MOSFET

Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

There is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying the channel and is, therefore, it is term as Insulated Gate Field Effect Transistor.

The most common type of insulated gate FET which is used in many other application of electronic circuits is called the Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

It is also a voltage controlled field effect transistor that varies from a JFET in construction is  that it has a “Metal Oxide” Gate electrode which is electrically insulated from the main semiconductor n-channel or p-channel by a very thin layer of insulating material mainly silicon dioxide, commonly known as glass so it is called insulated gate.

This ultra-thin insulated metal gate electrode can be considered as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high way up in the Mega-ohms (MΩ) region which thereby making it almost infinite.

As the Gate terminal is electrically isolated from the main current carrying channel between the drain and source, “NO current flows into the gate” and similar to JFET, the MOSFET also acts like a voltage controlled resistor or voltage controlled device where the current flowing through the main channel between the Drain and Source is proportional to the input voltage. Similarly, the JFET, the MOSFETs very high input resistance can easily accumulate large amounts of static charge result of this is MOSFET becoming easily damaged unless carefully handled or protected.

MOSFETs are available in two basic forms:

• Depletion Type–   In this mode, the transistor requires the Gate-Source voltage, (VGS) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
• Enhancement Type–   In this mode, the transistor requires a Gate-Source voltage, (VGS) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.

The symbols and basic construction for both configurations of MOSFETs are shown in below figure.

The MOSFET symbols above show an additional terminal called the Substrate and is not normally used as either an input or an output connection but instead it is used for grounding the substrate. It connects to the main semiconductive channel through a diode junction to the body or metal tab of the MOSFET.