So far our discussion was based upon the assumptions that channel
was long and wide enough, so that “edge” effects along the four sides
was negligible, longitudinal field was negligible and electric field at every
point was perpendicular to the surface. So we could perform one-dimensional
analysis using gradual channel approximation. But in
devices where channel is short longitudinal field will not be negligible
compared to perpendicular field. So in that case one-dimensional analysis
gives wrong results and we will have to perform dimensional analysis taking
into account both longitudinal and vertical fields. (which is out of the scope
this course)
When is a channel called a short channel? (i) When junction (source/drain) length is of the order of channel length. (ii) L is not much larger then the sum of the drain and source depletion width.
We have shown below the comparative graphs of I-V characteristics for both long channel and short channel length MOSFETs. From graph, it can be clearly concluded that when the channel becomes short, the current in saturation region becomes linearly dependent on applied drain voltage rather than being square dependent.
Figure 7.3: Comparison of ID vs VDS characteristics for long and short channel MOSFET devices
