Electron Velocity in Sub-50-Nm Channel Mosfets

Abstract

Inverse modeling of state-of-the-art NMOSFETs is used to investigate electron transport models and in particular to extract the effective velocity of electron injection from source to channel. It is found that this velocity is less than 50% of the maximum possible velocity, i.e. the thermal velocity of electrons in the source. Based on the Landauer formulation, as adapted by Lundstrom to silicon MOSFETs this indicates that modern NMOSFETs are quite far from their ballistic transport limit and therefore their current is still limited by momentum scattering as manifested in the electron mobility. Investigation of mobility in those transistors reveals that it is reduced with channel length, most likely due to Coulomb scattering by the ionized dopant atoms in the source and drain halos that are necessary for well-tempered ultra-short-channel MOSFETs, and possibly remote scattering by the source dopants.