Intrinsic Mechanism of Mobility Collapse in Short MOSFETs

The mobility degradation with channel length is examined from the viewpoint of electrostatic effects that alter not only the threshold voltage but also the transport properties. Even operated at low drain voltage, short MOSFETs are subject to a strong lateral electric field present over a significant portion of the channel. The lateral field, responsible for threshold voltage roll-off, causes a local degradation in the velocity and mobility of electrons and holes. The low-mobility regions located near the source and drain tend to merge in short devices, giving rise to an ineluctable mobility collapse that compromises the benefit of downscaling. This mechanism is fundamental and universal as it applies to all kinds of MOSFETs. A model of mobility collapse, totally different from existing theories, is proposed together with supportive numerical simulations.