What is IDSS of a FET Transistor?

IDSS (referred to as the drain current for zero bias) is the maximum current that flows through a FET transistor, which is when the gate voltage, VG, supplied to the FET is 0V.

When the gate voltage decreases for N-Channel FETs, or increases for P-Channel FETs, the drain current ID becomes smaller and smaller, until after a certain threshold, the transistor shuts off.

The current, IDSS, is important because it's the maximum current that a FET can reach without entering the restricted breakdown region. It is the maximum current in the tolerance range of drain-source voltages, VDS, that can be achieved.

IDSS is referred to as the drain current for zero bias, because the gate-source voltage requires no bias voltage to operate. The gate-source voltage is just zero. No voltage needs to be applied to it.