Extraordinary permittivity characterization of 4H SiC at millimeter-wave frequencies

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.