90 GHz Branch-line Coupler on GaN-on-Low Resistivity Silicon for MMIC Technology

We demonstrate a quadrature branch-line coupler operating at 90 GHz on GaN-on-low resistivity silicon substrates $(\rho \lt 40 \Omega cm$). To reduce the losses offered by the low-resistivity silicon at 90 GHz, a shielding technique is used where the silicon substrate is covered by a ground plane (Al metal). SiO2 dielectric of thickness $10 \mu \mathrm{m}$ is used as a spacer between the top metal and ground plane to further improve the performance of the coupler. Measured results showed return loss and isolation as low as -25 dB and -16 dB respectively, and coupling loss of $-4 \pm 0.5$ dB from 81 GHz to 101 GHz. The output amplitude imbalance achieved was less than 1 dB. The coupler validates the shielding MMIC technology on GaN-on-low resistivity silicon substrate.