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GaN on SiC MMICs: High Power AESA Radar Designs

Apr 22, 2020
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Gallium nitride (GaN), when implemented on a Silicon Carbide (SiC) substrate, is especially suitable for high-power applications like radar. Here, monolithic microwave integrated circuits (MMICs) based on the GaN-on-Silicon-Carbide technology facilitate bandwidths that are unachievable with Gallium Arsenide (GaAs) or laterally diffused metal-oxide semiconductors (LDMOS) commonly used in RF power amplifiers.

Besides leading to higher output frequency ranges, it’s a critical design consideration for the ground radars that are increasingly using the active electronically scanned array (AESA) technology to pack higher power in smaller packages. The solid-state designs in a matrix-node format enable AESA to offer wider fields of view, improving both accuracy and reliability.

The use of multiple transmitters means that the field of view can be steered to improve radar detection and accuracy. Moreover, this matrix-like design enhances reliability by facilitating a graceful degradation.

Below is a closer look at the benefits of AESA topologies and how GaN on SiC MMICs fulfill the AESA design promise.

AESA radar designs

The AESA-based radar designs support multiple modes to perform a variety of tasks such as real beam mapping, air-to-air search, and tracking; they also allow distribution of tasks amongst nodes. The AESA topologies use hundreds of nodes operating independently of each other, which, in turn, facilitates redundant systems without single points of failure.

Furthermore, AESA systems within a single pulse can operate at multiple frequencies across a wide band, which makes jamming attempts harder compared to traditional radar systems. The fact that AESA systems can quickly change frequency and pulse width in a random sequence also lowers the chance of detection by radar-warning receivers (RWR).

At the same time, however, AESA systems demand a higher level of integration to facilitate multiple stages placed within a single package. It’s to improve the gain of a single element and eliminate the need for multiple components in an RF chain.

That, first and foremost, mandates smaller packages to lower heat generation and boost energy efficiency. Next, the RF device must allow higher power per node to expand the frequency range and lower system complexity. Here, a GaN on SiC device fits the bill with application-specific packaging for AESA topologies.

GaN on SiC merits

It’s important to note that while using a single voltage is very beneficial in high-power RF designs like radars, traditional radar systems incorporating high-power designs often employ multiple stages operating at different voltages.

Here, GaN on SiC solutions combine multiple devices to achieve the overall power target in ground-based AESA systems. Take, for instance, Wolfspeed’s CMPA5259050F, which is specifically designed to enable AESA radar designs. It comes in a compact 0.5 × 0.5-inch package that enables multiple devices to be placed side by side.

The MMIC for radar power amplifiers, which operates at 28 V, models all thermal parameters using finite element analysis. That, in turn, leads to robust pulse capability and continuous wave (CW) operation. The higher power per node than competing GaN on SiC transistors also enhances the range and lowers system complexity.

Figure 1: The radar MMIC features 10× higher mean time to failure (MTTF) than competing solutions.

More importantly, it can operate at a junction temperature of 225°C, compared to 150°C for LDMOS and GaAs devices and 200°C for GaN-on-Si devices. The fact that CMPA5259050F operates at 25°C hotter than competing GaN-on-Si devices leads to more thermal headroom, and that significantly boosts efficiency and reliability. The thermal efficiency also leads to an MTTF of 10 years (Figure 1).

Wolfspeed has earned the U.S. Department of Defense Manufacturing Readiness Level 8 designation for its GaN on SiC radar devices. Additionally, CMPA5259050F provides more specific advantages compared to other GaN on SiC offerings in the market.

Wolfspeed RF models

The CMPA5259050F transistor facilitates the following three key benefits in radar designs. First and foremost, while many radar designers rely on the industry models to achieve first-pass success, Wolfspeed uses the same models internally to design application fixtures. That allows radar designers to quickly determine if the design will meet their needs and tune boards to the required frequencies.

Wolfspeed also makes available design files for all application fixtures in the AWR design suite. All simulation files, gerbers, schematics, and BOMs are available for reference designs to replicate, tune, and debug circuits quickly. The files typically use Modelithics libraries, so designers can replace them with standard s-parameter files from vendor websites in case they don’t have access to these files.

Figure 2: Hardware test fixtures quickly demonstrate MMIC performance in the lab.

Finally, Wolfspeed offers test fixtures for radar MMICs, which enable designers to evaluate the performance in the lab (Figure 2). There is also application support for new fixtures in different frequency bands.

First-pass designs

Wolfspeed operates one of the few GaN on SiC foundries. The fabrication process advantage, combined with the availability of internal designs using process design kits (PDKs), bolsters the promise of first-pass design success. That, along with design assistance and testing support, also enables radar system developers to execute a much faster design cycle.

These are crucial advantages for a wide-bandgap (WBG) semiconductor technology that continues to evolve and push boundaries toward higher power density in smaller, lighter, and more reliable designs.

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