GaN on SiC: The Optimal Solution for 5G

Executive Summary

End users have a seemingly insatiable appetite for data. Cisco’s annual Visual Networking Index predicts global IP traffic will more than triple between today and 2022, reaching 4.8 zettabytes per year by 2022, with traffic from wireless and mobile devices accounting for 71 percent of that total.

To deliver the bandwidth needed to meet this demand, the wireless industry is moving from today’s 4G networks towards the next generation of network that promises faster data speeds and reduced latency: 5G. As part of the process, however, mobile network operators know they must provide the best customer experience possible by offering high-quality, ptra-reliable services while containing both capital and operation expenditures. As such, they need to utilize infrastructure and technologies that deliver on three key requirements: performance, efficiency and value.

There are a few different semiconductor types available to RF designers as they build 5G base stations, including those based on:

• Laterally diffused metal oxide semiconductor (LDMOS), a wide bandgap semiconductor material with high-output power capability that utilizes silicon wafer substrates
• Gallium nitride (GaN), a wide-bandgap semiconductor material known for its high levels of thermal conductivity, heat capacity and hardness, and low sensitivity to ionizing radiation, that utilizes either silicon (Si) or silicon carbide (SiC) wafer substrates

This whitepaper will discuss the challenges the wireless industry faces in bringing 5G networks to fruition, and why, when it comes to technology decisions for 5G macro and small cell wireless base stations, gallium nitride on silicon carbide (GaN on SiC) is the clear choice for semiconductors.

Riding the 5G Wave

The move towards 5G has been building for many years, and in 2019 and 2020, we’ll finally begin to see the fruits of the industry’s labor. Early service rollouts and the first round of 5Genabled devices will make their way to market. Wider commercial deployments will take place starting in 2020 and continue to roll through the next several years. Analyst firm CCS Insight predicts there will be 1 billion 5G users globally by 2023—one-seventh of the world’s poppation. Cisco’s Visual Networking Index says nearly 12% of global mobile traffic will be on 5G cellpar connectivity by 2022.

The availability of higher network bandwidth, lower latency and incredibly fast data speeds promised by 5G will spur a wealth of new applications across virtually every industry sector, from manufacturing to energy to transportation and beyond. Smart cities, smart manufacturing, autonomous vehicles and connected transportation can all be realized through the widespread availability of 5G connectivity.

5G, however, also demands a new way of thinking about the technologies and infrastructure design needed to deliver connectivity that meet these new requirements. It will require densification not only on the macro level with the installation of more base stations, but also densification of power on the device level. Today's telecommunications infrastructure design requires technologies that can meet a number of criteria for the application, including temperature, speed, power, efficiency, size and cost.

5G Challenges

Why are the technology requirements of 5G different from previous generations of wireless technologies? There are three key reasons:

• Wider bandwidth is required. The rapid increase in mobile data is well documented; Cisco’s Virtual Networking Index says demand for mobile data will exceed 49 exabytes per month by 2021. The volume of data is large, but so too is the need for higher speeds to keep pace with the real-time demands that applications like autonomous vehicles and other use cases will place on networks. To support that level of data at higher speeds, additional wireless spectrum needs to be utilized. The wider the spectrum band, the faster data can travel. 5G standards allow for an order of magnitude wider bands (1 GHz+), allowing for much faster throughput.
• New frequency ranges are being utilized. 5G incorporates two new frequency bands at 3.5 GHz and 4.8 GHz to help meet the growing demand for throughput, and that means new requirements on the technologies being used. The higher the frequency band, the shorter the distance signals can travel. Unlike previous generations of wireless base stations, 5G base stations need to be much closer together, or “denser” than their earlier counterparts—in some cases, within a few hundred feet of each other. Imagine that today, especially in urban areas, with the size of base stations that power current mobile networks.
• 5G base stations are significantly smaller. Because more base stations are needed for 5G, they have stringent size requirements so that they can slide easily into their already welldeveloped surroundings. They’ll be placed on street lamps, sign posts, on the side of buildings, and anywhere else that is needed to relay 5G signals. The size of everything inside, therefore, also needs to be more compact.

GaN on SiC vs. GaN on Si

5G wireless base stations need to incorporate technologies that deliver in terms of performance, efficiency and value. Gallium nitride (GaN) solutions have emerged as a vital component. However, when evaluating GaN solutions, a common debate emerges: Which is better for RF applications, GaN on silicon (GaN on Si), or GaN on silicon carbide (GaN on SiC).

