13.2. I-V Characteristic of MOSFET

Ok, so after the break so we are back here. So, let me continue the graphical interpretation of the I-V characteristic and as an exercise I have asked you to make rewrite this expression of the current. As I said that if VSG is less than threshold voltage, for practical purposes you may say that this is equal to 0.

In this section, we discuss the I-V (current-voltage) characteristics of a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor). The first thing to note is that there is a threshold voltage (Vth), which is a critical voltage level. If the gate-source voltage (VSG) is below this threshold, the current passing through the MOSFET can essentially be considered as 0 because the transistor is off. Basically, a MOSFET will not conduct until the voltage applied at the gate exceeds this threshold.

Think of a light switch: the switch remains off until you press it down (threshold voltage), at which point the light (current) turns on. If you don’t apply sufficient pressure, the light doesn’t turn on at all.

On the other hand if VSG is higher than Vth and VSD is less than |Vth|, this is referred to as the linear region of operation or triode region. If pinch-off occurs, namely VSD exceeds VSG - Vth, the current has hardly any dependency on VSD.

The MOSFET can operate in different regions depending on the applied voltages. When the gate-source voltage is above the threshold and the drain-source voltage (VSD) is lower, the MOSFET is in the linear or triode region. This allows for a controlled flow of current, like a variable resistor. If VSD exceeds a certain value (specifically, VSG minus the threshold voltage), the MOSFET enters the saturation region where the current becomes relatively constant and is less dependent on VSD.

Imagine a water tap: when you slightly open it (linear region), water flows gradually. But if you open it fully, the flow remains constant regardless of how much more you opened it (saturation region).

Let us see what the graphical interpretation of this is, and to start with let me let you consider say for a given value of VSG let you observe I as a function of VSD. So, initially if VSD is less than this voltage which is referred as VSD(sat). So, till that point we may say that it is parabolic in nature, but then instead of really going this parabola beyond that, the current saturates.

In the graphical representation, the I-V characteristics of a MOSFET show that as you increase the drain-source voltage (VSD), the current (I) behaves like a parabola in the lower region, indicating a gradual increase. However, after reaching a certain point (VSD(sat)), the current levels off, indicating saturation, where increasing Volts further does not significantly increase the current.

Think of pushing a swing: initially, it moves higher with each push (increasing current with VSD). But after a certain height, pushing harder no longer makes the swing go higher (current saturation), it just swings back down.

If you decrease the VSD while VSG is above Vth, you will enter the cutoff region when VSD approaches 0, and the current will again be 0, aligning with the V-axis.

The cutoff region occurs when the drain-source voltage (VSD) is low enough that it no longer allows current to flow, even if VSG is above threshold. This behavior is part of the I-V characteristics, where, as you reduce VSD, the current returns to 0, akin to the 'off' state of the switch discussed earlier.

Relate this to a faucet: when you turn it significantly down (decrease VSD), the water stops flowing despite the fact that you've opened it (VSG > Vth).

For different values of VGS and VSD, the characteristics can shift, and the pinch-off will occur at different points, resulting in various operation regions.

Depending on the applied voltages for both the source-gate (VGS) and drain-source (VSD), the I-V curves can change significantly. This means that as you modify how you connect the voltages, you might experience different current values and transitions between the triode and saturation regions at different operating points.

Like tuning a radio: changing the frequency (altering VGS and VSD) causes the radio to pick up different stations (operating regions), each station representing a unique characteristic of the MOSFET.

There is an alternative way of representing this. Instead of plotting I versus VSD, you can plot I versus VDS keeping the standard conventions for p-MOS in mind...

The I-V characteristics can also be represented by plotting the drain-source voltage (VDS) instead of the standard representation. This shifts the graph, reflecting differences in the current flow direction depending on the type of MOSFET (n-MOS or p-MOS). Understanding this alternate representation helps in analyzing the behavior of MOSFETs under various conditions.

Think of mapping a path: each way you choose to draw the path may look different but ultimately leads to the same destination—the current characteristics of the MOSFET.

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