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Gate Voltage threshold and fringing effects

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IvanTheTerrible

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Voltage threshold

Below is an explanation for the
Threshold voltage increases for narrow NMOSFETs.

In the middle of the transistor, the field effect is similar to that for infinite
parallel plates. Near the edges, the fringing effect results in a higher required
voltage difference to achieve inversion. For narrow transistors, the
appropriate model moves from infinite parallel plates to a line and plate.

I don't really get it, can someone help me?
 

Re: Voltage threshold

As the gate length becomes very narrow, the fringed electric field area along the edges of the gate (at the source and drain) become the majority of the electric field between the gate and channel. The level 1 mosfet model used to do hand calculation does not account for the fringing effects, it assumes the electric field is straight up and down between gate and channel.

But if you want to create a model that accounts for this fringing effect, it would be a closer approximation to assume the gate is not a plate but a line of charge that has an electric field interacting with the channel below. It is the next simplest shape to model.
 

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