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GaN on SiC Range and Thermal Awareness Eases X-Band Radar Design

Oct 27, 2021
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Gallium nitride (GaN) offers many advantages over other semiconductor technologies as well as traveling wave tubes for amplification needs in a wide range of RF applications. These advantages are responsible for the exponential growth in its adoption by the industry.

Figure 1: Radar technology is used in a wide variety of both military and commercial applications.

According to market research by Strategy Analytics, the RF GaN revenue from the defense sector increased 75× from 2007 to 2019, driving change in defense philosophies and battlefield strategies.i Strategy Analytics expects the radar market to be the single-largest end-equipment segment for RF GaN, voraciously consuming all manner of devices, such as GaN-on-silicon-carbide (SiC) high-mobility electron transistors (HEMTs) and MMIC power amplifiers. 

Radar is used in a wide variety of applications (Figure 1), including fixed and mobile ground-based systems for air traffic control and automotive autonomous driving, respectively; airborne systems for surveillance, fire control, and weather monitoring; and naval systems for navigation, tracking, and surveillance. 

Performance parameters vary with application, and a radar system may need to operate at a frequency anywhere from below L band to beyond Ka band. The 8.5- to 11-GHz X band is rapidly becoming the dominant frequency range, finding use in marine navigation and in multimode active electronically scanned array (AESA) systems. AESA radars control thousands of transmit and receive chains to steer scanning/tracking beams without physically moving the antenna and achieve much higher performance and reliability than traditional architectures.

The GaN advantage 

The GaN market opportunity is driven by the following trends in radar applications

  1. Smaller size to increase portability and ability to combine large arrays at higher frequencies 
  1. Improved fidelity from higher output power to address the need for better target detection and reliability 
  1. Higher efficiency to help reduce system size and simplify overall system budget 
  1. Wider operating frequency to decrease susceptibility to detection 

GaN-on-SiC has the ability to deliver high output power from small, lightweight devices. GaN’s high breakdown field allows higher-voltage operation. GaN power amplifiers also draw less current, which translates to lower operating energy costs and less heat that needs dissipating by the cooling system. The high power density and the lower gate capacitance allows GaN to deliver much greater operational bandwidth than possible with silicon-based devices. GaN-on-SiC also tolerates much higher operating temperatures than silicon. 

The Wolfspeed advantage 

To meet the trends discussed above, Wolfspeed offers a broad portfolio of products to cover the entire satcom and X-band radar range of applications, with four recent product introductions shown as examples in Figure 2

Wolfspeed utilizes its G28V5 high-performance 28-V Foundry process that targets both high-frequency applications as well as lower-frequency operation for the highest-efficiency or wide-bandwidth requirements. The process makes it possible to use a small gate length of 0.15 µm, which decreases gate resistance and the gate-to-drain capacitance, thereby increasing gain and efficiency.ii 

The company has migrated products from metal ceramic packages to overmold to better capitalize on GaN-on-SiC’s size advantages. Parts are offered in QFN packages as small as 5 × 5 mm.

Figure 2: Wolfspeed's product portfolio meets satcom and radar requirements i metal ceramic as well as smaller overmold packages.

The heat from high power density 

The target X-band applications often must deliver multimode capability, operating under both continuous wave (CW) and short-pulse conditions. The additional need for a small form factor by the defense sector amplifies thermal management challenges. 

Although the G28V5 process enables higher efficiency at high frequencies, the increase in power density must still be considered and careful thermal planning during PCB design is essential. 

Thermal impact on reliability 

The device junction temperature, Tj, directly impacts the mean time to failure (MTTF), a measure of device reliability. Because Tj correlates to MTTF, Wolfspeed plots a curve of MTTF versus Tj for all its processes. All of Wolfspeed’s GaN technologies show an MTTF of greater than 10 years at the peak Tj of 225°C.

Figure 3: It is not possible to directly measure the peak temperature in the GaN channel, show as a red dot above.

Because Tj cannot be measured directly, it must be deduced using indirect methods (Figure 3). Infrared microscopy is first used to measure the package case temperature, and then finite element analysis is used to create accurate channel-to-case temperature differentials. From this, the junction-to-case thermal resistance, Rθjc, can be calculated. 

However, Rθjc varies with pulse width and duty cycle and, as earlier mentioned, today’s X-band radar applications often require multimode — both short-pulse and CW — operation. Rθjc increases with  
pulse width and tends toward a fixed CW thermal resistance value irrespective of the duty cycle. 

Engineers must therefore carefully use information from Wolfspeed datasheets to calculate Tj or find it through simulation. 

Mounting solutions for QFN 

Compared with a metal ceramic package, which is mounted directly to the module heatsink, a QFN MMIC typically sits on a multilayer PCB. Therefore, not only is the thermal resistance of the device important but also that of the layers between the device case and the heatsink. 

Although solder has a low thermal resistance, Rth, the PCB thermal resistance could be high and of the same order as the device itself, resulting in a significant rise from the baseplate to the device case. The fixture temperature would therefore need to be sufficiently low to maintain a case temperature within allowable limits. 

When using QFN devices in applications with power dissipation, Pdiss, lower than 30 W CW or those requiring pulses <500 µs and duty cycles <20% using via arrays as the thermal solution may be not only sufficient but cost-effective. 

While not as effective as copper pillars, conductive epoxy filling for the vias helps improve Rth, and tight spacing between vias can support both thermal and low stray inductance requirements. 

For applications with Pdiss higher than 30 W, such as those with a CW-like signal, a solution with lower Rth lies in using an embedded coin, typically copper. Press-fitted into the board, the coin pulls heat away from the package backside. The downside, compared with the via array, is an increase in processing costs and time.

Figure 4: Ansys 3D models of the via array and embedded coin-based mounting of a QFN device.

A Wolfspeed thermal simulation conducted under a CW signal, using Ansys 3D models like those shown in Figure 4, revealed that the embedded coin method achieves realizable fixture temperatures in a real-world application. The via array under those conditions would require a negative fixture temperature to achieve 85°C at the back of the case. 

Products & expertise to suit radar design 

Wolfspeed solutions are available across multiple platforms, with each providing unique benefits and performance capabilities. From MMICs with high power-added efficiencies to IM-FETs as high-power building blocks and GaN HEMTs for multiple power levels, Wolfspeed offers an unrivalled array of products that can be paired to support multiple gain-block transmit chains. 

Find the RF GaN-on-SiC solution for your X-band radar application here, or contact Wolfspeed experts for design guidance.

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