Ultracompact On-Chip Branch Line Coupler Based on Through-Silicon Via Technology

A compact branch line coupler with lumped elements on silicon substrate is realized by employing through-silicon via (TSV) technology, in which the lumped elements adopt high-density, high-Q 3-D solenoid inductors, and double-layer interdigital capacitors, and the proposed coupler is fabricated and measured. The results show that the return loss of the coupler is less than -19.7 dB, the isolation is less than -15.2 dB at 4.7–5.3 GHz, and the phase imbalance within the operating frequency range is within ±1.2°. In addition, the size of the branch line coupler is only $1.2\times 1.36$ mm² ( $0.069\times 0.078~\lambda _{\text {g}}^{2}$ ).