This paper presents a high‐performance on‐chip diplexer designed for K/Ka‐band applications, fabricated via 130‐nm SiGe BiCMOS across two distinct frequency bands: 17–21 and 27–31 GHz. A localized backside etching (LBE) technique is employed to create air cavities beneath the inductors, a method that fundamentally mitigates substrate‐related losses and significantly enhances the inductor quality ( Q ) factor. The diplexer, based on a compact low‐pass/high‐pass filter topology, achieves low insertion loss and high interband isolation. Measured results demonstrate insertion losses of 2.2 and 2.4 dB for the low band and high band, respectively, and an isolation greater than 40 dB. This experimental validation proves the effectiveness of the LBE technique for realizing compact, high‐performance RF front‐end components in silicon‐based technologies.
Greco et al. (Thu,) studied this question.