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

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 similar to 10(16) to similar to 10(18) 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.