Nitronex targets military radios

There is a growing demand for communication systems offering an increasingly rich user experience. In the commercial world, it is now the norm to rapidly download massive files via high-speed internet connections. And as smartphone sales extend beyond corporate users, consumers are starting to access high-definition video streaming, music downloading and live webchat via their handsets. Although it is often invisible to the end user, such feats of communication are the culmination of co-ordinated efforts by a multibillion dollar industry that is continually addressing network bottlenecks to improve bandwidth and coverage.

The US Department of Defense (DOD) demands even higher levels of system performance for military communications. In recognition of the needs across the various services, the US government initiated the Joint Tactical Radio Systems Joint Program Executive Office (JTRS JPEO) in 1997. More recently it has developed a vision that it describes as a tactical, mobile wireless global information grid.

Fulfilling this vision will enable all of the DOD and public safety communication systems – such as those used by firefighters and law enforcement agencies – to co-operate seamlessly, while exchanging vast amounts of information in a secure, reliable way. JTRS spans ground, air and space domains, and incorporates the needs of ground troops, mobile units (hummers and tanks), airborne platforms (manned or unmanned, surveillance, logistics and offensive mission objectives), maritime (carriers, cruisers and destroyers), fixed sites (command and control centers) and satellites. The move to JTRS will eliminate existing differences between various platforms, which has led to the diverse family of radios that are in operation today.

The engines of these wireless communication systems are RF PAs. Network performance is reflected in their capability, which is governed by the power transistors that are employed in the transmit chain of the amplifier. We believe that GaN HEMTs fulfill the JTRS requirements.

JTRS-compliant radios have to deliver power amplification across far wider bandwidths than commercial wireless infrastructure. Ultimately, JTRS will cover 2 MHz–2.5 GHz and enable universal software-defined radios. The interim goal, however, is for amplification at 30–1000 MHz and 1000–2500 MHz. At the antenna radio, power levels are 5–200 W, so output stages must produce a minimum of double this to compensate for the losses between the final stage and the antenna.

When the operating bandwidth exceeds an octave, trade-offs are needed between the transistor technology and the size and efficiency of the system for a given output power. Wider bandwidth requires an increase in the number of impedance transformation sections, leading to a larger, more complex, and more expensive and lossy overall system.

These trade-offs are based on the inherent properties of the materials that are used in the transistor, so a change in transistor technology is needed to substantially improve system performance. GaN HEMTs, which have a far higher power density than silicon equivalents, provide an immediate advantage to the broadband designer – higher impedances for a given power level. The higher impedance enables a less complex, more compact matching structure, and ultimately a smaller, lighter, more efficient and more capable radio.

We have developed a family of broadband discrete GaN HEMTs with these attributes. They deliver 5–200 W of RF power and satisfy the needs of JTRS radios (table 1). Our most powerful product in this family, the NPT1007, is the industry s first 200 W GaN HEMT. It aims to offer maximum broadband power to the designer in a small footprint.

Some new products are also being developed, including the NPT1010 and NPT1012 that will be launched in the first half of this year. These transistors deliver a 20–30% increase in thermal performance over our previous generations of products, which leads to higher broadband power. The improvement stems from the combination of a 50 µm thick GaN die and the latest packaging and die attach technologies, which unite to deliver industry-leading thermal performance for broadband communications. Our customers have already verified power performance over the broadband frequency needed for JTRS radios.

One of the key strengths of GaN HEMTs is their ability to operate at high voltages, such as 48 V. This increases the device s power density, which tends to increase the impedance and lower the output capacitance per unit of output power. Thanks to this benefit, it is possible to simplify the design of high-power broadband amplifiers with a reduced size and improved system efficiency.

