GaN: beyond the basestation

As the GaN RF power amplifier penetrates wireless base stations, industry developments hint handsets could be next. Compound Semiconductor talks to Qorvo to find out more.

GaN power amplifers are replacing LDMOS devices in basestations, is the technology set to reach handsets? [Ice base station tower/Senator88]

In a little over a decade, the mighty GaN transistor has transformed from novelty device into a must-have in a growing number of applications.

Thanks to US government initiatives, GaN - with its staggeringly high power density - is the star technology in radar, electronic warfare and satellite, and is infiltrating more routine spaces, most notably wireless basestations.

Sumit Tomar, general manager of wireless infrastructure at Qorvo, is confident this passage to the more ordinary will continue.

He believes GaN has already uprooted LDMOS, as the technology of choice for RF power amplifers in the high capacity base stations of 4G LTE network providers. And the endless need for greater network capacity and faster services, driven by rising mobile data demand, will only fuel this move.

"Next generation base station platforms will go into production by the end of 2016, and we believe these will be based on high power GaN RF amplification," he says. "LDMOS is limited beyond 2.4 GHz, so GaN comes into play here. At 4.5G and 5G we expect significant GaN adoption in base stations, particularly at frequencies greater than 1.8 GHz."

Tomar is also certain that network operators will opt for GaN-on-silicon carbide power amplifiers, over GaN-on-silicon versions. GaN's impressive power density means devices can be housed in relatively small packages and still dissipate heat, but factor in silicon and this can change.

"Silicon has a poor thermal conductivity, compared to silicon carbide, and our customers are worried that GaN-on-silicon power amplifiers cannot dissipate heat as efficiently," he asserts. "This could reduce overall life cycle, raise the risk of field failure, which is a major issue for the basestation OEM customer."

"For power levels over 100W, all major players in the basestation market are not looking at anything on silicon for precisely this reason," he claims.

Right now, major GaN RF device providers, Cree, Sumitomo, Freescale, and of course Qorvo, are pursuing GaN-on-SiC processes. In contrast, MACOM prides itself on being the world's only provider of GaN-on-silicon technology for RF applications, with its devices being used in military and communications applications.

However, earlier this month, the company started sampling a GaN-on-silicon wideband RF transistor for operation up to 100W, and reckons the device delivers a performance to rival GaN-on-silicon carbide.

As Michael Ziehl, vice president of marketing RF and microwave at MACOM said at the time: "We expect to see ramping commercial adoption of our GaN technology in other RF applications in the future, including 4G/LTE base stations."

Still Tomar is confident leading systems providers, Freescale, NXP Semiconductor and Infineon are all looking to adopt GaN-on-SiC over GaN-on-silicon.

"These players dominate the power market and are all pursuing GaN-on-silicon carbide. GaN-on-silicon has a play at power levels less than 100W, but at more than 300W, the technology just isn't there," he says.

A different device

But basestations aside, industry players are beginning to explore the opportunities for high power density GaN power amplifiers in wireless handsets. Earlier this year, a group of researchers from Intel, led by co-director of Components Research in Technology and Manufacturing, Robert Chau, revealed an enhancement-mode GaN MOS-HEMT.

Designed for RF power amplifiers, as well as voltage regulators, in low power mobile silicon-on-chips, the researchers claim the transistor out-performs industry-standard GaAs RF power amplifier transistors on power output and efficiency.

Chau declined to comment on the latest development, but stated in his paper: "This work shows for the first time that the application space of GaN electronics can be expanded beyond the existing high-voltage power and RF electronics to include low power mobile SoCs."

Intel is the first company to publicly share such results, but doesn't appear to be alone in its development of GaN RF power amplifier for next generation handsets.

Tomar is quick to highlight Qorvo's GaAs process development, saying: "It gets tricky to put a higher supply voltage amplifier inside a handset but we have a unique pHEMT GaAs process that we can scale to serve the need of 5G handsets."

But at the same time, GaN HEMT development for power amplifiers in future handsets is clearly underway. "We are also working on GaN processes that I believe can go as low as 10V and in the future will go lower."

"I do not want to discuss this publicly," he adds. "But we will keep looking at different flavours of GaN and as well as other processes, and will see where the market goes."