On
pages 144 and 145 you discuss how VDS,sat
is not
exactly
VGS
-
VTHN . Why
is the channel
charge distribution, Q'I (y) not
exactly
constant as you move away from
the source? Why does
Q'I
(y)
become zero earlier at the drain?
When
we increase the drain potential we
attract electrons to
the
surface of the MOSFET and enhance
the channel (this is
drain
induced barrier lowering, DIBL,
discussed on page 154).
This
causes the channel charge
distribution to become uneven
and
lowers the value of VDS
needed to saturate the
device.