Integra Turbo-charges Its S-band Transistors With Gallium Nitride
Since the late 1990s Integra has been manufacturing pulsed, S-band silicon transistors for radar. It has recently expanded its portfolio with GaN versions that are smaller, more efficient, and deliver gain over a far broader bandwidth. Richard Stevenson tracks this new venture.
GaN is a great material for producing RF transistors. It can yield devices with high efficiency and excellent gain over a broad bandwidth, characteristics that have encouraged many III-V chip makers within our community to develop high-frequency devices based on this material.
But these companies, which include the likes of Cree, RFMD, TriQuint and Eudyna, will not have this market to themselves. The allure of GaN has also piqued the interest of silicon RF manufacturers, including Integra Technologies, a company with almost 100 staff that makes high-power, pulsed transistors for the aviation industry.
Based in El Segundo, CA, Integra is renowned for its S-band transistors - it claimed that they delivered unparalleled performance when the company was founded in 1997.
Today Integra is Raytheon’s sole supplier of this class of device, which the US defense giant uses to fulfill its a long-term contract to supply radar systems to US airports that help pilots land their aircraft. “I know that London Heathrow has been upgraded with our devices too," says John Titizian, company founder and president.
Integra has several reasons for taking its decision to develop GaN transistors for radar applications. Turning to this wider bandgap transistor improves the resolution of radar systems and their target tracking capability. “And instead of having different frequency radars, it is possible to have one radar that covers a lots of bands," enthuses Titizian. He reveals that Integra’s decision to diversify into GaN was also motivated by the capability of this material to operate at higher frequencies. According to the company, silicon devices cannot deliver a good enough performance beyond the S band. By turning to GaN, Integra believes that it will be possible to produce products for the C-, X-, and Ku-bands.
The company’s foray into GaN can be traced back to its attendance at a compound semiconductor conference held at Hyatt Grand Champions Resort and Spa in Indian Wells, CA, in 2005.
At that meeting several firms, including Northrop Grumman, extolled the virtues of GaN. “We were debating whether to go with GaAs or GaN," reminisces Titizian. “After the information we got at that conference we decided that we were going with GaN."
Efforts began in earnest in the latter half of 2008. At that point the company set itself the target of sampling its first GaN products in 2010. It is a goal that has been completely fulfilled following the company’s announcement of two GaN HEMT products operating in the S-band at the IEEE MTT-S International Microwave Symposium that was held this May in Anaheim, CA.
At the IEEE MTT-S International Microwave Symposium in Anaheim, CA, from 23 to 28 May, Integra released a pair of GaN HEMTs. The more powerful of the two Sband products, which is shown here, covers the 2.7-3.1 GHz range
Initially Integra’s GaN development involved just two senior staff – a process engineer and a designer. Since then the team has expanded to five, with three employees working on the processing of the device and another two focusing on circuit development. This appears to be a remarkably small number of engineers for driving the development of a transistor employing a material entirely new to the company.
Titizian, however, is not surprised by the tremendous progress: “ We are semiconductor experts, and we understand semiconductor physics. It’s true that GaN presents some different challenges, but we understand the device physics."
When founded, the company’s expertise lay in silicon bipolar devices. However, more recently it has expanded its knowledge base, first with the development of silicon LDMOS technology, which is not so far removed from GaN HEMTs. “If we had to go from bipolar to GaN, then that may have been a bigger jump, " says Titizian, who reveals that extensive design iteration was not required for the development of the company’s HEMTs. Although there is not as much software available for aiding the design of GaN HEMTs as there is for silicon devices, Titizian says that what exists is adequate.
Integra has developed two products operating in the upper S-band (3.1-3.5 GHz). These devices are targeting military radar. The transistor shown here produces 120 W. The other device, which produces over 10 W, is an internally matched part with 50 ohm impedance at the leads
Outsourcing epiwafer growth
Integra decided against developing in-house, GaN-on-silicon growth expertise. Instead it evaluated material from several epiwafer suppliers. In some cases these wafers were too brittle, or yielded devices with leakage currents that were far too high. However, these problems did not plague the material provided by a Japanese manufacturer, which is now Integra’s sole supplier of epiwafers.
