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# Basic question regarding the operation of n MOSFET

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#### analog_prodigy

##### Full Member level 2
Hi, This is the basic question regarding the mosfet operation.

In the mosfet I-V characteristics derivation,

The charge in the channel is Q = -W Cox (Voxide - Vtn) colomb/cm
where Voxide = Vgs - V(x)
'x' is the position along the length. At x=0 => V(x)=0; At x=L => V(x)=Vg-Vd

i.e., At x=0 => Voxide = Vgs and Q = -W Cox (Vgs - Vtn) colomb/cm
At x=L => Voxide = Vgs - Vds and Q = -W Cox (Vgs - Vds - Vtn) colomb/cm

This indicates that charge per unit length in the channel of a pinched off transistor decreases as one proceeds from source to drain.

However, we know that current entering at drain terminal is equal to the current exiting at source terminal because gate & bulk currents are zero (for normal operation).

How can a constant current exists everywhere in the channel between drain and source if first statement is indeed true?

Current is a chargue displacement. Current is charge in movement.

In that expression you can see charge distribution in channel, but those charges are in movement. That charge distribution defines a current path that is equivalent to a constan resistance. Then you have a constant current, you have a constan charge flux from D to S, but in each instant, chargue distribution inside the channel is not uniform. That means that not all thargues travel at the same speed, or that not al chargues are in movement.

Points: 2