greatly improves the power density.
The trench structure almost eliminates the JFET region, which greatly reduces the input capacitance of the device, improves the switching speed, and reduces the switching loss.
The resistors in the JFET region are also eliminated, and the device's RDSON capability can be further improved to lower currents.
Compared with planar gate SiC MOSFET devices, trench SiC MOSFETs have higher power density, faster switching speed, lower on-resistance and lower losses, so they have attracted great attention from industry companies.
In layman's terms, trench gate SiC MOSFET can be understood as "digging a pit" on the basis of a flat surface, and international SIC manufacturers are using trench gate to maximize the potential of SiC. However, although each is "digging a pit", but the way is slightly different, looking at it, some manufacturers dig a pit, some dig two pits, and some are digging obliquely, various technical structures emerge in endlessly, a hundred flowers bloom.
Schematic diagram of several trench gate SiC MOSFETs in the industry.
To this end, SiC chip manufacturers, especially international manufacturers, are giving full play to their respective skills and have begun to explore trench SiC MOSFETs.
Trench SiC MOSFET, multi-faceted attack.
Among the leading manufacturers of SiC devices, they have basically begun to deploy trench gate MOSFETs.
ROHM and Infineon were the first to move to trench SiC MOSFETs. According to the YOL report, the trench SiC MOSFET camp has expanded from the original ROHM and Infineon to a number of leading manufacturers, such as Sumitomo Electric, Mitsubishi Electric, Denso, Qorvo (UnitedSiC), ST, Wolfspeed, ON Semiconductor, etc., are all transforming from planar MOSFETs to trench structures.
Rohm: Double-grooved structure
In 2015, ROHM developed and mass-produced the world's first SiC MOSFET with a trench structure, and a two-groove structure. Up to now, ROHM's trench SiC MOSFETs have developed into a dual-trench structure with both source and gate trenches.
ROHM dual-trench SiC MOSFET structure.
Source: Rohm).
In a typical single-trench structure, the electric field is concentrated at the bottom of the gate trench, so long-term reliability is always an issue. The dual-trench structure developed by ROHM also has a trench structure in the source region to alleviate the concentration of electric field at the bottom of the gate trench, which successfully reduces the electric field and prevents the destruction of the oxide layer at the gate trench, ensuring long-term reliability and improving the performance of the device.
It is understood that in the fourth-generation SiC MOSFET, ROHM has further improved the dual-trench structure, and successfully reduced the on-resistance by about 40% compared with the third-generation product on the premise of improving the short-circuit withstand time. At the same time, by significantly reducing the gate leakage capacitance, the switching loss was reduced by about 50% compared to the third-generation product.
Comparison of on-resistance and switching losses between the fourth-generation SiC MOSFETs and the third-generation (Source: ROHM).
ROHM**, the fourth-generation SiC MOSFET, will gradually increase its share in its sales mix from 2023 onwards until 2024-2025.
Compared to other competitors who are still challenging for the first series production trench gate products, ROHM is already several positions ahead. According to its product roadmap, the on-resistance of the 5th and 6th generations to be launched in 2025 and 2028 is expected to be reduced by another 30%.
ROHM's SiC MOSFET technology roadmap.
Infineon: Half-pack trench construction
As we all know, "digging holes" is an ancestral craft of Infineon.
In the era of silicon-based products, Infineon's trench IGBTs and trench MOSFETs are unique in the world. With the advent of the SiC era, most of the SiC MOSFETs on the market are planar cells, and Infineon continues the trench structure route.
Schematic diagram of Infineon's half-pack trench SiC MOSFET structure.
In 2017, Infineon reported a trench SiC MOSFET device with a half-side turn-on structure that creates a conductive channel on one side of the gate trench. As can be seen from the image above, the area in the adjoining trench is asymmetrical, the left wall of the trench contains the MOS channel, which is aligned to the A-plane plane to achieve optimal channel mobility, and a large portion of the bottom of the trench is embedded in the P-shaped region below the bottom of the trench.
The structure can protect the corner of the trench from the peak of the electric field, improve the reliability of the device, and further improve the withstand voltage of the device, so that the switching control is good and the dynamic loss is very low. In particular, this feature is essential to suppress additional losses caused by parasitic conduction in topologies using half-bridges.
Infineon's Coolsic MOSFET trench discrete family, which uses Infineon's unique trench approach, brings a number of benefits to its system design, including high reliability, increased efficiency, high switching frequency and high power density, reduced system complexity and total system cost.
