Low-Loss Coplanar Waveguides on GaN-on-Si Substrates

Low-loss coplanar waveguides (CPWs) on AlGaN/GaN HEMT heterostructures grown on high-resistivity Si substrates (GaN-on-Si) are demonstrated. Performance has been evaluated from 0.1 to 20 GHz, and loss as low as 0.27 dB/mm at 20 GHz is achieved. The performance of CPWs fabricated with ion implant- and mesa etching-based processes are compared, and finite element simulations are used to analyze the CPW loss mechanisms. In this frequency range, the dielectric loss associated with the GaN-on-Si substrate is below 0.065 dB/mm, compared to approximately 0.01 dB/mm for reference GaN-on-SiC CPW lines. The extra loss in the GaN-on-Si CPWs is due to a parasitic conductive layer with a sheet resistance of 2 × 10
⁴
Ω/□. The low loss obtained, combined with the economic and integration advantages of GaN-on-Si substrates, makes them promising for GaN monolithic microwave integrated circuits.