# Channel shape of NMOS in saturation

According to the Sedra/Smith Microelectronic Circuits textbook, 6/7 eds., increasing $v_{DS}$ beyond $v_{OV}$ has no effect on the channel shape and charge. Why is that? It seems as if (thinking naively) increasing $v_{DS}$ should move the point at which the channel is pinched towards the source, thereby decreasing the charge in the channel. Also, if $v_{DS}$ is large enough, wouldn't we have some sort of breakdown?

The textbook says further that any increase in v_{DS} above the saturation threshold appears as a voltage drop across the depletion region. Why is that?

A related question: why is the bottom of the channel a straight line?

• I actually remember that the channel becomes trapezoid from the same book, but cannot remember the condition for it
– C K
Feb 10, 2017 at 20:53