Introduction to MOSFET-Metal Oxide Semiconductor Field Effect Transistor,
Metal-oxide-semiconductor field-effect transistor (MOSFET) is an important semiconductor device and is widely employed in many circuit applications. Since it is constructed with the gate terminal insulated from the channel, it is sometimes called insulated gate FET (IGFET). Like, a JFET, a MOSFET is also a three terminal (source, gate and drain) device and drain current in it is also controlled by gate bias. The operation of MOSFET is similar to that of JFET. It can be employed in any of the circuits covered for the JFET and, therefore, all the equations apply equally well to the MOSFET and JFET in amplifier connections. However, MOSFET has lower capacitance and input impedance much more than that of a JFET owing to small leakage current. In case of a MOSFET the positive voltage may be applied to the gate and still the gate current remains zero.
MOSFETs are of two types namely
(i) Enhancement type MOSFET or E-MOSFET and
(ii) Depletion enhancement MOSFET or DE-MOSFET.
In the depletion-mode construction a channel is physically constructed and a current between drain and source is due to voltage applied across the drain-source terminals. The enhancement MOSFET structure has no channel formed during its construction. Voltage is applied to the gate, in this case, to develop a channel of charge carriers so that a current results when a voltage is applied across the drain-source terminals.