why slope of Id-Vds curve is not equal to gds at weak inversion range?

[cqmyg5]

Hi,

I run a simple DC simulation for a single NMOS to get Id-Vds curves, then plot the derivative of the Id-Vds curve to get slope value, it is equal to gds value in simulation output file at strong inversion range; However at weak inversion range or median inversion range, the slope value is 50% or more lower than gds.

Anybody know why?

 

[jimito13]

Hi cqmyg5,

As the inversion coefficient decreases (that means you are going from strong to weak inversion progressively) while holding drain current and channel length fixed,the transistor's rout decreases as well,so gds increases.This is the correct behavior.

Regards,
Jimito13