By adopting an outsourcing model, Integra slashed its capital expenditure on new equipment needed to provide its fab with the capability to process GaN-on-silicon HEMTs. Its only recent addition is a chorine-based, inductively coupled plasma (ICP) etcher. However, there have also been some adjustments to processing tools on the 6 inch line, so that they are capable of handling 4-inch GaN-on-silicon wafers.
The epiwafers that Integra imports have a silicon (111) foundation, because this orientation is better at accommodating the lattice and thermal mismatches between silicon and GaN than the more widely used (100) cut. Processing this material presents no problem whatsoever for Integra, because the company’s bipolar products are also grown on the silicon (111) orientation.
Integra has also been able to draw on its silicon technology for packaging its GaN HEMTs, which share the housing employed for LDMOS devices. Problems associated with parasitics are negated by taking sufficient care with the assembly process.
The pinnacle of all this effort-to-date has been the launch of products at the MTT-S show: a pair of single-ended devices housed in a ceramic flanged package, which deliver either a 25 W or 50 W peak output in the 2.7-3.1 GHz range. Both HEMTs produce at least 12 dB of gain and have a breakdown voltage in excess of 200 V. These performance figures were recorded using 300 μs pulses, a 10 percent duty cycle, and an operating voltage of 50 V.
What’s the pecking order?
Although the performance of these devices is impressive for a firm that only started developing GaN products two years ago, these HEMTs are not state-of-the-art commercial products.
“We’re getting 3.5 W/mm, and there are devices out there by Cree and Eudyna in the 5 W/mm range," admits Titizian. “We could design devices with higher W/mm, but we’re focused on offering a reliable device that will not drift in performance with time."
One avenue that Integra can explore to improve the performance of its HEMTs is to shorten gate length. The company is going to do just this, and has a target of 0.3 μm in its sights. “I’m absolutely convinced that we’ll have comparable devices [to the leading GaN HEMT makers] by next year," claims Titizian.
To ensure a high reliability for all of its products, Integra employs a gold metallization process. This element has an incredibly high activation energy, reducing the likelihood that it will undergo electromigration, which could ultimately cause premature ageing of the transistor.
Over the next year or so Integra plans to introduce several more GaN products. It has already started adding to its family of GaN S-band products and it is also producing cousins operating at higher frequencies. It has one device operating in the C-band, and over the next few years more devices will be launched in both this band and the Xband.
To protect its product’s commercial future, Integra is looking at the intellectual property surrounding GaN transistors, and deciding whether it needs to patent its technology. “There is a lot of public information out there. You can put together a device without infringing on other people’s IP," says Titizian.
Performance at a price?
The costs associated with manufacturing Integra’s GaN S-band products are significantly higher than those associated with production of silicon transistors operating in a similar part of the microwave spectrum. However, Titizian expects that this cost gap will close as products migrate to larger wafers: “Gallium nitride on 6 inch silicon will probably be available next year, and at some point GaN-on-silicon could be available on much larger wafers, such as 12-inch wafers." This will drive down manufacturing costs, and Titizian believes that there should come a time when GaN is comparable to silicon, in terms of the dollar-per-Watt metric.
Integra is also starting to develop GaN-on-SiC devices, which it says are the only wide bandgap RF technology that the US military is currently willing to deploy. “We are going to offer both technologies to our customers, and we believe that GaN-on silicon may be cheaper in the long run." Although GaN-on-SiC tends to have the edge in terms of reliability and output power, Titizian says that many applications may be adequately served with the silicon-based equivalent. Titizian reveals that processing GaN-on-SiC wafers does not require any major modifications to the fab. “With the ICP etcher we can do the vias – that is not an issue any more."
Sales of Integra’s GaN-based products are expected to grow significantly over the next five years, and account for over half of the company’s revenue in 2015. However, that does not mean that Integra will be turning its back on its customers wanting to buy silicon products.
“We are not going to abandon silicon because we’ve made commitments to our customers," says Titizian. “Most of these systems have a long life cycle that we need to support, and we will support that."
Keeping customers happy has long been acknowledged as one of the keys to a successful business, alongside the launching of new, superior products. Integra is executing on both these fronts, and it looks to be assured of a bright future.
Integra has one HEMT operating in the C-band: the IGN4450M100. This transistor produces more than 100 W of output power at 4.4-5.0 GHz