An On-Chip K/Ka-Band Diplexer with Enhanced Inductor Q Using Localized Backside Etching

This work reports a high-performance fully integrated diplexer targeting K-/Ka-band applications, implemented in a 130 nm SiGe BiCMOS technology. The circuit operates over two separated frequency ranges, namely 17-21 GHz and 27-31 GHz. To reduce substrate-induced losses, a Localized Backside Etching (LBE) process is adopted to form air cavities underneath the on-chip inductors, leading to a substantial improvement in their quality factor. The diplexer employs a compact low-pass/high-pass filter architecture, enabling low insertion loss and excellent isolation between the two bands. Experimental results show measured insertion losses of 3.1dB in the lower band and 3.3 dB in the upper band, while the inter-band isolation exceeds 35 dB. These results experimentally confirm the effectiveness of the LBE approach for the realization of compact, high-performance RF front-end building blocks in silicon-based technologies.