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An enhancement mode Mosfet's current-voltage equation in saturation mode is given by: $$I_D = \frac{\mu C_{ox}L}{2W}(V_{GS}-V_{th})^2$$

The graph:

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My question: I studied that the transfer characteristics can be obtained as long a \$V_D > 0\$. However, isn't it incorrect to just assume that the MOSFET is in saturation?

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    \$\begingroup\$ The formula should be MU * Cox * W /L ...... \$\endgroup\$ Mar 10 '18 at 17:26
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The necessary condition for an NMOS to be in saturation region is: VDS > VGS - VTH or VDS + VTH > VGS

You can see the catch here. If you get your VGS too high, you will be in triode region.

I remember a problem in "Design of analog CMOS integrated circuits" by Behzad Razavi, in which he asks how transfer characteristics will change if VDS is low. Give it a read.

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