Exploring the Potential of Semiconductors at Millimeter-Wave Frequency: From Permittivities to Applications

Precise modeling and characterization of material permittivity are essential for high-frequency devices operating above 110 GHz. However, existing data at millimeter-wave frequencies are limited, and measurements from different laboratories and techniques on the same material often vary by much as 10%. This article reviews challenges and methodologies for permittivity measurements, using high-purity semi-insulating 4H SiC—a material essential for high-frequency and harsh-environment applications—as an example. We highlight advancements utilizing Fabry-Perot resonators and substrate-integrated waveguides to measure ordinary and extraordinary permittivities, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ε</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ε</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">‖</sub>, and their respective loss tangents across 55–330 GHz. Without measurement artifacts, the measured values of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ε</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> = 9.77 ± 0.01 and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ε</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">‖</sub> = 10.20 ± 0.05 show negligible frequency dependence, while the measured tan <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> of (4.9 ± 0.1) × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−6</sup> <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> increases linearly with the frequency <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i>. The value tan <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">δ</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> = 3 × 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> is the lowest we have measured on any solid and is lower than the previous low-loss standards like sapphire. Applications in millimeter-wave circuits, including state-of-the-art D-band power amplifiers, validate these findings, underscoring their importance for next-generation communication systems such as 6G.