Fully conductive band tails cause the subthreshold swing to saturate at temperatures above 1 K. However, recent measurements indicate that below 1 K, the subthreshold swing in certain MOSFET structures resumes a linear scaling with temperature. Following this ultra-steep behavior, a new type of plateau has been measured below 1 K. In this letter, we show that hybrid band tails, with both traps and mobile states, explain this new plateau. Furthermore, hybrid band tails explain various non-saturating behaviors above 1 K. Remarkably, for entirely non-conductive band tails, the simulations and theory predict a third type of plateau below 10 mK. We hypothesize that this represents the lower bound of subthreshold swing at sub-Kelvin temperatures, which is a testable prediction from the theory.