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 |
W |
1 mm |
|
Channel length |
L |
1 mm |
|
m n |
100 cm2/V-s |
|
Channel doping |
Nd |
1017 cm-3 |
|
Channel thickness |
|
|
|
Built-in potential |
f i |
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.