Vast bandgap semiconductors have taken each energy electronics and excessive frequency circuits by storm, changing so many purposes that had been beforehand dominated by silicon-based units, e.g., LDMOS HPAs in base stations, IGBTs in excessive voltage DC/DC conversion and so on. Particularly inside energy electronics, it’s no secret that sure purposes are demanding energy dense options that function at excessive switching frequencies to reduce switching losses. From traction inverters, onboard chargers, and excessive voltage DC-DC converters in EVs to uninterruptible energy provides (UPSs) and solar energy converters in industrial/industrial purposes; WBG semiconductors have carved out an in depth area of interest for a lot of subsequent era electronics. 

The SiC substrate has established itself for EV and a few industrial purposes. Nonetheless, a bit extra lately GaN has surfaced as a powerful choice for a lot of overlapping purposes. Understanding the key variations between these substrates within the context of excessive energy circuits and their respective manufacturing concerns may make clear the way forward for these two widespread compound semiconductors. 

WBG advantages

WBG supplies are inherently capable of function at increased switching frequencies and with increased electrical fields than the standard Si substrate. When a semiconductor is heated up, its resistance tends to go down attributable to thermally excited carriers which can be extra ample at increased temperatures, inflicting conduction. Increased bandgap semiconductors would require increased temperatures (extra power) to excite electrons throughout the bandgap from the valence band to the conduction band. This interprets on to extra energy dealing with capabilities and better machine efficiencies. 

This may be seen in Desk 1 the place SiC and GaN exhibit a lot increased breakdown electrical area, electron mobility, saturation velocity, and thermal conductivity than Si—all components that improve switching frequency and energy density. Nonetheless, excessive switching frequencies will result in extra losses and a decrease effectivity FET, that is the place optimizing the facility machine determine of advantage (FoM) [R_ds(on) x Q_g], or optimizing the channel resistance and gate cost for decrease conduction and switching losses, is essential.  

Desk 1 Properties of Si, SiC, and GaN.

Typically, GaN FETs max out at round 650-V with energy purposes round 10 kW whereas 750-V and 1200-V SiC FETs are usually not uncommon and purposes can vary from 1 kW as much as the megawatts (Determine 1). SiC’s glorious thermal conductivity permits for comparable energy rankings in considerably smaller packages. Nonetheless, GaN units are capable of change quicker (be aware the considerably increased electron mobility) which, in flip, can translate to a better dV/dt, doubtlessly permitting for extra converter effectivity. 

Determine 1: Energy versus frequency plot for numerous energy units. Supply: Texas Devices

Manufacturing concerns

SiC, the latest golden little one of energy electronics, gained large traction after Tesla’s announcement utilizing completely SiC within the Mannequin 3 again in March of final 12 months. Since SiC MOSFETs had been commercialized by Cree in 2010, the demand for SiC has steadily ramped up with key gamers profiting from out there tax credit from the CHIPS act to develop operations and drive down the fee per wafer. Wolfspeed (previously Cree), as an example, lately invested a complete of $5 billion in a brand new manufacturing facility, the John Palmour (JP) manufacturing middle to develop 200 mm (~8-inch) wafers. 

Nonetheless, it isn’t that easy: getting a foothold in SiC fabrication requires costly tools that’s completely used for SiC. SiC boules are grown  at  temperatures in extra of 2700℃ at a fee not less than 200 occasions slower than Si, which requires a considerable amount of power. GaN alternatively can largely use the identical tools as Si semiconductor processing the place GaN epitaxial wafers might be grown on its respective substrate (typically Si, SiC, or sapphire) at a temperature of 1000 to 1200℃—lower than half that of SiC. SiC wafers are additionally almost 50% thinner than Si wafers (as much as 500 μm), resulting in a reasonably brittle materials that’s vulnerable to cracking and chipping—one other high quality that requires specialised processing tools. 

In keeping with Gregg Lowe, CEO at Wolfspeed, 6-inch SiC wafers price ~$3,000 in 2018, a value that has been trimmed all the way down to ~$850 for a 7-inch wafer simply 6 years later in 2024. And, as SiC energy units proceed to mature, prices per wafer will proceed to go down. A significant leap in optimizing prices are rising wafer sizes and growing the variety of units per wafer. For GaN-on-Si, that is comparatively easy, bigger diameter fabs can produce hundreds of 8-inch wafers per week with glorious line yields (98%) afforded by CMOS course of management. Nonetheless, comparable economies of scale might be utilized to SiC wafer manufacturing as firms now advance towards 8-inch wafers the place simply ten years in the past, mass manufacturing of 150 mm (~6-inch) wafers had been actually simply on the horizon. And, whereas the SiC units themselves could also be dearer than Si and GaN counterparts, the actual fact is, far much less energy units are required to take care of the identical efficiency. On the system degree, this implies much less gate drivers, magnetics, and different peripherals units which may in any other case be utilized in an Si-based design. 

GaN transferring past 700 V

Due to its glorious excessive frequency traits, GaN has already established itself as an acceptable III-V semiconductor for prime frequency circuits resembling MMICs, hybrid microwave circuits, and so on., together with different compound semiconductors resembling gallium arsenide (GaAs) and indium phosphide (InP). GaN is especially related for prime energy amplifiers (HPAs) within the transmit sign chain. Most of the GaN foundry providers presently out there usually deal with excessive frequency purposes with GaN-on-SiC nonetheless, extra lately, foundries are shifting their focus in the direction of GaN-on-Si for each RF and energy purposes. Desk 2 highlights a number of the GaN course of applied sciences for various firms globally. Notice the desk doesn’t embrace all GaN foundries resembling World Foundries or UMC which can probably be main contenders in Gan-on-Si applied sciences.

Desk 2: Choose GaN foundries and specs on their expertise.  

