A field-effect transistor (FET) is a voltage-operated transistor. Unlike a BJT, a FET requires very little input current, and it exhibits extremely high input resistance. There are two major classes of field-effect transistors: junction FETs (JFETs) and metal-oxide semiconductor FETs (MOSFETs), also known as insulated-gate FETs (IGFETs). FETs are further subdivided into P- and N-type devices. FETS are unipolar transistors because, unlike the BJT, the drain current consists of only one kind of charge carrier: electrons in N-channel FETs and holes in P-channel FETs.
FETs and MOSFETs are both made as discrete transistors, but MOSFET technology has been adopted for manufacturing power FETs (see “Power Transistors” later in this section) and ICs. There are both NMOS and PMOS ICs. When both P- and N-channel MOSFETs are integrated into the same gate circuit, it is a complementary MOS (CMOS).