Mitsubishi Electric and Coherent Joint Development of 8-inch SiC Substrates

Transition to 8-inch Reshapes the Economics of SiC

Mitsubishi Electric and Coherent, a major player in optical components and semiconductor materials, have agreed to jointly develop 8-inch (200mm) SiC wafers. The transition from the current mainstream 6-inch (150mm) to 8-inch is more than just an increase in diameter. It marks a turning point where the cost structure of manufacturing fundamentally changes due to an increased number of chips per wafer.

Cost has been one of the biggest hurdles hindering the widespread adoption of SiC power semiconductors. It is widely recognized across the industry that reducing wafer costs is essential to further deploy SiC, which remains more expensive than silicon (Si) devices, in the EV, industrial equipment, and renewable energy markets. The move to 8-inch is one solution that various companies are competitively pursuing. So, what position does the Mitsubishi Electric and Coherent partnership hold within this competition?

Why is "8-inch" Garnering So Much Attention?

When the wafer diameter expands from 6 inches to 8 inches, the area increases by approximately 1.78 times. If the chip size remains the same, the number of chips obtainable from a single wafer will also increase by roughly the same ratio. Since fixed manufacturing costs do not change significantly, this translates to a lower wafer cost per chip.

This chart illustrates that the transition from 6-inch to 8-inch results in an approximately 77% increase in wafer area. Even considering factors like yield and edge losses, the potential for cost reduction per chip is substantial, fundamentally altering the premise of procurement cost negotiations.

In the world of silicon semiconductors, 300mm (12-inch) wafers have long been the standard. However, SiC's challenging crystal growth process has made it difficult to achieve larger diameters. Even in the early 2020s, 6-inch wafers were the center of mass production, with 8-inch wafers being the "next step" that companies are rushing to develop. The timing and quality of this transition will significantly impact the competitiveness of device manufacturers.

Coherent and Mitsubishi Electric—What This Partnership Means

Coherent, originally known for lasers and optical equipment, became the current entity when II-VI acquired Coherent (the former company) in 2022. The present company possesses a semiconductor materials business, including SiC wafers. While Wolfspeed has maintained the top market share in SiC wafers for many years, Coherent is also increasing its presence as a key supplier and has shown an aggressive stance in the development of 8-inch wafers.

Mitsubishi Electric has a long history in the development and manufacturing of SiC power semiconductors, with a proven track record in inverters, railways, and industrial equipment. While collaborations between device manufacturers and wafer suppliers are becoming more common in recent years, this particular partnership is noteworthy due to Mitsubishi Electric's unique design technology for trench SiC-MOSFETs.

Mitsubishi Electric has demonstrated an improvement in short-circuit withstand time (SCWT) by introducing a p-type protective layer in trench SiC-MOSFETs. The precision of such device structures implies a high demand for wafer quality. 8-inch wafers tend to be more challenging to manage regarding crystal defect density compared to 6-inch wafers, making it reasonable for device manufacturers to be involved from the early stages of wafer development. This approach, which involves collaboratively defining specifications with an eye on device characteristics rather than simply "buying large-diameter wafers," is believed to ensure yield and reliability.

How it Differs from Competitors' Approaches

Mitsubishi Electric and Coherent are not the only ones moving towards the development and mass production of 8-inch SiC wafers. Wolfspeed operates a vertically integrated model, handling everything from wafer manufacturing to devices, and is progressing with the mass production transition to 8-inch. STMicroelectronics, while having a wafer supply agreement with Wolfspeed, is also advancing its in-house SiC manufacturing. onsemi is implementing a procurement strategy that includes capital investment in wafer manufacturers.

This comparison reveals that in the transition to 8-inch, the strategic divergence lies not only in "who to buy wafers from" but also in "at what timing and to what depth to engage in development." The joint development between Mitsubishi Electric and Coherent can be clearly positioned as a prime example of the latter.

However, this approach also presents challenges. Joint development requires time and investment, posing a risk of delaying the transition to mass production compared to competitors. The key focus going forward will be how quickly they can catch up to players like Wolfspeed, who possess their own mass production lines.

What Changes from the Perspective of SiC Device Selection?

While the discussion around 8-inch wafers may seem like a market and business affair, it also provides insights relevant to both design and procurement.

When selecting SiC devices, specifications such as on-resistance (Ron), breakdown voltage, and short-circuit withstand time (SCWT) serve as basic evaluation criteria. SCWT is particularly important, indicating the time until the device fails during a load short circuit and serving as a grace period for protective circuits to operate.

Regarding the impact of the 8-inch transition on device quality, attention should be paid to crystal defect density and the variation in device characteristics. As wafer diameter increases, managing variations across the wafer surface tends to become more difficult. SiC, in particular, is more challenging to grow crystals than silicon, and dislocation defects (micro-pipes, stacking faults, etc.) directly affect device characteristics. One of the intentions behind Mitsubishi Electric's joint development with Coherent is likely to define these quality requirements from the wafer development stage.

Furthermore, the trade-off between short-circuit withstand time and on-resistance is not unrelated to wafer quality.

A reduction in wafer defect density can make it easier to achieve the required current carrying capacity even with a smaller chip area. This directly leads to an improvement in RonA (on-resistance × area) and may contribute to mitigating the trade-off with short-circuit withstand time. Cost reduction through the transition to 8-inch and improvement of device characteristics through enhanced wafer quality are inextricably linked.

How to Interpret the Impact on Procurement and Development Plans

It will take time for this joint development announcement to translate into actual mass production shipments. Mass production of 8-inch SiC wafers is still in its nascent stages across the industry, and 6-inch wafers will remain dominant through 2025-2026. From the perspective of procuring and adopting Mitsubishi Electric devices, it is necessary to consider medium-term changes in the supply system in addition to short-term specification comparisons.

Specifically, several points will serve as decision-making criteria. First, whether the device part numbers after the 8-inch transition will be compatible with current ones. Changes in the manufacturing process may alter chip size and characteristics, so it is important to confirm the necessity of design changes early on. Second, the duration of supply for 6-inch products during the transition period. Whether to adopt a multi-supplier approach or wait for the switch to 8-inch with a single supplier will depend on the mass production schedule of the application.

From the perspective of the overall supply structure of the SiC device market, the coexistence of Wolfspeed's fully integrated model and joint development models like Mitsubishi Electric + Coherent can be viewed as leading to diversification of procurement sources. A reduction in dependence on specific wafer suppliers is expected to impact medium-to-long-term price negotiation power and supply stability.

While this announcement is currently at the agreement stage of joint development, details on mass production timing and performance targets have not yet been disclosed. However, the strategic differences in "how to approach the 8-inch transition" may result in clear differences in device lineups and price ranges in two to three years. This announcement is valuable to track as context for observing which direction these differences will lead.