  We report on the modeling and experimental characterization of Si CMOS
detectors of terahertz radiation based on antenna-coupled field-effect
transistors (TeraFETs). The detectors are manufactured using TSMC's 65-nm
technology. We apply two models -- the TSMC RF foundry model and our own
ADS-HDM -- to simulate the Si CMOS TeraFET performance and compare their
predictions with respective experimental data. Both models are implemented in
the commercial circuit simulation software Keysight Advanced Design System
(ADS). We find that the compact model TSMC RF is capable to predict the
detector responsivity and its dependence on frequency and gate voltage with
good accuracy up to the highest frequency of 1.2 THz covered in this study.
This frequency is well beyond the tool's intended operation range for 5G
communications and 110-GHz millimeter wave applications. We demonstrate that
our self-developed physics-based ADS-HDM tool, which relies on an extended
one-dimensional hydrodynamic transport model and can be adapted readily to
other material technologies, has high predictive qualities comparable to those
of the foundry model. We use the ADS-HDM to discuss the contribution of
diffusive and plasmonic effects to the THz response of Si CMOS TeraFETs,
finding that these effects, while becoming more significant with rising
frequency, are never dominant. Finally, we estimate that the electrical NEP
(perfect power coupling conditions) is on the order of 5 pW/$\sqrt{\rm{Hz}}$ at
room-temperature.
