The nonresonant THz response of CMOS FET has been analyzed based on static nonlinearities of the transistor channel. For the applied gate-to-source and drain-to-source signals, the significance of the second-order nonlinear terms is investigated as a function of dc bias conditions. A cross-coupled design for differential excitation of the THz detector is also proposed, and it has been shown, based on analysis as well as through measurements, that a cross-coupled detector topology can give the highest responsivity in an unbiased-drain operation, among the possible detector topologies. A minimum optical noise equivalent power (NEP) of 29 pW/v Hz is measured for the cross-coupled detector topology at 500 GHz with unbiased drain. The change in the polarity of the detector response under certain bias conditions is also explained. This paper gives a comprehensive picture of the behavior of the CMOS THz detector by discussing the design considerations such as readout modes, optimum bias points, and device dimensions with respect to both responsivity and NEP.