The field effect transistor, or FET, is a semiconductor device that serves as an alternative to the bipolar junction transistor. FETs are available in two broad types: the junction FET, or JFET, and the metal oxide semiconductor FET, or MOSFET.
It is best not to think of FETs as either better or worse than the BJT. They have different characteristics and lend themselves to applications where BJT performance might be wanting. The inverse is also true and for some applications the judicious use of a combination of BJTs and FETs can produce superior performance when compared to either device used alone. Like the BJT's NPN and PNP variants, FETs comes in two “flavors”: the Nchannel type and the P-channel type. We shall cover JFETs first and then discuss MOSFETs in subsequent chapters. In this chapter we will cover the internal structure of the JFET, its theory of operation and biasing techniques. In the next chapter we shall discuss small signal JFET amplifiers; both voltage amplifiers and voltage followers.
The JFET is fundamentally different from a bipolar junction transistor. While the JFET, like the BJT, relies on the PN junction for operation, the JFET is modeled as a voltage-controlled current source while the BJT is modeled as a current-controlled current source. Further, the BJT relies on a forward-biased base-emitter junction for proper operation while the JFET achieves current control via a reverse-biased junction. Consequently, JFET biasing circuits tend to be incompatible with BJT biasing schemes and one device cannot be swapped out for the other.