Regions of JFET operation

Cut-off Region: -

The transistor is off. There is no conduction between the drain and the source when the gate-source voltage is greater than the cut-off voltage (I_D = 0 for V_GS > V_GSoff)

Active Region (also called the Saturation Region): -

The transistor is on. The drain current is controlled by the gate-source voltage (V_GS) and relatively insensitive to V_DS. In this region the transistor can be an amplifier.

In the active Region I_D = I_DSS (1 – V_GS / V_GSoff) ²

Ohmic Region: -

The transistor is on, but behaves as a voltage proportional to the source-drain voltage and is controlled by the gate voltage.

I_D = I_DSS [ 2 (1 – V_GS / V_GSoff) V_DS / - V_GSoff - (V_DS / V_GSoff) ² ]

In the Ohmic Region: R_DS ≈ V_GSoff / 2I_DSS (V_GS - V_GSoff) = 1 / g_m