Threshold voltage, V_{T}, 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 DrainSource voltage at a gatesource voltage of 0.2, 0.4, 0.6 0.8 and 1.0 Volt for a silicon JFET with builtin 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 cm^{2}/Vs 
Channel doping 
N_{d} 
10^{17} cm^{3} 
Channel thickness 


Builtin 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 V_{G} = f_{i} 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.