Beyond the horizon

As GaN storms radar applications, Qorvo is taking the semiconductor to ever-higher frequencies, reports Compound Semiconductor.

Thanks to its high power, high frequency performance, the GaN transistor is the leading light in electronic warfare.

As the military's thirst for GaN semiconductors endures, defence manufacturers are making the most of the industry-wide migration from the incumbent vacuum electronic device to the mighty III-V transistor.

In late Summer this year, Northrop Grumann signed a deal to sell GaN radar to the US Marine Corps. And only weeks later, Raytheon revealed a contract with the Missile Defense Agency to retool its long-range radar line to support GaN semiconductors.

Clearly, the steady spate of military contracts spells good news for III-V device providers, with Wolfspeed, Qualcomm, MACOM and more, delivering transistor after transistor for high power radar. Case in point is Qorvo, having released two new GaN-on-SiC power amplifiers for advanced defence and civilian radar systems.

A 500W L-band device operates in the 1.2 to 1.4 GHz frequency range and targets high-power phased array radar, including Active Electronic Scanned Array (AESA) systems. Meanwhile a 450W power amplifier targets higher frequency 3.1 to 3.5 GHz, S-Band radar systems.

As Dean White, Market Strategy Director at Qorvo, highlights: "What is key here is that we have a GaN process that has high gain and high efficiency."

"[It enables] lower power consumption and allows a large phased radar array to be air-cooled and not liquid-cooled," he adds. "The weight associated with air-cooling can be half that of liquid-cooling, and this means a lot to the customer."

Targeting radar

Traditionally, TWTs have been the technology of choice across a broad range of radar frequencies, spanning from the L-band at 1 to 2 GHz to the X-band that covers 8 to 12 GHz. But thanks to its high voltage, high power, high frequency performance, and dogged military development, the GaN transistor is the leading light in electronic warfare applications.

"I know it sounds crazy as the traditional travelling-wave tube has always been touted as being very broadband but we can take GaN and design very broadband power amplifiers now," points out White.

Prior to the merger of RFMD and TriQuint, the latter once delivered record-setting power density levels with its GaAs devices, but for Qorvo, GaN is now king. "We reached 1 W per mm with GaAs but with GaN we get greater than 5 W per mm," says White.

As the Director highlights, the smaller chip size that ensues means AESA radar systems can be taken to higher and higher frequencies. "With the smaller chip-size we can make smaller, lightweight arrays for fighter aircraft and unmanned aerial vehicles," he says. "And beyond the device-level we can also design MMICs with even higher efficiencies as GaN offers power and efficiency in a small space."

Importantly, the impressive power density twinned with a SiC substrate allows Qorvo to package devices in relatively small housing while dissipating heat effectively. "We can really pull the heat out [of these packages] to keep the devices cooler, and the cooler the device the better it performs," says White.

With legacy systems in mind, Qorvo can also integrate its GaN-on-SiC PAs into its so-called Spatium RF power modules, offering a straightforward replacement for TWTs in myriad military systems. These high efficiency power amplifier modules are said to deliver longer service lifetimes as well as size- and weight-savings.

"This tube replacement looks like a tube but operates at a much lower voltage, so you don't have the power supply reliability issues you have with tubes," says White. "You can take the [package] and fit it into the same spot as your TWT."

Spatium module: looks like a travelling wave tube but operates at a lower voltage. [Qorvo]

And Qorvo is still seeing steady demand for its GaN-on-SiC devices in legacy and new defence programmes. "We still sell a lot of GaAs PAs but this is generally for legacy programmes," he highlights. "I would say around 90% of the time whenever a retrofit or new platform is built, the technology of choice for the power amplifier will be GaN, as opposed to GaAs or even LDMOS."

Beyond legacy devices, up and coming radar markets are offering more opportunities for GaN devices. US-based analyst, Markets and Markets, recently forecast the global X-band radar market to swell from today's US$4.58 billion to US$5.61 billion come 2021, following government investment in higher frequency radar and demand from land and naval security sectors.

White concurs, highlighting how Qorvo is now seeing demand for GaN in X-band airborne systems as well as X-band commercial radar. "As you go to these higher frequencies, the difference between GaN and GaAs becomes greater," he says. "But we are one of the few companies in the world that has production-ready GaN processes, that operate up to 60 GHz."

Given these X-band radar market developments, the company recently delivered GaN-on-SiC discrete FETs that can be used for power amplifiers, low noise amplifiers (LNAs) and drivers for RF applications at these frequencies and higher. "Everybody thinks of GaN as being high power and asks why would I use that as a low noise amplifier?" says White. "But this high density transistor also has a very good noise figure - on par with a p-HEMT device; so this is a new market and customers are getting used to this."