Saturation and pinch off in enhancement mosfets

  • Thread starter Genji Shimada
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In summary, Pinch-off is the edge of saturation in MOSFETs, where the inversion layer underneath the gate is pinched off at the drain. As VDS is increased, the channel remains pinched off but the depletion region between the inversion layer and the drain gets larger, causing the effective length of the MOSFET to decrease. This results in a slight increase in current, even in saturation region. The value of Vds that causes pinch-off is VGS-VT, which is the definition of saturation. This slight increase in current is due to channel length modulation, which is caused by the depletion region getting larger as VDS increases.
  • #1
Genji Shimada
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Hello! Is there a difference between satutation and pinch off in nmos transistors? Because my research in the internet for Mosfet pinch off leave me think that there is no difference between the two. Which confuses me, because pinch off means that there is no current at all, and saturation means that there is, but it has reached its maximum value. Also i don't think i fully understand the process of pinch off in enhancement mosfets. My understanding is that once the inversion layer is formed and a voltage is applied between the drain and the source, current start flowing. If we increase Vds too much, the strong electric field of the drain began to sink electrons from the chanel and it shrinks. If we increase it even more, the drain sinks so much electrons that basicaly a depletion layer is formed. Is that correct? Also how to determine at what value of Vds pinch off will occure?
 
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  • #3
Hi Genji,

Pinch-off is the edge of saturation (the point where the triode region goes to the saturation region). it is not the current being pinched-off but rather the inversion region underneath the gate (it is pinched off right at the drain). As you increase VDS while in saturation region, the channel remains pinched off but the depletion region between the inversion layer and the drain gets larger.

Remember that in triode region the MOSFET works as a linear resistor, and when you increase VDS the inversion layer under the gate gets thinner and thinner near the drain. The current goes up also when you increase VDS because of Ohm's Law. When the inversion layer right below the gate gets so thin it disappears this is called the edge of saturation.

The value of Vds that causes pinch off is just VGS-VT since this is just the definition of saturation (VGS > VT & VGD > 0).
 
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  • #4
analogdesign said:
Hi Genji,

Pinch-off is the edge of saturation (the point where the triode region goes to the saturation region). it is not the current being pinched-off but rather the inversion region underneath the gate (it is pinched off right at the drain). As you increase VDS while in saturation region, the channel remains pinched off but the depletion region between the inversion layer and the drain gets larger.

Remember that in triode region the MOSFET works as a linear resistor, and when you increase VDS the inversion layer under the gate gets thinner and thinner near the drain. The current goes up also when you increase VDS because of Ohm's Law. When the inversion layer right below the gate gets so thin it disappears this is called the edge of saturation.

The value of Vds that causes pinch off is just VGS-VT since this is just the definition of saturation (VGS > VT & VGD > 0).
Thank you!
 
  • #5
It is said that the current saturates at a certain Vds, but actually it doesn't. There is still a very little increase in the source to drain current, may be very little , almost negligible but still there are some increment. Why is that? I got it in my lab project for MOSFET while observing the output characteristics of MOSFET.
 
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  • #6
sua217 said:
It is said that the current saturates at a certain Vds, but actually it doesn't. There is still a very little increase in the source to drain current, may be very little , almost negligible but still there are some increment. Why is that? I got it in my lab project for MOSFET while observing the output characteristics of MOSFET.

In an ideal MOSFET, the current does saturate. However, an ideal MOSFET as infinite output resistance and real MOSFETs have finite output resistance, as you have observed in your lab project. I gave the reason in an earlier comment:

analogdesign said:
As you increase VDS while in saturation region, the channel remains pinched off but the depletion region between the inversion layer and the drain gets larger.

This is called "channel length modulation" and you can look it up. The core reason is that when you increase VDS the depletion region gets larger and it makes the effective length of the MOSFET shorter. Since the drain current in saturation is proportional to W/L this means the current increase.
 

What is saturation in an enhancement MOSFET?

Saturation in an enhancement MOSFET occurs when the transistor is fully turned on and there is a constant flow of current between the source and drain terminals. This results in a low resistance state and the transistor acts as a closed switch.

What is pinch off in an enhancement MOSFET?

Pinch off in an enhancement MOSFET refers to the point at which the channel between the source and drain is fully constricted, limiting the flow of current. This is caused by the depletion region between the gate and channel expanding, effectively blocking the flow of electrons.

How does the gate voltage affect saturation and pinch off in an enhancement MOSFET?

The gate voltage is used to control the current flow in an enhancement MOSFET. When the gate voltage is increased, it allows more electrons to flow through the channel, resulting in a lower resistance state and saturation. As the gate voltage is further increased, the depletion region expands and eventually pinches off the channel, limiting the current flow.

What is the difference between saturation and linear regions in an enhancement MOSFET?

The saturation region in an enhancement MOSFET is when the transistor is fully turned on and there is a constant flow of current between the source and drain. In the linear region, the transistor is partially turned on and the current increases proportionally to the gate voltage. The transition between these two regions is known as the pinch off point.

How can saturation and pinch off be controlled in an enhancement MOSFET?

Saturation and pinch off can be controlled by varying the gate voltage. Increasing the gate voltage will move the transistor into saturation, while further increasing the voltage will result in pinch off. Additionally, the physical dimensions of the transistor, such as the width and length of the channel, can also affect saturation and pinch off.

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