5G and GaN: The shift from LDMOS to GaN

Article By : Roger Hall

This article explores how the industry is innovating to satisfy the new demands 5G puts on embedded systems, driving a shift from traditional LDMOS amplifiers to GaN-based solutions...

In the previous article in this series, we reviewed the Massive MIMO technologies that are driving 5G implementation across the country. While the potential of mmWave frequency applications will eventually be realized, for the next few years 5G service will be defined by signals transmitted over Sub-6GHz bands. To make this possible, the next generations of base station solutions will require significant improvements to RF front end performance.

Engineers are being asked to develop base stations that account for better RFFE integration, size reductions, lower power consumption, higher output power, wider bandwidth, improved linearity, and increased receiver sensitivity. All that is in addition to satisfying the tighter coupling requirements between the transceiver, RFFE, and antenna. It’s a very tall order. The only way to meet these needs and successfully implement Massive MIMO will be with small, highly efficient, cost-effective power amplifiers that can be used in these expanding antenna arrays.

Powering Sub-6 Massive MIMO

The RF power amplifier space has been defined by laterally-diffused metal-oxide semiconductor (LDMOS) devices since the technology entered the market in the 1990s, particularly in frequencies below 2GHz due to their low cost. Its largest competition came from gallium arsenide (GaAs) amplifiers that was better suited for higher frequencies, but with lower power-transmission levels and at higher costs. When 2G digital mobile networks rolled out, LDMOS achieved market dominance in RF base station that it still holds today. As 3G and 4G networks were introduced however, LDMOS power amplifiers haven’t reached the same power efficiency levels of previous generations. Despite performance boosts from using Doherty topologies and envelope tracking, equipment manufacturers and operators started turning to gallium nitride (GaN) as a next generation semiconductor for RF power applications during the 4G LTE deployment across China in 2014.

GaN is a relatively new technology compared to other semiconductors, but it has become the technology of choice for high-RF, power-hungry applications like those required to transmit signals over long distances or at high-end power levels – making it ideal for Sub-6 5G base stations. Its high output power, linearity, and power-efficiency have driven network OEMs to switch from using LDMOS technology for PAs to gallium nitride. LDMOS technology still holds the greatest market share in RF base stations today, but GaN is expected to continue to displace it in 5G Massive MIMO deployments.

GaN Performance Advantages

The primary advantage of GaN is its higher power density. This is due to a band gap between the conduction and valence bands that is higher than in LDMOS technologies, which provides both high breakdown voltages and power densities. It allows a signal to be transmitted with more power that widens the coverage areas of base stations. The high-power density of GaN PAs also enables smaller form factors that require less PCB space. In a given area systems designers can produce more power than with another technology. Or, for a given power level systems designers can shrink the size of the RFFE and reduce costs.

This higher power density also allows for GaN power amplifiers to operate at temperatures as high as 250 degrees Fahrenheit – a level silicon-based technologies can’t reach. GaN’s improved thermal dissipation simplifies heat-sink and cooling requirements of systems, further reducing size and cost. Given the steep infrastructure expenditures facing MNOs, smaller, less expensive equipment will go a long way to making 5G available nationally.

GaN’s increased power efficiency also contributes to reducing the expense of running base stations. Carriers are looking to minimize network power consumption and are pushing OEMs to design for system efficiency and overall power savings. To meet that need, engineers are increasingly turning to GaN. In a Doherty PA configurations, GaN attains average efficiencies up to 60% with 100-W output power, significantly reducing the energy required to run power-thirsty Massive MIMO systems.

GaN’s efficiency at high frequency and over wide bandwidths can also help shrink Massive MIMO systems. Although the improvements in LDMOS amplifier characteristics allow for frequency ranges up to 4 GHz, GaN-based amplifiers can achieve frequencies up to 100 GHz at power densities up to five times higher. The higher efficiency and output impedance, along with lower parasitic capacitance, gives GaN devices easier wideband matching and scaling to very high output power. While the mmWave applications are more obvious, this can benefit carriers in Sub-6 by transmitting over multiple bands simultaneously. Carriers won’t need multiple narrowband radios, they’ll just need one wideband radio platform that serves multiple bands. GaN offers the range and flexibility to make these systems possible, while also easily scaling to deliver the high frequencies of mmWave transmissions of the future.

That’s not to say that GaN is always the right choice for every RF power application. LDMOS is often available at a lower price point and delivers very competitive linearity at certain frequencies. GaAs also has its own efficiency advantages in certain market niches. However there’s a reason that many major players in LDMOS shifting to GaN production: they recognize how critical GaN is to helping carriers and base-station OEMs achieve their goals for Sub-6 GHz Massive MIMO.

Because of GaN’s wide adoption in base stations, along with broadening applications in other industries like defense and aerospace, the volume of GaN being produced grows year over year. More volume equals greater economies of scale, making GaN a more affordable solution. That is without taking into account the savings achieved from increased energy efficiency, smaller form factors, or multiband applications. Linearity is also set to improve. It’s important to remember that GaN is only on its second generation of offerings for base stations. Mature technologies like LDMOS are on generation 15. It’s currently the most active research area in the GaN space, causing many in the industry to anticipate market-leading linear efficiency in the short term.

As the constraints limiting GaN from wider application are addressed, it now becomes critical for systems designers to understand how to apply the semiconductor to their own applications.

What Embedded Designers Need to Know

GaN offers a lot of performance advantages to embedded designers, but there are certainly design best practices that are unique to the material. The next article in this series will detail what embedded systems designers need to know to harness the full potential of GaN. It will correct common misperceptions, offer design solutions, and explore what’s next for GaN technology both in and outside RF applications.

— Roger Hall is the General Manager of High Performance Solutions at Qorvo, Inc.

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