What is a MOSFET?

MOSFET stands for Metal Oxide Silicone Field Effect Transistor or Metal Oxide Semiconductor Field Effect Transistor. It is a type of IGFET, which means Insulated Gate Field Effect Transistor. A MOSFET has four-terminals namely gate (G), drain (D), source (S), and body (B). The body of the MOSFET is connected to the source terminal and it forms a three-terminal device. It is used in both the analog and digital circuits.

Principle of MOSFET

A field-effect transistor (FET) is a 3-term semiconductor device, in which the output is controlled through an electric-powered field generated by way of the input energy. The main principle of the MOSFETs is to control the current flow and voltage between the source and drain terminals. It has the ability to change its conductivity with the amount of voltage that is supplied.

MOSFET has become the most common transistor due to its small size and high input impedence.

Structure of MOSFET

The structure of a MOSFET is similar to an FET. An oxide layer is applied to the substrate to which the gate terminals are attached. This oxide layer acts as a protective layer (separated from the substrate by SiO₂), which is why MOSFETs have another name, IGFETs. This is called pinch-off on the power MOSFET. If the drain voltage is equal to the saturation of the threshold voltage, the drain voltage becomes large enough to make the gate voltage less than the threshold voltage.

The two types of MOSFET are:

Depletion Type: These MOSFETs are normally on. The current flows from drain to source even without any gate voltage. Maximum current flows from drain to source when no difference in voltage exists between the gate and source terminals (V_GS=0). The operation of depletion-type MOSFET is very similar to a JFET.
Enhancement Type: These MOSFETs are normally off devices. The conduction of current will be on applicaton of gate voltage. As the gate-source voltage increases, the drain-source channel of the transistor becomes less resistive and current from drain to source increases.

Regions of Operation

The operation of MOSFETs happens mainly in three regions and those are as follows:

• Cut-off Region is where the device is in the OFF condition. No current flow through the device and it works as a switch.
• Saturation Region is where the devices will have constant drain to source current V_DS. This condition happens once when the drain to source voltage increases more than the value of pinch-off voltage. In saturation Region MOSFET functions as a closed switch.
• Linear/Ohmic Region is the region where the drain to source current I_DS increases linearly with drain to source V_DS voltage. When the MOSFET devices function in this linear region, they work as an amplifier.

N-channel MOSFET

The N-Channel MOSFET has an N-channel in between the drain and source terminals. The substrate is p-type while the drain and source are heavily doped N+ region. The current flows due to electrons in N-Channel MOSFET.

On applying positive voltage at the gate terminal, the holes present under the oxide layer are pushed downward into the substrate. The depletion region is then filled with bound electrons which are associated with the acceptor atoms. The positive gate voltage also attracts electrons from the N+ source and drain region into the channel thus an electron reach channel is formed.

P-channel MOSFET

The design and operation of PMOS is similar to that of NMOS. The P- channel MOSFET has a P- Channel region located in between the source and drain terminals. The substrate is of n-type and drain and source are heavily doped p+ region. The direction of flow of current is towards positively charged holes.

On applying negative voltage at the gate terminal, the electrons present under the oxide layer are pushed downwards into the substrate. The depletion region is then filled with holes which are associated with the donor atoms. The negative gate voltage also attracts holes from the p+ source and drain region into the channel region.

Common Mistakes

Remember that, BJT switching time is low compared to voltage & current with high alternating frequency.

Context and Applications

MOSFET is a voltage-controlled device and has many applications. There are a few advantages to using MOSFET in electrical circuits. Some important MOSFET applications are provided below:

• MOSFET is used to switch applications on an electronic tool.
• Used in different enlarging circles.
• Used in chopper circuits.
• MOSFET can be used as an excessive-frequency amplifier.
•  Can be used in voltage manipulate circuits.
• Used as an inverter in some electrical circuits.

In each of the expert exams for undergraduate and graduate publications, this topic is huge and is mainly used for:

• Bachelor of Technology in the electrical and electronic department
• Bachelor of Science in physics
• Master of Science in physics

Related Concepts

• FET
• PNP junction diode
• NPN junction diode

Practice Problems

Q1 The controlling parameter in a MOSFET is_____________.

(a) V_ds

(b) I_g

(c) V_gs

(d) I_s

Correct option- (b)

Explanation- The gate-source voltage Ig is the controlling parameter in a MOSFET.

Q2 Which of the following terminals are not MOSFET?

(a) Drain

(b) Base

(c) Gate

(d) Source

Correct option- (b)

Explanation- MOSFET is a three-terminal device Drain, Gate & Source.

Q3 Choose the correct statement.

(a) MOSFET is a unipolar, voltage controlled, three-terminal device.

(b) MOSFET is a bipolar, current-controlled, two-terminal device.

(c) MOSFET is a unipolar, voltage controlled, two-terminal device.

(d) MOSFET is a bipolar, current-controlled, three-terminal device.

Correct option- (a)

Explanation- MOSFET is a three-dimensional tool, Gate, Fountain & Drainage. It is controlled by voltage unlike BJT and the current flow of electrons only.

Q4 Which terminal does not belong to MOSFET.

(a) Gate

(b) Drain

(c) Source

(d) Base

Correct option- (d)

Explanation- MOSFET has three terminals: Gate (G), Drain (D), and Source (S)

Q5  The MOSFET icon arrow indicates _______________.

(a) The direction of electrons

(b) The direction of conventional current flow

(c) Both

(d) All of these

Correct option- (a)

Explanation- The arrow is to indicate the direction of the electrons (as opposed to the direction of the normal current flow).