Conventional silicon MOSFETs are used for THz detectors in order to facilitate fabrication of cost-efficient circuits with high integration density. Resistive mixers based on NMOSFETs are investigated by drift diffusion simulations, which include the time derivative of the current densities usually neglected in TCAD codes. Different time-integration schemes are investigated for transient simulations and the modified bacicward differentiation formula is found to be the most CPU-efficient method for the periodic steady-state. By comparison with the Boltzmann transport equation it is shown that the drift-diffusion model can capture the salient aspects of transport in the THz range. The features of the device simulator are demonstrated by investigation of the current and voltage responsivity together with the noise equivalent-power for a resistive mixer based on a quarter-micron NMOSFET. (C) 2016 Elsevier Ltd. All rights reserved.