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  1. For millimeter-wave power applications, GaN high-electron mobility transistors (HEMTs) are often grown epitaxially on a high-purity semi-insulating c-axis 4H-SiC substrate. For these anisotropic hexagonal materials, the design and modeling of microstrip and coplanar interconnects require detailed knowledge of both the ordinary permittivity ε⊥ and the extraordinary permittivity εǁ perpendicular and parallel, respectively, to the c-axis. However, conventional dielectric characterization techniques make it difficult to measure εǁ alone or to separate εǁ from ε⊥. As a result, there is little data for εǁ, especially at millimeter-wave frequencies. This work demonstrates techniques for characterizing εǁ of 4H SiC using substrate-integrated waveguides (SIWs) or SIW resonators. The measured εǁ on seven SIWs and eleven resonators from 110 to 170 GHz is within ±1% of 10.2. Because the SIWs and resonators can be fabricated on the same SiC substrate together with HEMTs and other devices, they can be conveniently measured on-wafer for precise material-device correlation. Such permittivity characterization techniques can be extended to other frequencies, materials, and orientations. 
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    Free, publicly-accessible full text available July 3, 2024
  2. Currently, lacking suitable test structures, little data exist for the permittivity of hexagonal materials such as GaN and SiC at millimeter-wave frequencies, especially for the extraordinary permittivity ε || as opposed to the ordinary permittivity ε ⊥ . This paper demonstrates for the first time that it is possible to characterize ε || of c-axis 4H SiC using on-wafer measurements of substrate-integrated-waveguide resonators. In fact, measurements on eleven resonators yield a relative ε || of 10.27 ± 0.03 and a loss tangent tanδ<0.02 over the D band (110-170 GHz). The on-wafer measurements of resonators and other devices fabricated on the same SiC substrate can allow material property to be closely correlated with device performance. The present approach can be extended to materials of other types and orientations. 
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    Free, publicly-accessible full text available January 22, 2024