We are developing the technology needed to support 48 V operation. This requires new device designs to manage the higher electric fields and dissipated power, so that the transistor s lifetime can be guaranteed to commercial and military standards. Meeting customer requirements Not all radios contain a high voltage supply and the lower-power JTRS radios, such as Handhelds and Manpacks that are used by foot soldiers, typically supply lower voltages. This means that they cannot always take advantage of the high voltage capability of GaN HEMTs. Employing voltage converters to supply higher voltage is not a worthwhile solution either, because this reduces the overall system efficiency. Although there may come a time when higher-power radios on larger platforms can operate GaN HEMTs at 48 V, smaller radios currently use 14–28 V operation, with the exact voltage depending on the individual radio design.

Consequently, it is essential for GaN HEMTs to deliver good performance across a wide range of operating voltages, rather than a great performance at a single voltage. We have addressed this issue with a transistor design that produces high gain and efficiency across the 14–28 V range (figure 1). With this approach, designers can use the drain voltage as a variable to meet their system s unique needs, and maximize power and efficiency at linearity for a given thermal constraint.

Ruggedness is another key requirement for next-generation military tactical radios, which are often subjected to high-voltage standing wave ratio (VSWR) conditions. This can occur when a radio is turned on without the antenna connected or, in an extreme case, when an antenna attached to a vehicle is damaged in combat while the radio is transmitting. Such events create an infinite VSWR condition and the radio, and hence the transistors, have to survive without catastrophic failure.

GaN HEMTs have a far higher breakdown voltage than silicon LDMOS devices, and radios built with the wide-bandgap semiconductor are far less likely to fail under harsh voltage conditions at a stable operating temperature. Our devices are designed to be immune from catastrophic failure at high VSWR ratios and tests show that they can survive with minimal performance change when subjected to a 10:1 VSWR at the 3 dB compression point with the package flange at 90 °C.

In addition to the requirements for power and efficiency over bandwidth, inherent ruggedness, and a small footprint coupled to low weight, the DOD has started to push for lower costs. We are well positioned to meet the goal for long-term cost reduction by driving down costs associated with development, production and operation through improvements in system efficiencies and lifetimes.

Designers and business managers generally want more than just performance guarantees before fully adopting a new semiconductor technology – reliability, for example, must be proved before a designer will be willing to initiate a design effort. To address this particular issue, our products are subjected to a series of stress tests as part of the qualification process. JEDEC and MIL standards, in combination with customer input, are used to define the tests and conditions. Unlike every other GaN HEMT manufacturer, we perform a full qualification of GaN HEMT products and share our results in the public domain – they are available on our website.

Our ability to perform high-quality manufacturing and product qualification stems from the development of a process to manufacture GaN HEMTs on a high-quality 100 mm silicon platform. We have produced 15,000 GaN HEMT wafers on our 100 mm line, which is a sufficiently high volume to provide statistically significant sample sizes. The large number of growth runs has also helped to reduce development costs and improve product quality.

Other potential concerns for business managers include the health of the supply chain, long-term costs, redundancy in capacity and customer service. Many GaN HEMT manufacturers produce their devices on SiC, which means that they are dependent on a couple of suppliers of high-resistivity SiC substrates. By working with 100 mm silicon since 2002 we have avoided this issue, while benefiting from numerous substrate vendors, thanks to the use of a common platform. Looking in both directions We have just passed our 10-year milestone, and during this first decade our company has devoted a great deal of effort to systematically solving problems in epitaxy, fabrication, device design, reliability and product development. We have now reached the position where GaN HEMTs can fulfill their promised benefits to wideband power amplifier designers. A new generation of products will be released shortly, satisfying our customer s demand for significantly improved thermal performance.

We believe that the focus of attention will soon switch from the performance of the GaN HEMTs to the capability of the systems in which they are deployed. These systems could be serving soldiers in battle in the hot, dusty deserts of Iraq and Afghanistan, assisting unmanned airborne vehicles transmitting videos back to a control center or aiding a firefighter trying to communicate with armed services while dealing with the aftermath of a natural disaster. GaN HEMTs will contribute massively to the efforts of all of our services.   

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