Extreme GaN

As Cobham joins forces with RFHIC to drive GaN radar forward, Compound Semiconductor learns this is only the beginning.

Cobham intends to take solid state GaN transmitters to air traffic control, long range surveillance and more.

Thanks to its high voltage, high power, high frequency performance, and a decade of military development, the mighty GaN transistor has become the front-runner in power amplifier applications including radar, electronic warfare and more.

Silicon-based amplifiers struggle to operate at the higher frequencies of 4 GHz, while the GaAs devices just don't produce the peak power levels demanded by most pulsed applications.

And although the manufacturers of vacuum electronic devices for even higher frequency applications have been developing ever-smaller travelling wave tubes to fend off the competition, GaN is making in-roads to 4 GHz and beyond.

These developments in pulsed RF power device markets have not been lost on GaN device and system manufacturers alike. From Wolfspeed, Microsemi and Qorvo to BAESystems, Raytheon and more, myriad GaN-based components and circuits have been delivered.

And in the last few weeks UK-based defence and aerospace systems manufacturer, Cobham, joined forces with RFHIC, Korea, a designer and manufacturer of RF and microwave components, to muscle in on the action.

GaN development

Radar frequencies are divided into different categories; L-band and S-band radars operate at 1 to 2 GHz and 2 to 4 GHz frequencies, respectively. Meanwhile C-band and X-band radars cover the 4 to 8 GHz and 8 to 12 GHz frequency ranges.

To date, many GaN applications have targeted S-band radar. Indeed, in a solid state transmitter development program dubbed 'SOLSTx', Cobham recently unveiled a 35 kW prototype S-band GaN transmitter for air traffic control and weather radar applications.

Now, working with RFHIC, Cobham intends to soon deliver a 175 kW solid-state transmitter prototype for long range radar surveillance, based on the 35 kW power amplifier modules. And the next step will be to develop 175 kW modules to form the building blocks of systems that provide multi-megawatts of power for meatier defence applications.

As Ralph Marrone, product line director for SOLSTx says: "RFHIC is a major commercial telecoms GaN provider with strengths in high power amplification."

"Part of our system for SOLSTx requires high power amplifiers so RFHIC brings strong commercial and competitive knowledge," he adds.

And Cobham means business. While GaN is the undisputed champion of performance for power amplification applications, hefty materials and processing costs have stymied industry adoption.

Increasing production volumes is critical to shaving costs, and as Marrone points out: "[RFHIC] understands volume manufacturing."

"They actually have one of the highest volume capabilities for power amplification manufacture that I have ever seen," he says. "And this will be key to solid state amplification."

RFHIC has been working with GaN for more than a decade, forging a strategic partnership with a GaN foundry service provider in 2004 and then joining forces with Cree in 2008 to develop GaN-on-SiC products.

Delivering a host of GaN-based amplifiers and modules, the company now has links with several other foundries and remains close to Cree's company, Wolfspeed.

"It is publicly known that RFHIC packages for Cree, now Wolfspeed, in various locations and supplies most of these devices back to the company which are then supplied commercially," says Marrone. "So our development with RFHIC [involves] GaN-on-SiC, but this doesn't mean we are limited to [this technology]; other suppliers are working with GaN-on-diamond and GaN-on-silicon."

"We are 'technology agnostic' and will find the best match depending on customer requirements and applications ," he adds. "The choice also depends on frequency and at low frequencies we may use silicon LDMOS but at the S-, C- and X-band, GaN is the technology of choice at this time."

Time to failure

What about the industry incumbent? Today's radar transmitters use a host of vacuum electronics devices including travelling wave tubes, magnetrons and klystrons. Indeed, recent figures from DARPA indicate more than 200,000 vacuum electronic devices are now in service in the Department of Defense alone, powering critical communications and radar systems.

But according to Marrone, these devices have high operating costs, demand very high voltage power supplies and crucially, can be beaten on reliability. As he points out, the latest 35 kW solid state transmitter prototype delivers a significant increase in mean time before failure (MTBF), compared to the travelling wave tube.

"This [metric] is very application specific but you might get up to a ten times increase in reliability here," says Marrone. "People always ask how do you prove reliability but I think with GaN devices in the last ten years, reliability, with a high MTBF, has been proven."

The product director also points out how industry players can be concerned about TWT replacement saying: "There is a strong installed base in vacuum tube electronics and [customers] want to understand the swap; can we fit our systems into the package they need at the right price point?"

"We're always going to get questions about future technology, but solid state GaN is going to offer higher reliability and lower overall cost of ownership," he adds.

What's more, Cobham is setting its sights on taking its solid state devices to ever-higher frequencies. Key applications for its SOLSTx products, operating in S- and C-bands, are going to be air traffic control and land and marine long-range surveillance, but the future will hold more.

"We are focusing on the S- and C-bands right now but that doesn't mean we won't look at higher frequencies," says Marrone. "We have product road maps that look across all the bands, all the way up to the X-band."