Substrate-dominated RF losses in 200 mm GaN-on-Si coplanar waveguides up to 150 °C

We present an experimental and simulation study of radio frequency (RF) losses in gallium nitride (GaN)-on-Si coplanar waveguides (CPWs) fabricated on 200 mm GaN-on-Si(111) with high-resistivity silicon substrates. The effects of Au metallization thickness (0.5–1.5 μm), substrate resistivity, and temperature (25–150 °C) are systematically examined to separate conductor and substrate contributions to loss. Reference measurements on bare Si confirm that substrate resistivity dominates attenuation, while the properly optimized GaN buffer and epitaxial layers have a negligible influence. Temperature-dependent results show that increased RF losses at elevated temperatures originate from reduced Si resistivity and Au conductivity. Finite-element simulations using an extended CPW model reproduce these trends and quantify the relative roles of conductor and substrate. When Au thickness exceeds 1 μm, measured losses are 0.10 dB/mm at 10 GHz and 0.17 dB/mm at 40 GHz, matching bare-Si performance and approaching GaN-on-SiC levels. These findings establish 200 mm GaN-on-Si as a scalable, low-loss, and thermally robust platform for RF passives and millimeter-wave MMIC integration.