Aspects of the present disclosure are directed to a photonic integrated circuit (PIC) having a resistivity-engineered substrate to suppress radio-frequency (RF) common-mode signals. In some embodiments, a semiconductor substrate is provided that comprises two portions having different levels of resistivity to provide both suppression of common mode signals, and reduction of RF absorption loss for non-common mode RF signals. In such embodiments, a bottom portion of the semiconductor substrate has a low resistivity to suppress common mode via RF absorption, while a top portion of the semiconductor substrate that is adjacent to conductors in the IC has a high resistivity to reduce RF loss.