Cryogenic CMOS is a crucial subcomponent of quantum-technological applications. However the standard BSIM4.5 model is only applicable for temperatures between 230 K and 420 K. In this work, N-type MOSFETs with different dimensions in a 65-nm CMOS technology were characterized at room temperature and 4 K, and the measurements were compared to corresponding simulations from the BSIM4.5 model in Cadence’s Spectre simulator. A Mathematica model of drain current in triode region was constructed, allowing key parameters to be modified. By adjusting the temperature-dependent parameters, the modified model predicted the triode region currents with greatly improved accuracy.