The subthreshold swing (SS) of MOSFETs decreases with temperature and then saturates below a critical temperature. Hopping conduction via the band tail has been proposed as the possible cause for the SS saturation. On the other hand, numerical simulations have shown the source-to-drain tunneling (SDT) current limits the SS at low temperatures. It has been argued which transport mechanism dominates the cryogenic subthreshold current. Hence, for the first time, this letter presents an analytical model of the SDT current and the corresponding SS, which is validated by cryogenic measurement on devices from an advanced 16 nm FinFET technology.