Infineon introduced the first generation of the CoolSiC family of SiC MOSFETs in 2016 and updated the second generation in 2022 with 25 to 30 percent higher current carrying capacity compared to the first generation.
In terms of production capacity, Infineon currently reduces the waste of material in the ingot cutting process mainly through its unique "cold cutting" technology, and can obtain twice as many silicon carbide substrates in the same ingot in the future to increase production capacity. On the other hand, Infineon announced last year an investment of more than 2 billion euros in the expansion of its fab in Malaysia specifically for silicon carbide wafers.
STMicroelectronics: Tapping into the potential of the plane and laying out the trenches
According to YOLE statistics, the manufacturer with the highest market share of silicon carbide power devices in the world is STMicroelectronics (ST), and with the cooperation with Tesla, ST's SiC MOSFET products are also the first to be used on a large scale in electric vehicles, since the beginning of the Model3 model, Tesla has been using ST's silicon carbide modules on a large scale.
In terms of chip design, ST continues to explore the technical potential of planar-design SiC MOSFETs, and in 2022 launched the fourth generation of planar-gate SiC MOSFETs. The performance of the 4th generation planar gate silicon carbide has improved compared to the previous generation, including a 15% reduction in on-resistance and a doubling of the operating frequency to 1MHz.
The previously planned trench gate product will be ST's fifth-generation SiC MOSFET, which should be in the R&D stage and is expected to be mass-produced in 2025.
STMicroelectronics' SiC MOSFET roadmap.
Source: ST).
Compared to planar SiC MOSFETs, trench SIC MOSFETs can have smaller on-resistance, lower parasitic capacitance, and stronger switching performance.
In terms of production capacity, ST previously planned to invest US$2.1 billion in fiscal 2022 to expand production capacity, including the expansion of the original 6-inch silicon carbide wafer fab and the Singapore 6-inch silicon carbide wafer fab that will be put into operation in 2022. At the same time, Norstel, a Swedish silicon carbide substrate manufacturer acquired by ST in 2019, has also begun testing 8-inch silicon carbide materials, which are expected to be applied in the 8-inch production line in Singapore around 2025.
ON Semiconductor: Trench products are coming
In the third quarter of 2021, with the approval of the acquisition of GTAT, a substrate manufacturer, onsemi has built a vertical integration model from silicon carbide ingots, substrates, device production to module packaging.
Although the technical strength of some of these projects is still far behind that of leading companies in various fields, their overall strength is more balanced: compared with the substrate leader Wolfspeed, onsemi's module packaging and testing and mass production experience is slightly better; Compared with Infineon, which has excellent device design capabilities, onsemi has an addition from GTAT silicon carbide materials.
From the perspective of product structure, onsemi's first-generation silicon carbide MOSFET technology (M1) adopts a flat design and has a withstand voltage rating of 1200V. Later, 900V and 750V withstand voltage specifications were derived, and the microstructure was changed to a HEX cell design, and the two changes were superimposed to reduce the on-resistance of the SiC MOSFET by about 35%. Most of onsemi's SiC products are based on the M1 and its derived M2 platform.
The latest generation of silicon carbide technology (M3) still uses planar technology, but has been changed to a patent-protected strip cell design, which improves the conduction performance by an additional 16% compared to the previous generation of derivatives. This generation of products will gradually become the company's main automotive silicon carbide platform, covering the mainstream 400V and 800V platforms of electric vehicles in terms of voltage specifications.
It is understood that onsemi's next-generation technology platform M4 will be upgraded from a planar structure to a trench structure. Compared to the original SiC technology, the trench structure of the SiC MOSFET can reduce the chip area by a considerable amount for the same current carrying capacity. Add to this the possibility that the M4 platform will be produced on 8-inch wafers, and the cost of the M4 is expected to be significantly lower than before.
In fact, onsemi has been working on trench gates for many years, and with many samples being tested in-house, it believes that the only problem is that there is a certain risk in terms of reliability from premature introduction of trench gate products. Therefore, onsemi is optimizing reliability and improving the utilization rate of trench grids.
At the same time, in terms of improving reliability, onsemi is also conducting a thorough survey of the trench gate, adding some test points that are considered risky on the basis of standard tests, and trying to clarify the risks.
