Current source
A single FET can be used as a current source when $V_{s}$ and $V_{g}$ are held at constant values and as long as the difference between them is $>4 U_{T}$ or ($|Vd−Vs| ≥ 100mV$). (in saturation region)
$I_{ds} = I_{0}e^{\frac{(\kappa V_{g}-V_{s})}{U_{T}}}$
The drain current is independant of the drain voltage. A device that supplies an output current that is independent of the voltage applied to the same terminal is called a current source (The Early effect makes the transistor deviate from an ideal current source, but by making larger and longer transistors we can counter this deviation).