# For the same JFET why are there two curves in a Transfer characteristics graph?

I am trying to learn about n-channel JFETs from David. A. Bell's book. In the transfer characteristics graph, there seem to be two curves as shown. Note that VGS(off) = Vp

As you can see, one curve is drawn for the minimum values of VGS(off) and IDSS and the other curve is drawn for the maximum values of VGS(off) and IDSS. The book shows an excerpt from a data sheet and says that the manufacturer has mentioned the values of minimum and maximum of VGS(off) and IDSS and uses this formula to draw the curves:

I can't understand how there can be maximum and minimum values for the cut-off voltage and the drain current. From the input characteristics graph, I understand that cut-off voltage is nothing but the Pinch-off voltage when there is no voltage applied between the gate and the source(ie) when VGS=0. Now, if there is a maximum and minimum cut off voltage then that means for VGS=0 there are two pinch-off voltages which doesn't make any sense. Also what does having a minimum "saturation" current even mean?

• FETs are analogue devices, there is no voltage where it is completely off and 1e-190V later it is completely on, there are gradually increasing electric fields that interact. Commented Oct 12, 2017 at 8:03
• @PlasmaHH Are you implying that at the minimum of cut-off voltage the JFET starts to have near zero current and at the maximum of cut-off voltage it actually switches off? Commented Oct 12, 2017 at 8:28
• From Electronic Devices and Circuits, second edition, page 271: One of the problems in using FET's is that each device type does not have a single transfer characteristic. This is because Idss and Vp cannot be specified accurately. Instead, the manufacturer specifies maximum and minimum values for each parameter. Referring to Fig. 12-9, it is seen that, for the 2N5457 FET, Idss(min) = 1 mA, and Idssmax=5mA. Also Vp, which is listed as Vgs,off has a minimum level of 0.5 V and a maximum of 6 V.
– Bart
Commented Oct 12, 2017 at 10:27
• Electronic Devices and Circuits is the book VenkiPhy6 is referring to.
– Bart
Commented Oct 12, 2017 at 10:38
• @Bart I actually have the fifth edition(Indian version). I don't see this explanation here. So thanks for going through the trouble of finding the book and letting me know! Your comment also checks out with Neil_UK 's answer... Commented Oct 12, 2017 at 11:10