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 (ID = 0 for VGS > VGSoff)

Active Region (also called the Saturation Region):

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

In the active Region ID = IDSS (1 – VGS / VGSoff) 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.

ID = IDSS [ 2 (1 – VGS / VGSoff) VDS / - VGSoff - (VDS / VGSoff) 2 ]

In the Ohmic Region: RDS ≈ VGSoff / 2IDSS (VGS - VGSoff) = 1 / gm

Common Circuit Application(JFET Diode)

The JFET pn gate junction can be used as a diode by connecting the source and the drain terminals. This is done if very low reverse leakage currents are required. The leakage current is very low because the reverse leakage current scales with the gate area. Small gate areas are designed into JFETs because it decreases the gate-source and the gate-drain capacitances.