The JFET consists of a lightly doped N-type channel diffused into a P- substrate of P-type material. Basically, there are three wires connected to the device, one at each end of the channel. One wire is connected to the substrate. JFET is a bit like a PN-junction diode, except that we`ve connected two wires to the N-type side.
The n-type section is known as the channel, and the two ends are known as the source and drain. The connection to the p-type region is known as the gate.
There is an ohmic connection made to one end of the channel called source, through which majority carriers enter channel.
Electrons can move along the channel, so when we apply a voltage between the two end-wires a current will flow along the channel. We can continue this by injecting electrons in one end (the source) and removing them at the other (the drain). The effective resistance between the two ends will depend upon the size & shape of the channel and the properties of the N-type material.
The electrons moving in the channel will be repelled by the fixed charges in the P-type substrate. Therefore, current doesn`t fill the whole channel.It avoids the depletion zones near the walls.