In addition, from a packaging perspective, onsemi offers a variety of different packaging options, and will also introduce the next generation of packages with strong designs, which can be adapted to different needs through continuous iteration of packaging.
Mitsubishi Electric: Unique electric field limiting structure
In 2019, Mitsubishi Electric also developed a trench SiC MOSFET, and in order to solve the problem of fracture of trench-type gate insulating film at high voltage, Mitsubishi Electric developed a unique electric field limiting structure based on advanced simulation conducted in the structural design stage, which reduces the electric field applied to the gate insulating film to the conventional planar level, so that the gate insulating film can obtain higher reliability at high voltage.
Schematic diagram of Mitsubishi Electric's new trench SiC MOSFET structure.
Source: Mitsubishi Electric).
Mitsubishi Electric ensures device reliability with a unique electric field confinement structure that protects the gate insulating film by injecting aluminum and nitrogen to alter the electrical properties of the semiconductor layer.
Specifically, aluminum is injected in the direction of the vertical trench to form an electric field limiting layer at the bottom of the trench, and then aluminum is injected obliquely through its new technology to form a side grounding connecting the electric field limiting layer and the source, and nitrogen is injected obliquely, and then a high-concentration doped layer that is easier to conduct electricity is formed locally. The electric field limiting layer reduces the electric field applied to the gate insulating film to the level of the traditional planar structure, ensuring the withstand voltage and improving the reliability of the device. The electric field limiting layer and the side of the source are connected to the ground, and high-speed switching operation is realized, reducing switching losses.
Compared with the planar structure, the cell pitch of the trench device is smaller, so the power device can arrange more cells. The high-density arrangement of cells increases the amount of current flowing, but if the spacing between the gates is too small, the path becomes narrower and the current flow is difficult. Nitrogen is injected obliquely to form a highly concentrated doped layer that is easier to conduct electricity locally, so that the current in the current path becomes easy to transmit, thereby reducing the resistance of the current path. The resistivity is reduced by about 25% compared to the unused high-concentration layer.
Wolfspeed: Planar Gate SiC MOSFET benefits are not exhausted
As a company with more than 30 years of experience in the SiC industry, Wolfspeed and its predecessor Cree introduced the first mass-produced silicon carbide substrate in 1991. Wolfspeed's silicon carbide substrate performance and quality are so strong that competitors such as STMicroelectronics, Infineon, and ON have had to spend hundreds of millions of dollars on it. As a result, Wolfspeed's silicon carbide products have gained a crucial first-mover advantage and become the vane of the entire silicon carbide industry.
In terms of design, Wolfspeed's SiC MOSFETs are in their third generation with a planar design, covering multiple voltage specifications between 650V and 1200V. Compared to the previous two generations, the Gen3 planar MOSFETs feature a hexagonal unit cell micro-design with a 16% lower on-resistance than the previous generation of strip cells.
The Wolfspeed Gen3 SiC MOSFETs feature HEX Cell's planar technology (**Wolfspeed).
It is understood that Wolfspeed's next generation product will be a trench gate design, and the Gen4 trench gate MOSFET is still under development, and the specific mass production time has not been disclosed.
Although trench structures are also being laid out, Wolfspeed, which has been working on the development of silicon carbide diodes and MOSFETs from the beginning, believes that the technical advantages of planar gate SiC MOSFETs are far from exhausted.
John Palmour, co-founder of Wolfspeed, once said, "Because trench MOSFETs have better on-resistance, that's a key performance metric. As long as we're far ahead of the trench SiC MOSFETs in terms of on-resistance, I see no reason to change that, and we're going to continue to improve planar SiC MOSFETs. The customer should not care if it is a planar MOSFET or a trench MOSFET, what matters is the specific on-resistance. In fact, we don't care which technical route we have, we only focus on which design will bring the greatest benefit to the customer. ”
In short, there is still room for deep digging in the plane structure, and there is also a market for reliability.
Fuji Electric: Full SiC trench MOSFET
As early as 2016, Fuji Electric developed a 1200V SiC trench MOSFET for all SiC modules, achieving 3A low specific resistance of 5m cm2 with a threshold voltage of 5V while maintaining high reliability as a "channel" for opening and closing currents.
As a result, the resistivity has been reduced by more than 50% compared to the conventional planar structure. In addition, Fuji Electric has developed a high-current-density dedicated SiC module with a unique pin connection structure that makes full use of the advantages of SiC devices. Fuji Electric has implemented an ALL-SIC module using this device.
