Drain current versus drain voltage

You might expect that current ID, passing through channel of a JFET, would increase linearly with the increasing drain voltage ED. But this is not, generally what happens. Instead, current ID rises for awhile, and then begins to level off.

The drain current ID (which is same as channel current) is often plotted as a function of drain voltage, ED, for the various values of gate voltage, EG. The resulting set of curves is known as family of characteristic curves for device. The graph of figure shows the family of characteristic curves for the hypothetical N-channel JFET. Engineers make use of these graphs when deciding on best JFET type for the electronic circuit. Also of importance is curve of ID vs EG, one example of which is shown in the Figure given below.

                                   Figure--A family of characteristic curves for a hypothetical N-channel JFET.