MOSFET

CONSTRUCTION OF MOSFET:

• Metal oxide semiconductor field effect transistor.
• Gate is insulated from a thin layer of SiO₂ , It can be operated in depletion and enhancement modes.
• MOSFET Is a semiconductor device and is widely used in many circuit applications.
• The input impedance of MOSFET Is much more then that of a JJFET Because. of very small gate leakage current.
• The MOSFET Can be used in any of this circuit covered for the JFET. Therefore all the equations apply equally well to the MOSFET and JFET In amplifier connections.
• Inter electrode capacitance is 10 pF.

It is similar to JFET with the following modifications: there is only one P region, called substrate.
•A thin layer of metal oxide (SIO) Is deposited over the left side of the channel a metallic gate is an insulator therefore, the gate is insulated from the channel. For this reason, MOSFET is also called insulated Gate FET or insulated Gate transistor.
• like JFET MOSFET has three terminals source drain gate

WORKING OF MOSFET:

Instead of a gate diode as in JFET. A gate capacitor is formed here. One plate of this capacitor is the gate and the other plate is the channel with metal oxide as the dielectric.
When negative voltage is applied to the gate electrons accumulate on it. These electrons repel the conduction band electrons in the N channel. Therefore lesser number of conduction electrons are made available for current conduction through the channel. The greater the negative voltage. On the gate the lesser in the current conduction from source to drain. If gate is given positive voltage more electrons are available in the channel. Consequently current from source to drain increases.
• In a MOSFET the source to drain current is controlled by the electric field of capacitor formed at the gate.
• Unlike the JFET a MOSFET has no gate diode. This makes it possible to operate the device with positive or negative gate voltage.
• As the gate forms a capacitor therefore, negligible gate current flows whether positive or negative voltage is applied to the gate. Consequently, the input impedance of MOSFET is very high ranging from 10 3 M ohm 10 5 M ohm.

There are four types of MOSFET. They are:
• In channel enhancement(E only) MOSFET
• N channel depletion (DE) MOSFET
• P channel enhancement (E only) MOSFET
• P channel depletion (DE) MOSFET