Paul Beasly

For radar and communication driven military applications, system designers are under continuous pressure to achieve the aggressive size, weight and power (SWaP) profiles that can help ensure a sustained, strategic battlefield advantage. But achieving higher power and smaller, lighter components using conventional silicon and GaAs-based power transistors is an ever mounting challenge. For these devices, limitations in component power density, breakdown voltage and thermal reliability are introducing increasingly problematic performance constraints, with significant implications for system reliability, ruggedness and functionality to meet new mission objectives.

This problem is impeding the evolution and proliferation of small form factor radar and communication systems optimized for mobile and airborne applications, particularly UAVs. In these domains, parallel advances in radar and sensor fusion innovation are driving greater operational autonomy, situational awareness and responsiveness for remote military personnel, vehicles and aircraft – which are critical advances to sustain strategic battlefield advantage.

Meanwhile, for sea-borne applications, momentum is gathering behind the development of a new generation of more versatile, multifunction radar and communication systems that consolidate large numbers of co-located antenna masts within a single streamlined, multifunction Active Electronically Scanned Array (AESA) aperture. This naturally reduces system size and complexity while enhancing battlefield performance and versatility.

Meaningful forward progress on these strategic initiatives hinges on the ability to develop and manufacture smaller, wider bandwidth, lighter and functionally more flexible power transistors that promote multifunction integration. What is needed in all of these cases is a new approach to power transistor design and packaging technology that provides greater overall power performance in a smaller form factor with the greatest possible ease of assembly.

Achieving The Promise of GaN

The recent emergence of GaN-based power amplifiers is equipping radar system designers to achieve high-power operation using smaller power transistors while improving efficiency, frequency bandwidth and reliability. These new GaN-driven capabilities are yielding a new generation of more agile, ruggedized radar systems optimized for increasingly demanding performance and multifunction flexibility requirements.

The higher breakdown voltage performance of GaN semiconductor technology allows scaling to a higher operational voltage, which minimizes power loss, reduces power supply demand, allows for wideband impedance matching, enables system designers to use smaller energy storage capacitors and reduces current handling within the power supply system. High breakdown voltage significantly improves ruggedness under load mismatch conditions and allows for greater flexibility in signal wave-forms for multifunction roles. GaN-enabled power efficiency gains improve overall thermal performance, which provides greater flexibility in pulse and CW operational modes.

GaN semiconductor technology has clearly set a new standard in power performance for power transistors targeted at military radar and communication systems. Going forward, it is the implementation of this GaN technology – the packaging and assembly – that will require our focused innovation if we are to unlock the full benefits of GaN.

To date, most vendor approaches to applying GaN to power amplifiers have relied on packaging techniques found in earlier generation ceramic, flange-mount packaged devices like Si LDMOS and Si BJT. By replacing the silicon material in these packages with GaN, improvements in power density and efficiency have been achieved.

But GaN is not Si LDMOS or Si BJT, nor should it be thought of as a mere enhancement of a power semiconductor. GaN on SiC offers the chance to make a complete paradigm shift based on the fact it can dissipate heat better and run hotter more reliably than any other power semiconductor technology to date. The continued reliance on conventional ceramic packaging has not yielded meaningful reductions in component size or weight, or for that matter performance and cost – nor has it taken advantage of advances in commercial, surface-mount packaging technologies. Metal-ceramic based flange mount packages have a number of limitations compared to plastic, surface-mount packages, including larger size, manual assembly, higher cost, more challenging impedance matching and additional board manufacturing.

Benefits of GaN in Plastic Packaging

The landscape of the GaN power transistor market has shifted radically with the introduction of new GaN in plastic power transistors. Ideally suited for high-performance military radar applications, GaN in plastic-based power transistors defy the power, size and weight limitations of competing GaN-based offerings to enable a new generation of high-power, ultra-compact radar systems for use in mobile and/or multifunction battlefield electronics systems.

