In Figure 7.6 the ΔId,lin over time during hot-carrier stress in the linear regime of various n-channel MOSFETs with different channel lengths is shown. From this figure and the literature [180], it can be concluded that hot-carrier stress is a severe problem in short channel devices too. This can be explained in terms of the charge carrier energy distribution and is still an active area of research. As discussed for the temperature dependence, in shorter channels the charge carriers can accelerate without being scattered as much as in a long channels. This reduction in the number of scattering events per second in scaled MOSFETs reinforces the cumulative probability of the carrier ensemble to dissociate Si-H bonds at the semiconductor-oxide interface, since there are more carriers with sufficient kinetic energy. Additionally, electron-electron scattering is more pronounced in shorter channel devices, where more carriers exceed the threshold to significantly elevate the high energy tail of the distribution function than in long channel devices. In summary the average number of hot electrons is increased by electron-electron scattering, which has a significant influence especially for shorter channel devices, since the carriers scatter less with phonons and impurities.
| Figure 7.6: | Hot-carrier degradation expressed as ΔId at room temperature for various drain voltages and channel lengths. The device under test was an n-channel MOSFETs with an EOT of 16nm at a gate voltage of 2.0V. From the plot it can be seen that the general trend over drain voltage is the same for various channel lengths. Nevertheless, the distance between the curves grows with decreasing channel lengths [174]. |