Sumitomo Japan: V-shaped groove
In 2016, Sumitomo developed a sample of a V-trench SiC MOSFET device with a thick bottom oxide layer, which further improved the gate oxygen reliability and threshold stability of the device.
Cross-sectional view of Sumitomo Electric's SiC VMOSFET.
Source: Sumitomo Electric).
Sumitomo Electric has developed a new V-slot trench MOSFET using a unique crystal plane. V-MOSFETs have excellent characteristics such as high efficiency, high blocking voltage, and high stability in harsh environments, and realize high current (200A on a single chip), making them suitable for EVs and HEVs. In addition, Sumitomo Electric is collaborating with the National Institute of Advanced Industrial Science and Technology to develop next-generation V-MOSFETs with the world's lowest on-resistance.
Denso: U-shaped groove
In March 2023, Denso announced that it had developed the first inverter using SiC semiconductors.
Among them, Denso's unique trench-type MOS structure uses its patented electric field mitigation technology and trench gate semiconductor devices to improve the output of each chip because they reduce power losses caused by heat generation, and the unique structure realizes high-voltage and low-on-resistance operation.
Denso's trench gate structure (Source: Denso).
According to some information, Denso is similar to Sumitomo's groove structure, but it has been changed to a U-shaped groove.
Source: Songge Power.
Qorvo: High-density trench SiC JFET structure
Qorvo's SiC technology is based on the acquisition of UnitedSiC in 2021, and now SiC is a top priority for Qorvo's future development.
It is understood that unlike the traditional SiC MOSFET design, Qorvo has a new approach, its SiC MOSFET adopts a high-density trench SiC JFET structure, and the channel resistance Rchannel in the SiC MOSFET is replaced by the resistance of the low-voltage silicon MOSFET in the SIC FET, which has a much better inverted layer electron mobility, achieving ultra-low on-resistance per unit area, so the loss is also lower. The structure is co-packaged with low-voltage SI MOSFETs, and the SIC FETs also have a relatively small die area.
Comparison of SiC MOSFET (left) and Qorvo's SiC FET (right) architecture (Source: Qorvo).
Qorvo has expanded its 1200V product portfolio to bring its breakthrough SIC FET technology to higher voltage applications, ranging from 23M to 70M, targeting applications such as 800V EV on-board chargers (OBC) and DC converters.
Renesas: Variant two-stage trench MOSFETs
It is understood that Renesas Electronics has just applied for a patent in 2023 and is ready to study the silicon carbide trench structure, abbreviation"Periodic Connection, Variation Two-Stage Trench MOSFETs".
Source: Silicon carbide chip study notes.
Write at the end. In summary, several important metrics to improve the performance of SiC MOSFETs, including smaller cell pitch, lower specific conduction resistance, lower switching losses, and better gate-oxygen protection, almost all point to the trench gate structure.
From the perspective of the industry as a whole, the current mass production of trench SIC MOSFETs is mainly carried out by international SIC manufacturers such as Europe, the United States and Japan. From the perspective of the layout of international manufacturers, trench gate SiC MOSFETs will be a more competitive solution in the future.
Nearly 9 years have passed since the launch of the first mass-produced trench gate SiC MOSFET product in 2015, and many companies are developing trench gate products, but there are not many manufacturers on the market that can launch mass-produced products.
Of course, the design and manufacture of high-performance trench gate SiC MOSFETs is also a top priority for the development of domestic SiC power devices, and some companies have shifted the focus of research to trench gate SiC MOSFETs. However, it should be noted that international SiC giants have been in the field of SiC MOSFET for many years and have accumulated a lot of patents. The high patent barrier of the groove structure is also a hurdle for domestic manufacturers to overcome.
According to the author of "Silicon Carbide Chip Study Notes": "The complete set of technology and structure IP of trench SiC MOSFET is the ticket to the silicon carbide competition in the next ten years!" "In the current period of sustained and rapid growth of the overall SiC market, it is necessary to lay out the appropriate technical route in advance to have the opportunity to take the lead in the new application market in the future.
References: 1] Summary of the 2023 Trench Year – Trench MOSFET Development Roadmap of 13 SiC Companies
2] Flat type or trench type, who has the right to speak in the industry? The future of silicon carbide (SiC) MOSFETs.
3] No war on the Western Front, the smoke of the silicon carbide five.
The science and technology of the great powers are in