# Pinch-Off and Saturation Of MOSFET

• Dr_Pill
In summary, in this conversation, the speaker is asking for clarification on the concept of pinch-off in MOSFETS. They also mention two issues they are having with understanding the topic. The first issue involves the interaction between VDS and VGS in pinch-off, while the second issue is about the direction of the electric field in the depletion region. Another person suggests changing the perspective and analyzing VGD instead of VDS, and explains how the MOS transistor works in different modes. Finally, the concept of electric field and its role in driving electrons is mentioned.
Dr_Pill
Hi,

I don't understand pinch-off in MOSFETS.
I have two big issues:

1.
VDS is pointing from drain to source, so the voltage works in the horizontal plane.VGS on the other hand,is in the vertical plane.
So how they can cooperate? How does increasing VDS till VGS - VTh get you in pinch-off.
Why does increasing VDS above VGS - VTh result in the moving of the pinch-off point to the left?
In what sense are VGS and VDS interacting with each other, I don't see it.

2.
I don't get how electrons are swept from the shortened channel bye the electric field of the depletion regio towards the drain.
With all the voltages from Gate and Drain and built in voltages of depletion regio, I don't know where the electric field is pointing at.

Here's a sketch, a messed up sketch from me in Paint:

Hope some people can explain it to me.

Hi.

Change the point of view of the voltages. Instead VDS, analyze VGD. So, you have VGS, VGD (so VDS = VD - VS = VGS - VGD).
Think about this: A MOS transistor is, in principle, purely simmetryc. (look into the schematic, you can switch S with D).

If VGS > vt, and VGD > vt , (then VDS = VGS - VGD > 0) the MOS works in triode mode (the channel is entirely formed across L).
Now, if you want to analyze another mode, for example saturation:

VGS > vt, VGD < vt (channel isn't formed in the drain-side) then:

VGD = VG - VD < vt ---> VS + VG - VD < vt + VS --> VSD < vt + VSG --> VDS > VGS - vt
(It's easier connecting S to 0 V)
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Why does increasing VDS above VGS - VTh result in the moving of the pinch-off point to the left?

This happen because you're forcing no channel forming en the D-Side.
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I don't get how electrons are swept from the shortened channel bye the electric field of the depletion regio towards the drain.

The Electric Field betweed the Drain and the channel is very large, and the density of electrons it's huge in the channel, so, it's a electrostatic force driving the electrons (some of them, see the ID vs VDS curve).

## 1. What is pinch-off in a MOSFET?

Pinch-off is a phenomenon in a MOSFET (metal-oxide-semiconductor field-effect transistor) where the channel between the source and drain regions becomes completely depleted of charge carriers, resulting in a high resistance state. This occurs when a sufficiently high voltage is applied to the gate, causing the depletion layer to expand and cut off the flow of current through the channel.

## 2. How does pinch-off affect the operation of a MOSFET?

When pinch-off occurs, the MOSFET is said to be in saturation. In this state, the current flowing through the channel is controlled by the voltage applied to the gate, rather than the voltage between the source and drain. This makes the MOSFET an efficient switch, as it can easily transition between low and high resistance states.

## 3. What is the significance of the saturation region in a MOSFET?

The saturation region is an important operating region for a MOSFET, as it allows for efficient switching and amplification of signals. In this region, the MOSFET acts as a voltage-controlled current source, making it suitable for applications such as amplifiers and digital logic circuits.

## 4. How is the saturation region of a MOSFET characterized?

The saturation region of a MOSFET is typically characterized by a plot of the drain current (ID) versus the drain-source voltage (VDS) at a constant gate-source voltage (VGS). This plot shows a linear relationship between ID and VDS, with a constant slope known as the transconductance (gm) of the MOSFET.

## 5. What factors can affect the pinch-off and saturation behavior of a MOSFET?

The pinch-off and saturation behavior of a MOSFET can be affected by various factors such as the voltage applied to the gate, the doping concentration of the semiconductor material, the physical dimensions of the device, and the temperature. Changes in these factors can alter the depletion layer width and affect the overall performance of the MOSFET.

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