MOSFET Circuit Symbols,
iD-vDS Characteristics.
There are two circuit symbols you may encounter for the
enhancement type MOSFET. For the n-channel, one symbol is

Referring to this circuit symbol:
The arrowed terminal indicates the source,
This arrow direction indicates n-type (direction of current)
The gap at the gate indicates the oxide layer.
However, the body is often connected to the source. This leads to a more common circuit symbol:

Similar circuit symbols are used for p-channel enhancement type
MOSFETS:

MOSFET iD–vDS Characteristics
Similar to a BJT, we can generate a set of iD–vDS characteristic curves for a MOSFET by setting vGS and varying vDS. This is shown in Fig. 4.11 for an n-type MOSFET:

There are three regions of operation:
(1) Cutoff. To operate an enhancement type MOSFET, we first must induce the channel. For NMOS, this means that
v_{GD}= V_{t} (induce)
If vGS < Vt there is no channel and the device is cutoff,
which we see in When the MOSFET is cutoff, iD = iS = 0.
2) Triode. To operate in this mode, we first must induce the channel as in (1) above. We must also keep vDS small enough so the channel is continuous (not pinched off):
V_{GD} >V_{t}
[Note how similar this last criterion is to V_{GD} >V_{t} for the channel to be induced. Here in (2), we have V_{GD} >V_{t} for a continuous channel at the drain end. This observation can help us to remember these criterion.] Another way of writing this criterion in (2) is in terms of vDS. Referring to this circuit element

we see that
V_{DS}=V_{GS}+V_{DG}
For a continuous channel, as required by (2), (3) becomesv
V_{Ds}-V_{GS}=V_{DG}<-V_{t}
Therefore,
V_{DS}GS-V_{t}(continuous)
We can use either (2) or (4) to check for triode operation of
the MOSFET. As given in the last lecture, in the triode region .
where rDS is defined as the (linear) resistance between the
drain and source terminals. The value of rDS is controlled
by vGS.
(3) Saturation. To operate in this mode we need to first induce the channel
V_{GS}³ V_{t}(pinch off)
then ensure that the channel is pinched off at the drain end
V_{GS}³ V_{t}(pinch off)
or equivalently
V_{GS}³V_{GS}(inch off)
As we saw in the previous lecture, the drain current in this
region is

and is not dependent on vDS.
A plot of iD versus vGS for an enhancement type NMOS
device in saturation is shown in

In the saturation mode, this device behaves as an ideal
current source controlled by vGS:

In reality, though, there is a finite output resistance (ro) that
should be added to this model:

where

This finite output resistance gives a slope to the iD–vDS
characteristic curves:

Example N26.1 (similar to text exercise 4.4). Given an
enhancement type NMOS with Vt = 2 V.

Determine the region of operation of this device for the
following VD. Use these criteria for the region of operation:
Cutoff: V_{GS}>V_{t}
Triode: V _{GS}³ V _{t} and V _{DS}GS-V_{t}
Saturation : V_{GS}³ V_{t}and V_{DS}³ V_{GS}-V_{t}

Example N26.2 (similar to text problem 4.16). An NMOS
enhancement type MOSFET has Vt = 2 V. If VGS ranges from 2.5 to 5 V what is the largest VDS for which the channel remains continuous?

V_{ GS} > V_{ t}, V_{ GS} . so the channel is always present.
Then for the channel to remain open at the drain end,
V_{ DS}< V_{ GS} - V_{ t} (triode)
Which VGS to use here? The smallest. Therefore,
V_{ DS}|_{ max} < 2.5-2= 0.5 V