MACOM has demonstrated the benefits of GaN in plastic packaging technology with new GaN in plastic power transistors that scale up to 100 W – among the industry’s highest power levels for this product category. Achieving this level of power performance requires sophisticated thermal dissipation techniques to ensure reliability that’s comparable to conventional ceramic-packaged GaN-based offerings.

By optimizing the transistor die layout and using advanced heat sinking and die attachment methods, these GaN in plastic power transistors have demonstrated less than 115°C average junction temperature (80°C base-plate) for a pulsed power output of 93 W, using a 1 mS pulse, 10 percent duty cycle. These performance metrics have been verified using stringent thermal imaging testing methodologies.

These transistors operate at 50 V drain bias resulting in outstanding power density and performance, higher efficiency and smaller impedance matching circuits due to improved device parasitics. The high voltage operation also benefits overall system design with smaller energy storage capacitors and lower current draw.

GaN in plastic-based power transistors are also extremely lightweight compared to ceramic-packaged GaN-based offerings. Combined with significant size reduction in the external application solution and aggregated across the hundreds of power amplifiers within a typical modern military radar system, this can reduce overall system weight considerably. The resulting weight reduction ensures greater ease of movement for mobile radar systems.

The first entries in MACOM’s GaN in plastic power transistor product portfolio include 90, 50 and 15 W transistors, all of which are available in standard 3 × 6 mm DFN packaging. The devices can be mounted on PCBs via ground/thermal arrays. Internal stress buffers allow these devices to be reliably operated at up to 200°C channel temperature. All of these transistors are capable of operating at frequencies up to at least 3.5 GHz.

Ease-of-Assembly Evolution

The GaN in plastic approach also allows for ultra-small, fully matched, integrated module solutions. The next evolutionary step is to develop high gain power modules based upon the GaN in plastic power transistors for the L- and S-Band radar markets. These modules can be fully matched with two stages of high gain and are realized using surface-mount technology (SMT) assembly on a compact RF board. The GaN power transistors are assembled using standard reflow techniques and the module can be easily integrated into a radar system front end.

The ability to offer a full SMT solution using GaN combines the best of advanced military power technologies and high volume commercial manufacturing expertise. With this combination, it is possible to break through the current boundaries of SWaP and realize a new level of performance and capability in future radar systems.

From Military to the Mainstream

As mass production and deployment of GaN in plastic power transistors accelerates, we can expect this class of product to follow a commoditization trajectory that aligns with most other plastic-packaged products, both military and commercial. Manufacturing efficiency gains and cost of material reductions drive reduced production and product costs, which drive volume demand – a historical trend followed by many product types after they translate to lead-frame based plastic packaging.

This naturally leads to penetration into other markets. With the advent of low-cost GaN in plastic power transistors, we can envision this technology being deployed in government and commercial applications including centralized and remotely-based weather and early-warning radar systems, and even radar-driven automotive applications such as advanced driver assistance.

AESA technology itself has already extended beyond the military domain into commercial communication link equipment, and it will find its way into other non-military applications in the years ahead. The accelerating proliferation of AESA systems, which can integrate hundreds to tens of thousands of active RF elements, will contribute to the growing ubiquity of low-cost GaN in plastic power transistors – and vice versa.

Of course, the military domain is more sensitive to dual-sourcing requirements than the commercial domain, and dual-sourced GaN-based components have been hard to come by to date. But this too is changing. Dual-source supply chains for high-performance GaN devices are now a reality with the recent partnership between MACOM and Global Communications Semiconductors (GCS). This dual-source agreement – an industry first – unlocks one of the key remaining barriers to mainstream GaN market adoption by providing a secure supply chain.

New Generation of High-Performance Military Radar Systems

Continued innovation in high-power GaN in space-saving plastic is enabling radar system designers to take full advantage of GaN technology and achieve new levels of power density while significantly reducing system size and weight. Utilizing sophisticated packaging and thermal management techniques, GaN in plastic power transistors, accompanied by industry small application solutions, are helping designers overcome challenging development hurdles and pioneer a new generation of high-performance, rugged radar systems for mobile and multifunction military radar applications that transcend the capabilities of systems based on conventional power transistors.