What is MOSFET? Type and Disadvantages

“MOSFET’s are voltage controlled devices which have insulated gates resulting in an input impedance many times higher than a JFET”

As well as the Junction Field Effect Transistor (JFET), there is another type of Field Effect Transistor available whose Gate input is electrically insulated from the main current carrying channel and is therefore called an Insulated Gate Field Effect Transistor or IGFET. The most common type of insulated gate FET which is used in many different types of electronic circuits is called the Metal Oxide Semiconductor Field Effect Transistor or MOSFET for short.

The IGFET or MOSFET is a voltage controlled field effect transistor that differs from a JFET in that it has a “Metal Oxide” Gate electrode which is electrically insulated from the main semiconductor N-channel or Pchannel by a thin layer of insulating material. This insulated metal gate electrode can be thought of as one plate of a capacitor. The isolation of the controlling Gate makes the input resistance of the MOSFET extremely high in the Mega-ohms region.

As the Gate terminal is isolated from the main current carrying channel “NO current flows into the gate” and just like the JFET, the MOSFET also acts like a voltage controlled resistor were the current flowing through the main channel between the Drain and Source is proportional to the input voltage.

Like the previous JFET, MOSFETs are three terminal devices with a Gate, Drain and Source and both P-channel (PMOS) and N-channel (NMOS) MOSFETs are available. The main difference this time is that MOSFETs are available in two basic forms.

MOSFETs Basic Types

Depletion Type

(Normally ON) – 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

(Normally OFF) – the transistor requires a Gate-Source voltage, ( VGS ) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.

MOSFET Configurations

Disadvantages of MOSFET Devices

One of the main disadvantages of MOSFET devices is their very high input impedance making them extremely sensitive to electrostatic discharge (ESD) on the gate terminal. The metal oxide insulating layer around the gate terminal is extremely thin and can be easily punctured by static so be careful when handling MOSFET devices.