While there are advantages to each approach, “infrastructure designers choose the solution that offers the best overall value,” says John Palmour, co-founder and CTO of Wolfspeed. “Silicon is a relatively cheap substrate compared with silicon carbide but has some distinct disadvantages as well. SiC devices lead to lower system costs and better performance compared with silicon and, because of that, GaN on SiC is proving to have the best overall value.”

GaN on SiC has several key characteristics that together make it the best option for use in the telecom and wireless industries:

• Thermal Conductivity. GaN on SiC has three times the thermal conductivity of GaN on Si, allowing devices to run at a much higher voltage and higher power density. Palmour explains: “If RF devices put out high watts per square centimeter, you also have to dissipate a high amount of heat per square centimeter. The better the thermal conductivity, the easier it is to remove that heat. Silicon carbide has great thermal conductivity – much better than silicon.”
• Materials Match GaN and SiC are latticematched, meaning the lattice structures between the epitaxial layers allows a region of band gap change to be formed without changing the crystal structure of the SiC substrate material. This creates a lower defect density of the crystals, reducing leakage, improving reliability and creating an overall superior product. In contrast, GaN on Si is a mismatched material system; the crystalline structure of silicon doesn’t align well with GaN. For GaN to grow on silicon, a more complicated epistructure is required to keep the wafer from warping, impacting time, cost and performance of the semiconductor.

The crystal defect density determines how many “good” devices can be derived from the wafer. GaN on Si delivers fewer good devices than GaN on SiC because it has a higher defect density. GaN on SiC can operate at a higher electric field than GaN on Si, and—because more good devices are derived—the GaN on SiC chip can be about 20 percent smaller than those utilizing GaN on Si, says Simon Wood, senior director of RF product development and applications at Wolfspeed. “We can put more watts on a 6-inch wafer than can be done with GaN on silicon. Our contention is that makes up any price difference with silicon,” he said.

This is a benefit when designing for 5G applications, where real estate inside the base station is at a premium.

Higher Efficiency, Lower Cost. While silicon has high resistivity at room temperature, wireless infrastructure generally operates “hot.” At high temperatures, silicon is conductive, and RF coupling to the substrate can occur. When it is cooled, the GaN will shrink more than the silicon substrate. With this, some RF power to the substrate is lost, decreasing efficiency. Because of SiC’s close match with GaN, GaN on SiC does not suffer from these same temperature change issues.

In addition, the cost to grow the GaN epitaxy on silicon is more than the cost to grow GaN epitaxy on silicon carbide. This gives GaN on SiC significant efficiency and cost advantages over GaN on Si.

GaN on SiC vs. LDMOS

Compared to LDMOS, GaN on SiC offers significant improvements in 5G base station performance and efficiency:

• Better thermal characteristics. Again, GaN on SiC has much better thermal conductivity compared to other materials, dissipating heat more efficiently and effectively, allowing devices to run at a much higher voltage and higher power density than other technologies.
• Smaller arrays possible with same performance. Because of GaN on SiC’s superior thermal characteristics, power per device can be much higher. This means a 32x MIMO array is feasible rather than a 64x MIMO array. In addition, because GaN on SiC semiconductors are more efficient, a GaN on SiC chip can be about 20 percent smaller. Both of these mean smaller base stations can be designed.
• Rugged enough for 5G demands. GaN on SiC is robust, reliable, and hardened, providing a high breakdown voltage with minimal performance degradation. It is used in some of the most demanding applications and conditions.
• Better efficiency at higher frequencies. LDMOS works best in lower frequencies. In the higher frequencies now being utilized for 5G, such as 3.5 GHz, GaN is 10% to 15% more efficient than LDMOS. Conventional LDMOS silicon technologies generally operate well below 3 GHz. “Once you are into these higher frequencies being opened for 5G, LDMOS doesn’t work well,” Wood said. In addition, “there is a tipping point at which SiC outperforms Si, and 2.7 GHz is that tipping point.”
• Significant runway for future optimization. Silicon-based technologies like LDMOS have been in use for years and are reaching the end of their optimization lifespan. By comparison, GaN is in early generations with significant room for improvement and extensions.

Understanding Total Lifecycle Costs

In the end, for service providers building out networks to support the continuously increasing appetite for data, it’s all about lifecycle cost— kilowatt hours and the energy they are burning.

GaN on SiC has demonstrated to be a better solution overall for wireless communications because of its thermal conductivity, materials matching, efficiency and total lifecycle cost.

“The GaN on silicon vendors say that SiC is more expensive, and if you are only measuring that topline cost, that may be true,” Palmour says. “But the advantages GaN on SiC brings from an overall value perspective makes GaN on Si and GaN on SiC comparable price-wise, with the undisputed technology advantages going to GaN on SiC.”

