Reverse Short Channel Effect

The reverse short-channel effect is a secondary effect describing the reduction in threshold voltage V_th in MOSFETs with non-uniformly doped channel regions as the gate length increases. Since drive current is primarily determined by V_th, longer channel devices have more driving ability.

Reverse short channel effect (RSCE) is a result of non-uniform channel doping (halo doping ) in modern processes. To combat drain-induced barrier lowering (DIBL), MOSFET channels are more doped near the source and drain terminals to reduce the size of the depletion region in the vicinity of these junctions (called "halo doping" to describe the limitation of this heavy doping to the immediate vicinity of the junctions). At short channel lengths the halo doping of the source overlaps that of the drain, increasing the average channel doping concentration, and thus increasing the threshold voltage. This increased threshold voltage requires a larger gate voltage for channel inversion. However, as channel length is increased, the halo doped regions become separated and the doping mid-channel approaches a lower background level dictated by the body doping. This reduction in average channel doping concentration means V_th initially is reduced as channel length increases, but approaches a constant value independent of channel length for large enough lengths.