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Substrate crosstalk into standard and trap-rich high resistivity silicon (HR-Si) substrates over a wide frequency range, from ultralow frequency (ULF) to extremely high-frequency band (EHF), is investigated using finite-element numerical simulations and experiments. It is demonstrated that low-frequency substrate crosstalk is strongly impacted by the presence of free carriers at the interface between the HR-Si substrate and the interconnection passivation layers. The efficiency of a trap-rich layer, a polysilicon layer thicker than 300 nm, placed at that inter face to recover the nominal high-resistivity characteristic of the Si substrate is theoretically and experimentally demonstrated. Finally, the wideband crosstalk behavior of the HR-Si substrate with and without a trap-rich layer is modeled by means of a simple equivalent lumped-element circuit. The proposed model shows excellent agreement with finite-element numerical simulations and experimental data for frequencies above 100 kHz. Due to the introduction of a trap-rich layer, HR-Si substrate behaves as a lossless dielectric substrate. In that case, a purely capacitive electrical equivalent circuit is sufficient to properly describe the substrate crosstalk characteristics.