8.1 The Junction Field Effect Transistor (J-FET)

Table of Contents - 1 2 3 4 5 6 7 8 9 R S ®

Threshold voltage, VT, is the voltage required to fully deplete the doped channel layer:

Two regions:

quadratic region where the depletion layer is less than the channel thickness d

saturated region where the depletion layer at the drain end equals the channel thickness d

The drain current become independent of the drain voltage and equals:

Fig.8.x Drain current versus Drain-Source voltage at a gate-source voltage of 0.2, 0.4, 0.6 0.8 and 1.0 Volt for a silicon JFET with built-in potential of 1 V. Channel parameters and device dimensions are listed in the table below.

Channel width


1 mm

Channel length


1 mm

Channel mobility


100 cm2/V-s

Channel doping


1017 cm-3

Channel thickness

100 nm

Built-in potential


1 V

Table 8.x JFET parameters

The transfer characteristic of a JFET is shown in the figure below and compared to a quadratic expression of the form

where is the average depletion layer width in the channel layer. The quadratic expression yields the same current at VG = fi for = 3d/8.

Fig.8.x Transfer characteristic of a JFET. Shown is the square root of the drain current of the JFET (solid line) and a quadratic fit with =3d/8.

8.1 ® 8.2

© Bart Van Zeghbroeck 1998