MOSFET

 

What is a MOSFET?

The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device that is widely used for switching purposes and for the amplification of electronic signals in electronic devices.  

A MOSFET is a four-terminal device having source(S), gate (G), drain (D) and body (B) terminals. In general, the body of the MOSFET is in connection with the source terminal thus forming a three-terminal device such as a field-effect transistor.

 MOSFET is generally considered as a transistor and employed in both the analog and digital circuits.

 

 

From the above MOSFET structure, the functionality of MOSFET depends on the electrical variations happening in the channel width along with the flow of carriers (either holes or electrons). The charge carriers enter into the channel through the source terminal and exit via the drain.

The width of the channel is controlled by the voltage on an electrode which is called the gate and it is located in between the source and the drain.

Working Principle of MOSFET

The main working principle of a MOSFET device is to be able to control the current flow and voltage between the drain and source terminals. It works almost like a switch.

• The operation of a MOSFET is dependent upon the MOS capacitor. The MOS capacitor is the heart of the MOSFET.
• The semiconductor surface can be inverted from n-type to p-type by applying either negative or positive gate voltages respectively.
• Positive gate voltage pushes holes deeper into the substrate, allowing electrons to flow.
• The depletion region is populated by bound negative charges associated with the acceptor atoms.
• A channel is developed when electrons are reached.
• Electrons are also attracted to the positive voltage from the n⁺ source and drained into the channel by that voltage.
• As a result, if a voltage is applied between the drain and source, current flows freely between them, and the gate voltage controls electron flow.
• If we apply a negative voltage, a hole channel will be formed under the oxide layer, instead of the positive voltage. MOSFET can function in two ways

1.       Depletion Mode

2.       Enhancement Mode

 

Depletion Mode

 

·         In a Depletion MOSFET, the channel between the source and drain is naturally present even when no voltage is applied to the gate.

·         A depletion-type MOSFET conducts by default, and applying a voltage to the gate creates a depletion region in the channel, reducing the current flow.

·         The threshold voltage for a depletion MOSFET is negative, and the device is turned off by applying a positive voltage to the gate.

 

 

 

Enhancement Mode

·         In an Enhancement MOSFET, the channel between the source and drain is initially non-conductive or weakly conductive when no voltage is applied to the gate.

·         An enhancement-type MOSFET requires a positive voltage on the gate to create an inversion layer or induce carriers in the channel, allowing current to flow.

·         The threshold voltage for an enhancement MOSFET is positive, and the device is turned off when no voltage is applied to the gate.