In our previous work [Sun et al., Appl. Phys. Lett. 100, 013506 (2012)], we inferred the existence
of localized self-mixing in an antenna-coupled field-effect terahertz detector. In this Letter, we
report a quasistatic self-mixing model taking into account the localized terahertz fields and its
verification by comparing the simulated results with the experimental data in a two-dimensional
space of the gate voltage and the drain/source bias. The model well describes the detector
characteristics: not only the magnitude, but also the polarity, of the photocurrent can be tuned. The
existence of strongly localized self-mixing in such detectors is confirmed.