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I want confirmation of whether a MOSFET is in saturation or not. I have calculated the condition using the formula \$V_{GS}-V_{tn}\$, and my answer is a negative value. Does it matter if the value is negative? If yes, what is the value actually showing?

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    \$\begingroup\$ What does \$ V_{DS} - V_{tn} \$ get you? I don't think I've heard an equation that requires you to subtract the threshold voltage from the drain-source voltage. I've heard, however, of subtracting the threshold voltage from the gate-source voltage, thus getting the drain-source voltage, which can determine the FET's mode of operation. Perhaps you mean \$V_{GS} - V_{tn} \$ instead? \$\endgroup\$ – KingDuken Apr 10 '17 at 3:34
  • \$\begingroup\$ Did you switch channels? from N to Pch_en \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 10 '17 at 6:18
  • \$\begingroup\$ yaa, it is actually Vgs not Vds, my mistake. i have edited it. \$\endgroup\$ – Haziq Mahmur Apr 11 '17 at 8:33
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If \$ V_{GS} \$ is smaller than your than your threshold, \$V_{tn}\$, then that means your FET is in cutoff region, meaning that your drain current \$I_D=0\$ and your FET will not conduct. Here's a good table from the Sedra\Smith textbook that gives a good summary of the regions of operations for the NMOS device, where \$V_{ov}\$ is the overdrive voltage.

enter image description here

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  • \$\begingroup\$ tq for the answer and the reference u gave.! \$\endgroup\$ – Haziq Mahmur Apr 11 '17 at 8:38
  • \$\begingroup\$ You're very welcome :) glad it was a typo. It confused me at first. Just remember to add channel length modulation to the formulas if the question asks to include it. This table also neglects the Body Effect because it's a three terminal device implying that the substrate is tied to the source. \$\endgroup\$ – KingDuken Apr 11 '17 at 15:06
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The check for saturation has two conditions:

\$V_{GS}>V_{th}\$ and \$V_{DS}\geq V_{GS}-V_{th}\$

To rearrrange the second equation for \$V_{GS}\$, you would get the following equation: \$V_{DS}+V_{th}\geq V_{GS}\$. If you somehow ended up with \$V_{DS}-V_{th}\$, that suggests a likely algrebra error.

If you find that \$(V_{GS}-V_{th})<0\$, then that means that you do not meet the other requirement, which is \$V_{GS}>V_{th}\$.

Your transistor is off (cutoff/subthreshold) and will be in neither linear mode or saturation mode.

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  • \$\begingroup\$ yaa it is actually Vgs not Vds. tq for the answer! \$\endgroup\$ – Haziq Mahmur Apr 11 '17 at 8:36

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