MOSFET Transistors – Understanding Triode and Saturation Regions

What do you mean by "input characteristics"?

Textbooks and datasheets describe the behavior of MOSFETs using two graphs:

• Output characteristics: \$I_D\$ versus \$V_{DS}\$ with \$V_{GS}\$ as parameter.

• Transfer characteristic: \$I_{D}\$ versus \$V_{GS}\$ at a given fixed \$V_{DS}\$ value (this latter is chosen so that the MOSFET is in saturation region).

There is no "input characteristic" (such as the \$I_B\$ versus \$V_{BE}\$ curve of a BJT) because the other input quantity besides \$V_{GS}\$, namely \$I_G\$, is virtually zero at DC (and all these curves assume DC operations). Therefore it wouldn't make much sense to plot \$I_G\$ versus \$V_{GS}\$, unless you wanted to analyze leakage gate current, but I assume you are not interested in that.

So it is clear (also by a comment of yours) that by input characteristic you mean the transfer characteristic (TC). Note that the TC is plotted with a fixed drain-source voltage that guarantees that the MOSFET is in saturation for each \$V_{GS}\$ value on the horizontal axis. This is done because the TC is useful when the MOSFET is in saturation, i.e. when the output current depends solely on the input voltage (not considering "Early effect"), for example when you want to use the MOSFET as an amplifier and you need to draw a load line to design its bias circuit.

If you plot the TC for different values of \$V_{DS}\$ you get a family of TC curves. For example consider this circuit simulation with LTspice:

Plotting the TC for different \$V_{DS}\$ values you get:

As you can see, the more you increase \$V_{DS}\$ the more the curve resembles a parabola, as you would expect for the TC in saturation. Notice that this part shows a threshold voltage \$V_{th} \approx 4V\$.

Let's consider what happens if \$V_{DS}\$ is not big enough to drive the MOSFET in saturation for every \$V_{GS}\$ value, like in the lowest blue curve (Note: to present a more revealing plot I selected the curve corresponding to \$V_{DS} = 2V\$, whereas the lowest blue curve above corresponds to \$V_{DS} = 1V\$):

As you can see, in saturation region you get a quadratic curve, whereas in triode region you get a linear curve. Everything as expected, except that real devices don't have an abrupt change between the two regions and that the linearity of the triode region is not perfect because of the device not being ideal (SPICE models usually take into account these effects).

If you see in your simulation an abrupt departure from this behavior it could be that you tried plotting the curves outside the range of the voltages/currents admissible for your device. Notice that I limited the first plot to max 14A/20V which are the absolute maximum ratings for the device I chose. If you don't keep this in mind you will destroy the device (in real life) or get odd results (in simulations).

EDIT (in response to a comment and a question edit)

You ask why the "perfectly" linear curve for \$I_D\$ versus \$V_{GS}\$ in ohmic region is not exploited. Here is some insight:

Why do you need a linear characteristic between input (\$V_{GS}\$) and output (\$I_D\$)? Usually to use the device as a (linear) amplifier. But what are the conditions that allows to have that linearity? \$V_{DS}\$ must be held constant. Therefore to make an amplifier this way you have to insert a load in the output circuit and still keep \$V_{DS}\$ constant. You can understand that such a load cannot be a simple resistor (which is the simplest kind of load). Therefore you need a much more complex circuit (with other active devices).

On the other side, you can use the same MOSFET biased in saturation and get a decent linear amplifier: even if the behavior of the device is not intrinsically linear, but quadratic, there are linearization techniques (e.g. employ simple feedback schemes, like a resistor in series with the source terminal) that allow the overall amplifier to become more linear.

why the saturation region of the MOSFETs has this name?

The post you link to does explain this, but in case it needs repeating and perhaps backing up with a textbook reference, the saturation region for a MOSFET is called so because the drain current saturates, i.e. basically stops increasing as Vds increases further.

You are correct that the active region of a BJT corresponds to the saturation region of a MOSFET when these devices are used as amplifiers.

The saturation region of a BJT (e.g. when turned on as a switch) corresponds to the triode/ohmic region of a MOSFET.

Some authors also call the saturation region of a MOSFET the "active mode", which does match the terminology used for BJTs. But they also call the triode/ohmic region the "linear mode" which perhaps doesn't help that much because "linear" suggests an amplifier rather than a switch. Linear here again refers to how the MOSFET characteristic looks like in that region rather than any external/use considerations. (Luckily, it seems nobody calls the BJT saturation region "linear mode".)

The only thing that's not confusing about this terminology is the cut-off region, which is the same for both. Here's a summary table for the correspondence (from an external/use viewpoint):

This summary also includes the reverse active region for BJTs, which is seldom used, but it doesn't include synonyms for the triode region; as I said "linear mode" or "ohmic region" are also used to denote the MOSFET triode region.