Influence of substrate impurity concentration on sub-threshold swing of Si n-channel MOSFETs at cryogenic temperatures down to 4 K

Abstract

The sub-threshold swing (SS) of Si n-MOSFETs is experimentally and systematically evaluated in a temperature range of 4–300 K with varying the substrate impurity concentration (N sub) from ~1016 to ~1018 cm−3, to obtain a physical understanding of SS at cryogenic temperatures. It is clarified that the temperature and drain current dependencies of SS in n-MOSFETs are well represented by a model composed of mobile tail states and localized interface states, irrespective of N sub. The densities of these states are found to increase with increasing N sub. A physical origin of band tail states is studied by experimentally examining the impact of substrate bias on these states, which can separate the effects of the N sub and the surface electric field. It has been clarified, as a result, that the band tail states can be explained by the impurity-induced model.