For what reason might one ever choose a JFET instead of a MOSFET?
According to use and cost.
The most important feature of any switching device is a high input impedance. the reason is to avoid any effect of the connected loads on the source, which would cause a loading effect.
That being said, the mosfet has a highly isolated (mega-ohm) gate terminal that provides a high input impedance, but the cost will be very high compared to the jfet.
jfet bjt are not less valuable, they are still widely used for their profitability and the use of their applications.
The multiple arrangement of the bypass and coupling filters added to the circuit will ensure filtering and impedance of the jets, making it quite similar in input impedance of the MOSFET.
in general, mosfets are easier to manufacture (ie mosfets are easily scalable) and have more desirable properties than jets, such as higher input impedance and lower leakage current .
However, there may be cases where a jfet is more useful than a mosfet, and they are:
1] in applications where one or the other device must be used as a power source, a jfet has an output resistance greater than that of a mosfet, in the saturation region.
The output resistance of a mosfet is generally lower because of the modulation of the channel length. Thus, a jfet offers a flatter vds-id curve in the saturation region than a mosfet and is a better approximation of a current source, since an ideal current should have its resistance tending to infinity.
2] firing noise strongly affects a mosfet: the carriers circulate in the channel located just under the oxide layer. the oxide has many imperfections (defects) which, in turn, affect the mobility of carriers. In high frequency applications, firing noise (also known as 1 / f noise or flicker noise) affects the performance of a mosfet.
in an airplane, the carriers do not undergo any noise of this type, because they cross the channel, bordered by the zone of exhaustion of variable width of both sides. thus, the carriers do not meet any defect contrary to the imperfections in oxide of mosfets. moreover, for applications where the current is of the order of ma, the firing noise has an impact on the fluctuation of the direct current due to the unequal mobility of the charge carriers, in the case of a mosfet. that is, the continuous drain current of the order of ma is more stable in a jfet for a fixed vds-vgs combination than for a mosfet.
These are two applications where a JFET offers a more satisfactory performance than the otherwise advantageous Mosfet.
