Drain current versus drain voltage

You might expect that the current I_D, passing through the channel of a JFET, would increase linearly with increasing drain voltage E_D. But this is not, in general, what happens. Instead, the current I_D rises for awhile, and then starts to level off.
The drain current I_D (which is the same as the channel current) is often plotted as a function of drain voltage, E_D, for various values of gate voltage, E_G. The resulting set of curves is called a family of characteristic curves for the device. The graph of Fig. 1 shows a family of characteristic curves for a hypothetical N-channel JFET. Engineers make use of these graphs when deciding on the best JFET type for an electronic circuit. Also of importance is the curve of I_D vs E_G.

23-6 A family of characteristic curves for a hypothetical N-channel JFET.