I was reading on a book (Thomas Lee, The Design of CMOS Radio Frequency Integrated Circuits) that in a short channel model, since the phenomenon of drift speed saturation is very relevant, it is a mistake to use the classical formula of the drain current of a long channel MOSFET, i.e. this one (we are supposing a MOSFET in pinch - off region):
It states that in a short channel MOSFET the correct expression becomes this one:
Then it states that, if VGS - VT is not so high, we may anyway use the first formula; if VGS - VT is quite high, we have to use the second one. I do not understand this last statement: in fact the difference between the two MOSFET is their length, and this determines that, with the same VDS applied, there is a high electric field in a short channel Mosfet (and so, speed saturation) and low electric field in a long channel Mosfet (and so, no speed saturation). Why should the choice of the model depend on VGS - VT?