confusion with Noise parameters (gdo) zero drain source conductance

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IvanTheTerrible

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One of the noise parameters gdo:

gdo is the zero drain source conductance.

Is it defined as the change in Id over change in Vgs while VDS =0?

And why isn't gdo zero for all values of Vgs? since when VDS =0 , there is no electric field and no current flow.
 

gd0 is not zero for Vds=0 if Vgs>Vth and it is defined by dVds/dId when Vgs>Vth. Because already the channel has been created. And when there is no current through a resistor, it does not mean that the value of resistor is zero. Also, For sure there is a vertical electric field due to Vgs in the channel.

When Vds is zero, the transistor is in deep triode and gdo=unCox(W/L)(Vgs-Vth).
 
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