However, if the gate becomes reverse biased with respect to the channel, the applied electric field will enlarge the depletion region, as shown to the left. As a result, the working channel width is reduced, and the effective channel resistance is increased significantly. In this manner, a small voltage applied to the gate can have a profound effect on the current flowing through the channel. Indeed, if the applied reverse bias becomes high enough, the depletion region can cover the entire width of the channel, and cut off current flow completely.
It is quite possible to reverse the semiconductor regions, in which case we would have a p-channel JFET. Of course, the applied voltages must be reversed in polarity for a p-channel device, and the current carriers within the p-type channel are holes rather than electrons. Otherwise, the basic behavior of the device is the same.
If the channel has a uniform concentration of impurities and the gate is placed in the middle of the channel, the FET is said to by symmetrical. In this case, the source and drain are interchangeable, which can be useful in some applications. However, many FETs are deliberately constructed to be unsymmetrical, to enhance certain parameters and behaviors. In these FET types, performance will be impaired if source and drain are interchanged.
JFETs, like any semiconductor device, have their advantages and disadvantages. Since they are controlled by the applied gate voltage, they draw no gate current (except for a small leakage current, which can be a disadvantage), and hence present a very high input resistance to any signal source. In addition, the reverse-biased junction can take a considerable amount of radiation damage without any appreciable change to the FET operation. This makes the JFET an excellent choice for operation in high-radiation environments.