What quantitatively is relevant to FETs?

This is a broad question-- but FET behavior can be quantitatively described by model equations (here for JFETs):

• The saturation voltage is given by $V_{ds({\rm sat})}=V_{gs}-V_t$, where $V_t$ is the threshold voltage for turn-on of current $I_d$.
• In the linear region where $V_{ds} < V_{ds({\rm sat})}$, $I_d = K(2 V_{ds({\rm sat})}-V_{ds}) V_{ds}$, where $K$ is a constant.

• In the saturation region where $V_{ds} > V_{ds({\rm sat})}$, we have $I_d = K V_{ds({\rm sat})}^2$.