Nd = n-type doping (donor) concentration, VP = pinch-off voltage. Then the drain current in the linear region can be approximated as In terms of , the drain current can be expressed as Constant-current region The drain current in the saturation or active or pinch-off region is often approximated in terms of gate bias as where IDSS is the saturation current at zero gate–source voltage, i.e. the maximum current that can flow through the FET from drain to source at any (permissible) drain-to-source voltage (see, e. g., the I–V characteristics diagram above). In the saturation region, the JFET drain current is most significantly affected by the gate–source voltage and barely affected by the drain–source voltage. If the channel doping is uniform, such that the depletion region thickness will grow in proportion to the square root of the absolute value of the gate–source voltage, then the channel thickness b can be expressed in terms of the zero-bias channel thickness a as where VP is the pinch-off voltage the gate–source voltage at which the channel thickness goes to zero, a is the channel thickness at zero gate–source voltage. Transconductance The transconductance for the junction FET is given by where is the pinchoff voltage, and IDSS is the maximum drain current. This is also called or (for transadmittance). See also Constant-current diode Fetron MOSFET MESFET References External links Physics 111 Laboratory -- JFET Circuits I Interactive Explanation of n-channel JFET Introduction to Junction Field-effect Transistors Transistor types Field-effect transistors