MOSFET Review

MOSFET has the following operation regions:

• Cutoff region: When $V_{GS} < V_{TH}$. MOSFET acts like open circuit.

• Triode region: When $V_{GS} > V_{TH}$ and $V_{DS} < V_{GS} - V_{TH}$.
  In this region, the current flowing from drain to source is $I_{D} = {k \over 2} [ 2 (V_{GS} - V_{TH}) V_{DS} - V_{DS}^2 ]$. When $V_{DS}$ is very small, this is approximately $I_{D} = k (V_{GS} - V_{TH}) V_{DS}$ and the MOSFET can be regarded as a resistor of resistance $R_{ch} = k (V_{GS} - V_{TH})$.
  It’s common to use this region in digital circuit because it behaves like a digital switch whose resistance is either 0 or $R_{ch}$.

• Saturation region: When $V_{GS} > V_{TH}$ and $V_{DS} > V_{GS} - V_{TH}$.
  In this region, the current flowing from drain to source is $I_{D} = {k \over 2} ( V_{GS} - V_{TH} )^2$. The MOSFET can be regarded as a voltage-controlled current source.
  It’s common to use this region in analog circuit because it provides the largest gain from input $V_{GS}$.

The above are for NMOS. For PMOS, change the sign of $V_{GS}$, $V_{TH}$ and $V_{DS}$ and other things are similar.