SiC and GaN serve very distinct components of the facility spectrum, nonetheless, can increased voltage GaN units be designed to creep up the spectrum and take care of SiC? The GaN pHEMTs that dominate GaN fabrication have breakdown voltages (~0.6 to 1.5 MV/cm) that usually cap out at round 650 V because of the inherent limits of its essential breakdown area [1-2]. Strategies of reaching the intrinsic limits of three MV/cm are being explored in analysis so as to enhance the breakdown traits of GaN units. 

Increasingly producers are showcasing their 700-V GaN options. There have been talks of a 1200 V GaN FET; Transphorm launched a digital design of their 1200 V GaN-on-Sapphire FET in Might of final 12 months. Outdoors of this a lot of the speak of GaN transferring up the facility spectrum has remained within the R&D area. 1200-V Vertical GaN (GaN-on-GaN) transistors are additionally being researched by NexGen Energy Programs with their Fin-JFET expertise [3], a hit that has allowed the corporate to obtain funding from the US division of power (DOE) to develop GaN-based electrical drive techniques. Nonetheless, many of those options are usually not GaN-on-Si.

GaN-on-Si merely might need the key benefit of bandwagoning on the silicon semiconductor business’s already established expertise maturity, nonetheless, utilizing the Si substrate comes with some design challenges. There are two main constraints: a big lattice mismatch and an excellent bigger thermal mismatch between the GaN epitaxial layer and the host substrate inflicting tensile and compressive strains on the 2 substrates leading to dislocations and better defect densities (Desk 3). Different substrates are being researched to beat this challenge, Qromis, as an example, has lately engineered a ceramic poly-aluminum nitride (AlN) layer that’s CMOS fab suitable and CTE-matched to GaN. 

Desk 3  Lattice and thermal mismatch between GaN and Si, sapphire, and SiC. Supply: [4] 

Entry to Gallium

Whereas GaN wafers are usually extra handy to fabricate, they do require a valuable steel that’s, by nature, in restricted provide. There was pressure on the gallium provide with the 2019 tariffs on Chinese language imports ratcheted up considerably inflicting a 300% improve in gallium steel imported from China in comparison with 2018 the place the excess was probably stockpiled. China’s restrictions on gallium exports in August of final 12 months additional diminished the already small quantity imported from China. The bans might have doubtlessly signaled an issue as China produces almost 98% of the world’s low-purity gallium. 

Nonetheless, the difficulty has not actually disrupted gallium-based wafer manufacturing (GaAs or GaN), largely because of the stockpiling and shifting to different sources for the uncommon steel (Desk 4). Many international locations now have the inducement to scale up the operations that, over a decade in the past, had been shut down attributable to China’s overproduction. Nonetheless, this can be one thing to think about if China additional restricts its exports within the brief time period. It might even be necessary to notice that since GaN wafers are produced by rising GaN crystals on high of quite a lot of substrates, comparatively small quantities of gallium are used per machine as in comparison with GaAs pHEMTs which can be grown on semi-insulating GaAs wafers. So, whereas this will likely have been one thing to think about given the latest historical past of restricted gallium provides, it has not likely impacted GaN manufacturing and sure received’t sooner or later.

Desk 4: US imports of unrefined gallium by nation or locality in response to USGS [5].

SiC and GaN

Because it stands SiC and GaN dominate distinct components of the facility spectrum and due to this fact distinct purposes with just some overlap. Nonetheless, if GaN FETs can efficiently improve in drain-source voltage with out stifling its present large manufacturing benefit, it might very effectively get away of its present place largely in shopper electronics (e.g., USB chargers, AC adapters, and so on.) into increased energy purposes that SiC energy units presently dominate. SiC manufacturing has not stagnated although, and regular progress is being made in wafer measurement and yield to drive down the price of SiC. 

Aalyia Shaukat, affiliate editor at EDN, has labored within the design publishing business for seven years. She holds a Bachelor’s diploma in electrical engineering, and has revealed works in main EE journals.

Associated Content material

• Silicon carbide (SiC) counterviews at APEC 2024
• APEC 2023: SiC transferring into mainstream, price main barrier
• GaN Energy Gadgets: Challenges and Enhancements
• Deciding on GaN or SiC units with a Deal with Reliability

References

1. Tian Z, Ji X, Yang D, Liu P. Analysis Progress in Breakdown Enhancement for GaN-Primarily based Excessive-Electron-Mobility Transistors. Electronics. 2023; 12(21):4435. https://doi.org/10.3390/electronics12214435
2. Exploring an Method towards the Intrinsic Limits of GaN Electronics. Sheng Jiang, Yuefei Cai, Peng Feng, Shuoheng Shen, Xuanming Zhao, Peter Fletcher, Volkan Esendag, Kean-Boon Lee, and Tao Wang. ACS Utilized Supplies & Interfaces 2020 12 (11), 12949-12954. DOI: 10.1021/acsami.9b19697
3. R. Zhang et al., “Vertical GaN Fin JFET: A Energy System with Brief Circuit Robustness at Avalanche Breakdown Voltage,” 2022 IEEE Worldwide Reliability Physics Symposium (IRPS), Dallas, TX, USA, 2022, pp. 1-8, doi: 10.1109/IRPS48227.2022.9764569.
4. Kaminski, Nando, and Oliver Hilt. “SiC and GaN Gadgets – Vast Bandgap Is Not All of the Similar.” IET Circuits, Gadgets & Programs, vol. 8, no. 3, 2014, pp. 227-236. https://doi.org/10.1049/iet-cds.2013.0223. 
5. “Gallium Statistics and Info.” U.S. Geological Survey, [last modified August 29, 2023], usgs.gov/facilities/national-minerals-information-center/gallium-statistics-and-information.  [accessed on  2023-10-26].

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