A study of the increased effects of hot-carrier stress on NMOSFETs at low temperature

It is well known that hot-carrier (HC) degradation of NMOSFETs is enhanced at low temperature (LT). Up to now, this has mainly been attributed to the greater mean free path of carriers at LT, which acquire more kinetic energy between two scattering events. It is shown that this is not the unique cause. It is demonstrated that the same density of HC generated defects modifies the electric characteristics of NMOSFETs at 77 K much more than it does at 300 K. There exist several possible explanations for the observed enhancement of degradation at LT: (1) the effect of Coulomb scattering by tapped charges on mobility degradation is relatively more important at LT; and (2) since at LT the Fermi level is closer to the conduction band in strong inversion, acceptor interface state levels near the conduction band are more populated with electrons. From simulations, it is concluded that (1) substantially enhances the effects of stress at LT, while (2) has no effect in weak inversion. Measurements show that the operating voltage of devices that work at 77 K should be reduced even when they are occasionally operating at 300 K.