Q. Explain the construction of JFET?

The basic construction of the p-channel JFET is shown in figure. The major part of the structure is the p-type material that forms the channel between the embedded layer of p- type material. The top of the p-type channel is connected through an ohmic contact to a terminal referred to as the drain, while the lower end of the same material is connected through an ohmic contact to a terminal referred to as source. The two n-type materials are connected together and to the gate terminal. The two n-regions  connected together are reverse biased with respect to the p-type substrate. A second battery, Vds is used to pull current out of the source, by applying a negative voltage between the drain and the source. The reverse biased n-p junctions creates a depletion region which extends into the p-type material through which the holes travel as they go from source to drain . By adjusting the value of Vgs, one can make the depletion region smaller or larger, thus increasing or decreasing the drain current. There is more reverse bias across the p-n junctions at the drain end of the channel than at the source end. Thus, a more accurate depiction of the JFET would be what is shown in figure 3. When the drain/source voltage gets large enough, the two depletion regions will join together, and, just as with the MOSFET, the channel pinches off, as shown in figure.