In electronics, short-channel effects occur in MOSFETs in which the channel length is comparable to the depletion layer widths of the source and drain junctions. These effects include, in particular, drain-induced barrier lowering, velocity saturation, quantum confinement and hot carrier degradation.

Electrostatic effects

Electrostatic short-channel effects arise in a planar NMOS) becomes short enough for the source and drain to exert significant electrostatic influence over the channel, reducing the gate’s relative control over the channel potential.). Under the simplified ideal long-channel conditions used here, the threshold voltage) is independent of the NMOS’s channel length and is represented as a function of the gate-oxide) and the acceptor doping concentration). The threshold-voltage dependence in a long-channel NMOS is thus represented by:

—the source–channel barrier strongly suppresses the flow of electrons into the channel, albeit a smaller subthreshold current can still flow. is sufficiently high, the source–channel barrier is lowered, and the conduction-band edge.

becomes sufficiently small, the depletion regions of the source and drain begin overlapping underneath the channel.), where:

) depend on the relative strength of lateral electrostatics versus transverse electrostatics. The gate controls the channel from above through transverse electrostatics, while the source and drain influence the channel from its ends through lateral electrostatics. The electrostatic problem therefore becomes two-dimensional in a short-channel NMOS. The central channel is now no longer well isolated from source and drain influence; these influences extend into the middle and the gate cannot control the channel potential as independently or uniformly. This is described as “electrostatic integrity”, which is the extent to which the gate maintains effective control over the channel potential. A short-channel NMOS thus has reduced electrostatic integrity as lateral source and drain electrostatics become stronger relative to transverse gate control.

The division of electrostatic control—the support of depletion charge beneath the gate—between the gate and the source and drain within an NMOS is known as "charge sharing". In a long–channel NMOS, the gate supports most of the depletion charge beneath it and has dominant control over the channel potential, and must provide most of the electrostatic influence required to reach the threshold condition in the channel, while the source and drain are far enough away that their electrostatic fields support only a small share of the central depletion charge. However, a short-channel NMOS sees both the source and drain having more electrostatic influence, resulting in less gate dominance and less gate voltage being required to reach the threshold condition, as the source and drain now support a larger share of this depletion charge.

as in a short–channel NMOS, the gate has a smaller electrostatic contribution to make before reaching the threshold condition in the channel owing to increased electrostatic influence from the source and drain. is therefore reduced. This is known as threshold-voltage roll-off.

Threshold-voltage roll-off is related to drain-induced barrier lowering in an NMOS whose length is held constant, causing the drain depletion region to expand, thus increasing the drain’s electrostatic influence and lowering the source–channel barrier...

See also

• Channel length modulation

• Reverse short-channel effect

References