This paper presents an experimental investigation, compact modeling, and low-temperature physics-based modeling of a commercial 28-nm bulk CMOS technology operating at cryogenic temperatures. The physical and technological parameters are extracted at 300, 77, and 4.2 K from dc measurements made on various geometries. The simplified-EKV compact model is used to accurately capture the dc characteristics of this technology down to 4.2 K and to demonstrate the impact of cryogenic temperatures on the essential analog figures-of-merit. A new body-partitioning methodology is then introduced to obtain a set of analytical expressions for the electrostatic profile and the freeze-out layer thickness in field-effect transistors operating from deep-depletion to inversion. The proposed physics-based model relies on the drift-diffusion transport mechanism to obtain the drain current and subthreshold swing, and is validated with the experimental results. This model explains the degradation in subthreshold swing at deep-cryogenic temperatures by the temperature-dependent occupation of interface charge traps. This leads to a degradation of the theoretical limit of the subthreshold swing at deep-cryogenic temperatures.