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GaN Gives Power And Flexibility To L-Band Radar

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Increased efficiencies, superior power-handling capabilities and higher breakdown voltages give GaN the upper hand over silicon LDMOS in L-band radar
BY DOUG CARLSON AND ERIC HOKENSON FROM MACOM
One sector where sales of radar are on the rise is air traffic control infrastructure. That is partly because in the developed world, a large swath of civilian radar infrastructure is nearing the end of its operational life, and a mix of upgrades and replacements are on the agenda. Developing nations are also having an impact, deploying their first air traffic control radar systems. These countries are actually in an enviable position, as they can take advantage of new technologies that will allow them to leap frog the capabilities of many legacy systems.

Increasing shipments of these air traffic control radar systems is spurring intensified research and development in the RF and microwave domain, with emphasis on improved performance in the L-band. Radar systems operating in this domain span 1.2 GHz to 1.4 GHz, a range of frequencies that are less prone to atmospheric interference. This asset makes L-band radar ideal for long-range monitoring and volume search capabilities that enable air traffic control stations to detect incoming and outgoing aircraft over vast distances. Augmenting this are S-band and X-band radar systems that cannot probe as far, but provide superior short-range resolution.

The growth of GaN

L-Band radar systems can require thousands of watts of pulsed power, so it is naturally advantageous to employ high-power RF transistors. Armed with high-power discrete transistor "˜building blocks', designers of radar systems have the flexibility to architect multi-kilowatt products with fewer components. This simplifies the design of the system, and cuts its manufacturing cost and complexity.

Various semiconductor materials can be used to construct RF power transistors. The most promising of late is GaN, which is enabling designers of L-band radar systems to realise breakthroughs in power output capability, temperature tolerance and efficiency. 


One of the great merits of GaN is that it offers an eight-fold hike in raw power density over the incumbent silicon LDMOS technology. Thanks to this, it is possible to slash RF component size, thereby allowing designers to harness higher power in smaller enclosures. This is simplifying the development of RF transmitters capable of scaling to 10 kWs and beyond, while maintaining or trimming the system footprint.

These improvements in chip performance lead to gains in the capability of the system. By utilising the high output power enabled by GaN-based RF components, it is possible to extend the surveillance range for L-band radar systems while simultaneously boosting target resolution, leading to better aircraft detection and identification.

Another strength of GaN over silicon is its higher breakdown voltage. This unlocks the door to higher operational voltages and ultimately increased efficiency, in both the device and in the overall power supply of the radar system. Note that these gains in efficiency are far from trivial: Compared to LDMOS technology, GaN delivers an improvement in efficiency ranging from more than 40 percent to 70 percent, depending on operating frequency. This trims the running cost of the system, which is anticipated to be in service for 20-30 years. In addition to the higher efficiency, the high voltage thresholds of GaN-based RF power components allow for increased wideband impedance matching, enabling an L-Band radar system to sustain high performance across the full frequency band.

Turning to GaN for constructing the transistor also leads to greater flexibility over the shaping of the RF signal pulses. Compared to silicon RF transistors, those based on GaN can produce far longer radar pulses "“ and this ensures the focusing of more energy on a target for improved resolution. While a conventional transmitter for L-band air traffic control radar will emit pulses in the range of 100 microseconds, that based on GaN can have a duration of thousands of microseconds.

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Increased flexibility of pulse characteristics is another benefit of GaN. Being able to control pulse length and duty cycle is highly desirable, because it unleashes an opportunity to vary the levels of energy of the radar system, leading to improved detection of a wider array of target objects. Equipped with this capability, civilian air traffic control radar can also serve homeland security applications: Being able to distinguish between a commercial aircraft, an unmanned aerial vehicle and other airborne objects entering the airspace enables increased airspace awareness and enhances threat detection.

Thanks to all the merits of GaN that have been outlined above, this wideband semiconductor is enabling the launch of a new generation of more agile L-band radar systems that are optimised to meet the increasingly demanding performance and multifunction flexibility requirements of modern air traffic control facilities. 

MACOM's 650 W GaN L-band radar transistor in industry standard ceramic flanged package

At MACOM of Lowell, MA, we are helping to drive this revolution in L-band radar by bringing to market a transistor with a record-breaking peak power output of 650 W. This forms part of our portfolio of GaN-on-SiC components that are offered as transistors and pallets and utilise a 0.5 mm HEMT process. This manufacturing technology yields devices with excellent RF performance with respect to power, gain, gain flatness, efficiency and load mismatch tolerance over wide operating bandwidths. 

We use SiC as the foundation for high-power GaN transistors because this substrate has superior thermal properties, making it ideally suited for applications requiring high power densities.

We are not the only manufacturer of GaN-on-SiC power devices, and efforts by several vendors have showcased the capability of this class of wide bandgap chip, particularly in the electronic warfare domain. In our view, the great performance on GaN-on-SiC makes it the clear leader for performance-driven applications.

Our record-breaking transistor is a relatively new member of our family of GaN-on-SiC transistors for L-band pulsed radar applications, adding to a line-up of products delivering 125 W, 250 W and 500 W of peak RF power. We have hit 650 W by assembling a higher number of GaN transistor and silicon capacitor chips into a single package using a eutectic die attach process. Gold wire connects input resonance networks, as well as the gate, source and drain wires; and "˜jumper' wires provide parallel connections to the multiple GaN and capacitor chips. 

With this approach, we have been able to take an industry-standard ceramic package and fill it almost completely with GaN transistor dies, assembling a large number of cells that deliver considerably higher power than their legacy LDMOS-based cousins. This type of transistor benefits from the higher thermal conductivity of the SiC substrate that is very effective at transferring heat laterally and vertically, and ultimately allowing the component to dissipate more power.

19.5 dB, and 60 percent drain efficiency. The device also boasts a very high breakdown voltage, allowing customers reliable and stable operation at 50 V under load mismatch conditions that are more extreme than those possible with older semiconductor technologies. 

One of the benefits of high gain is a reduction in the driver requirements of the final stage. This leads to a further reduction in the number of components and a decrease in the power required to realise the desired performance. 

Meanwhile, thanks to the high efficiency, overall power consumption is reduced, while the higher operating voltage and greater voltage standing wave ratio (VSWR) tolerance increase system efficiencies through bias advantages at higher voltage. Note that there is no significant risk of damage when operating this device in that regime, due to the higher breakdown voltage of GaN.

A pallet-packaged RF solution from MACOM with multiple transistors paralleled in a single modular device



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Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.

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