In this paper, we explain the THz detection mechanism in sub-threshold Si MOSFETs by exploiting the exponential dependence of channel electron density to the gate-source voltage. According to our theory, this high frequency non-linear dependence is the underlying mechanism for rectification of THz radiation. The maximum detection frequency is limited by dielectric relaxation time of the electrons inside the channel close to the source region, which lies well into the THz range. Extensive two-dimensional TCAD device simulations in time domain support our proposed theory.