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The spectrum congestion in the lower gigahertz region has caused many industries to seek technology solutions in the millimeter-wave spectrum. In fact, technology developments for millimeter-wave solutions in sensing and telecommunications have encouraged millimeter-wave research and development in almost every industry. Specifically, the military and aerospace industries are looking to millimeter-wave technology to increase connectivity and sensing for the next generation of tactical networks, security, electronic warfare (EW), and even active denial systems. This interest is being met with sponsored initiatives or product developments from military organizations, such as the Defense Advanced Research Projects Agency (DARPA), Air Force Research Laboratory (AFRL), and many military/security contractors.

Cleanroom

For its part, the AFRL—a dedicated organization within the United States Air Force—is tasked with developing warfighting technologies in the areas of air, space, and cyberspace. To meet these goals, the AFRL is delving into millimeter-wave technologies including high-power transmit sources, low-noise receivers, control component technologies, and multi-beam transmit arrays. The laboratory’s goal is to enable the Air Force to address the latest in increased bandwidth, data rate, and linearity requirements, thereby enhancing EW and communications systems with millimeter-wave technology (Fig. 1).

“Many of the components we currently work with and will be working with in the future use the benefits of shorter wavelengths to provide an advantage with compact solid-state and vacuum circuits,” states Lewis Kahias, Aerospace Components Technologies Division Principle Engineer for AFRL. “We are looking at opportunities that millimeter waves provide in terms of additional frequency spectrum. In general, we feel that moving up in frequency, whenever possible, will bring additional opportunities with higher data rates and more spectrum to work with.”

By taking advantage of the more compact solid-state and vacuum components available with millimeter-wave technology, the AFRL may be looking to reduce the size and weight of deployed systems. Of course, enabling such developments requires the use of solid-state transistor technologies, such as gallium nitride (GaN) and indium phosphide (InP). Innovative millimeter-wave vacuum-tube electronics also must be developed, which can handle high power and thus exceed the performance of traditional coupled cavity vacuum electronics. 

Kahias comments, “We are developing vacuum electronic devices (VEDs) mainly for their power-level capability. With vacuum electronics, we can do extremely high-power electronics—and it generally doesn’t matter if it’s lower frequencies or millimeter wave. In general, vacuum electronics provide much higher power than solid-state electronics. In a lot of our efforts, we are expecting hundreds of watts of power from VEDs, as opposed to tens of watts from solid state. Solid-state electronics do have advantages with SWaP. We expect low-cost performance from a lot of platforms, so solid-state electronics have a niche with low-power performance.”

In terms of millimeter-wave semiconductor material, GaN is increasingly attracting interest for high-frequency and wide-bandwidth power electronics. Its high power density and high breakdown voltage make it well suited for such applications. For mission-critical applications, the robustness of the device is considered as critical as its performance. While GaN carves its niche, InP technologies are mainly being pursued for low-power and receive devices that require extremely linear and low-noise properties. Although gallium-arsenide (GaAs) technology may not be considered a fit for the latest military applications, it may still find a niche in lower-millimeter-wave, low-power applications.

Realiability testing

All new technology approaches come at the cost of substantial research, which must be done to overcome challenges and limitations. Generally, these higher-frequency electronic materials degrade in performance and efficiency as frequency increases. The added challenge of losing efficiency at millimeter-wave frequencies is managing thermal dissipation and reliability in both solid-state and vacuum electronics in much-reduced dimensions. Modeling these technologies also is a challenge, as no accurate models exist that can predict design performance for high-heat, high-frequency, and small-size compact devices. Fabrication yields and thermal and mechanical reliability issues also increase design complexity while limiting performance predictability. As a result, rigorous testing at a statistically significant level is necessary to produce viable models and reliability data (Fig. 2).

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