Reducing the gate-length of a MOS transistor to dimensions smaller
than about 2 
alters the electrical behavior of the device
in a characteristic way.
The influence of the source and drain regions on the channel grow,
i.e., the fraction of the channel that is controlled by the gate
decreases with respect to the long channel device
,
where the transition region from the channel to the source and
drain regions is small in comparison to the gate length.
These short channel effects
cause considerable distress for the device designers, because
one-dimensional device equations - widely used in circuit-design
applications -
do not any more give accurate results.
In consequence, it is a declared goal of VLSI process design to
sustain long-channel behavior even for short-channel devices.