One well understood effect found at decreasing channel lengths is a reduction
of the threshold voltage 
.
Figure 9.4:
Short channel effect in a simulated NMOS transistor.
Figure 9.4 shows the
threshold voltage of an NMOS transistor versus the gate length,
with the gate length being varied from 
to 
.
For lengths smaller than a quarter of a micron, the device shown does not
turn off any more and no threshold voltage can be established.
The decrease of the threshold voltage is caused by the increased
fraction of the channel-depletion region that is occupied by the
space charge in the junction depletion regions between source and
channel, and channel and drain, respectively, and the resulting
decrease in gate charge needed to cause inversion in the channel.