Coplanar waveguide performance comparison of GaN-on-Si and GaN-on-SiC substrates

A comparison of coplanar waveguides (CPWs) for MMIC applications fabricated on AlGaN/GaN HEMT heterostructures grown on both high-resistivity Si (GaN-on-Si) and semi-insulating SiC (GaN-on-SiC) substrates is reported. In addition to the two substrate types, two fabrication process flows-one suitable for mesa-isolated MMICs and the other appropriate for MMIC flows incorporating implant isolation-were evaluated. The propagation and loss performance of the CPWs on the different substrate types was assessed from 100 MHz to 20 GHz. While the ohmic loss associated with the metal lines was comparable between the two substrate types, some differences in the dielectric loss were observed. For the GaN-on-Si substrates, the dielectric loss contributes ~0.1 dB/mm to the line loss, while the GaN-on-SiC substrates show less than 0.01 dB/mm. To gauge the impact for circuits, X-band λ/8 open-circuited stubs (for matching) and quarter-wave short-circuited stubs (e.g. for bias) were designed and compared. The obtained reflection coefficients suggest that while GaN-on-Si CPWs have more loss, matching network performance can be expected to be within ~0.3 dB of those for GaN-on-SiC. From this study, it appears that GaN-on-Si substrates have interconnect performance for MMICs that is nearly as good as those on GaN-on-SiC substrates, demonstrating excellent potential for high-performance GaN MMICs.