Why Wolfspeed

GaN on SiC has emerged as the frontrunner to take on all of the challenges and requirements brought about by the introduction of 5G networks. Simply stated: “If you want the best performance, you need to be using GaN on SiC for 5G,” Wood said.

Wolfspeed created the industry’s first GaN on SiC high-electron-mobility transistor (HEMT) more than 20 years ago and has chosen to focus on GaN on SiC and be the best in the world at it. That focus has allowed it to mature and produce high-performance, highly reliable semiconductor solutions. Here’s why Wolfspeed stands out amongst the competition:

• Vertically integrated. Wolfspeed controls all steps of the GaN on SiC development process (crystal growth, epitaxy, device processing), allowing it to push the technology forward quickly. Wolfspeed:
  • Designs both the wafer growth and epitaxy processes so they are optimized for each other, creating superior epitaxy
  • Controls its own SiC supply and has a major share of global SiC production, ensuring customers always have access to SiC
  • Produces a large volume of SiC and is increasing capacity, leading to lower SiC costs over time
• Best-in-class failure-in-time (FIT) rate. A FIT rate is defined as a failure rate of 1 per billion hours. Many FIT rates are measured in the hundreds. Wolfspeed’s is consistently in the single digits at 5-per-billion device hours, illustrating the industry-leading reliability and performance of Wolfspeed’s GaN-on-SiC devices.
• More field hours than anyone else. Wolfspeed has successfully fielded more than 15 million devices and clocked an industry-leading 170+ billion field hours, dwarfing the competition.
• Rugged enough for the U.S. Government. Wolfspeed is accredited as a Category 1A Trusted Foundry by the U.S. Department of Defense and is used in government projects like the U.S. Air Force’s Space Fence project (see Space Fence callout on next page) . “If it’s rugged enough for their most critical applications, Wolfspeed’s GaN on SiC is rugged enough for 5G,” Wolfspeed CTO Palmour says.

Wolfspeed GaN on SiC Leads the Charge

GaN on SiC RF devices are used throughout the world on projects in a broad range of applications, such as surveillance and weather radar, first responder communications, and others where downtime is not an option. Because of its technological superiority and low total cost of ownership, GaN on SiC is the perfect fit as the 5G market continues to gather momentum.

With its vertically integrated supply chain, industry-leading field hours, best-in-class FIT rate and proven ruggedness, Wolfspeed GaN on SiC is the natural choice to lead the wireless industry into 5G and beyond

Wolfspeed GaN on SiC in Action: Space Fence

GaN on SiC RF devices are used throughout the world on projects in a broad range of applications where downtime is not an option. But are they rugged and reliable enough to tackle 5G, especially with the harsh environmental conditions, and power and temperature fluctuations 5G base stations will face?

A look at a case study incorporating GaN on SiC technology from Wolfspeed may help put things in perspective. Lockheed Martin has partnered with Wolfspeed to provide GaN on SiC-based high-power amplifiers (HPAs) for the U.S. Air Force’s Space Fence system, a ground-based system located on the Kwajalein Atoll in the Marshall Islands designed to detect, track and catalog an estimated 500,000 objects in space, a.k.a., space junk.

Space Fence incorporates a scalable, solid-state S-Band radar with a high frequency that is capable of detecting much smaller objects than the previous system, and will thus improve accuracy, quicken response time and expand surveillance coverage.

Lockheed Martin uses Wolfspeed’s GaN on SiC, as it provides significant advantages for active phased array radar systems, including higher power density, greater efficiency and improved reliability over other technologies. Early in the process, Lockheed Martin conducted more than 5,000 hours of accelerated stress testing and demonstrated with greater than 99% confidence that Wolfspeed’s GaN HPAs would meet the long-term reliability goals for the Space Fence program, which is integral to meeting the project’s efficiency and availability goals.

5G Readiness

S-Band radar systems, such as those used in Space Fence, work at similar frequencies as those being developed for sub-6 GHz 5G systems. Although the technology is similar, radar applications are actually driven even harder for saturated power and are stressed more than is expected in 5G systems.

"Our high-performance commercial GaN products have been fielded for many years in a variety of military and commercial applications and have matured enough to support the mission-critical 24/7/365 coverage required by the Space Fence system and other missioncritical applications like 5G," says Jim Milligan, senior director of Wolfspeed’s Foundry, Aerospace and Defense Business Unit.

In other words, if it’s good enough for the world’s most critical applications, GaN on SiC is ready for 5G.

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