It has been observed through experimental computation that subthreshold slope (swing) (SS) for n-FET and p-FET operating at cryogenic temperatures, reaches a fluctuating saturation value violating the thermionic emission related Boltzmann limit that at T = 0 K, SS should pass through the origin when SS plotted on Y-axis and temperature T plotted in the X-axis. Some very plausible analysis have been presented and analytical model has been developed to support these device physics based analyzed concepts by these articles. Yet, the real cause for fluctuating saturation value of SS near 0 K need more cogent analytical understanding. The authors propose here that channel thickness variation caused by (1) random fluctuations in dopant density (predicted by atomisticity for low volume when the channel doping is lower) and their very severe incomplete ionization near 0 K leading to sparse channel carriers for n-FET and p-FET, (2) very steep surface band bending induced generation of electrons in the wider depletion region as T is reduced further and these electrons subsequently migrating to the channel attracted by the gate potential and forming a steady state, (3) deep density of interface trap D_it (number/cm²-eV) much greater than 10¹⁰/cm²-eV and their occupation by electrons in the band tails formation near conduction band for n-FET and valence band near p-FET, subsequently screening the gate field to penetrate into the depletion region and making it wider rather than almost fixed. This fixes the depletion capacitance C_D and (4) trap assisted tunneling (TAT) from interface traps occupied electrons to source sided channel at steady state, also keeps a certain number of electrons near the source side that cannot be removed by the thermal voltage V_T = kT/q placed on the gate potential as T is reduced to 0 K.

Cryogenic temperature, Subthreshold swing saturation, Channel thickness variation physical insights, Density of states band tails, Interface states, Trap assisted tunneling