velocity saturation in devices

[rp276]

why velocity saturation occurs in short channel device and not in long channel device..????:?:

 

[rahdirs]

@rp276: At highfield strengths, the carriers fail to follow the linear model of vn = un*E. For p-type silicon, the critical field at which electron saturation occurs is around 1.5*10^6 V/m (or 1.5 V/um).This means that in an NMOS device with a channel length of 1 um, only a couple of volts between drain and source are needed to reach the saturation point.

So,voltage required to velocity saturate is given by V = E*l(assumed linear).
when,l is small(small channel),V required to saturate is small.(V = 1.5V/um * 1um = 1.5V)
But,when l is large,the voltage required to velocity saturate it is significant and you usually don't work with those kind of voltages.(V = 1.5V/um *100um = 150V)
The above calculations are approx,but should clear your idea.