Q.What is I_DSS? Write the relation between I_D, I_DSS, V_GS, and V_P.

$I_{DSS}$ is a fixed reference current of a JFET, and the four quantities are linked by Shockley's equation. …

[!TLDR]

$I_{DSS}$ is the maximum drain current at $V_{GS} = 0$, and the drain current at any $V_{GS}$ is $I_D = I_{DSS}\left(1 - \tfrac{V_{GS}}{V_P}\right)^2$.

$I_{DSS}$ stands for the drain-to-source saturation current. It is the largest drain current a JFET can pass in the saturation region, and it occurs when the gate-to-source voltage is zero, i.e. when the gate exerts no controlling reverse bias. As the reverse $V_{GS}$ is increased the channel narrows and $I_D$ falls below $I_{DSS}$. The way $I_D$ depends on $V_{GS}$ is given by Shockley's equation,

$I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_P}\right)^2$ …