MOSFET – Metal Oxide Silicon Field Effect Transistors

MOSFET or Metal-Oxide-Silicon Field-Effect Transistor is a type of field-effect transistor (FET) that is commonly fabricated using silicon. It features an insulated gate, the voltage of which determines the conductivity of the device. MOSFETs are used for amplifying or switching electronic signals and are known for their high input impedance, which results in high switching speed.

In this article, we will discuss MOSFET in detail including its types, working as well as application.

What Is a MOSFET?

MOSFET is the abbreviation of Metal Oxide Field Effect Transistor. MOSFET was developed to improve the disadvantages present in FETs such as high drain resistance, moderate input impedance, and lower operation speed. Therefore, a MOSFET is the improved version of FET.

In some cases, MOSFETs are also called IGFET (Insulated Gate Field Effect Transistors). They have three primary terminals—the source, drain, and gate—and can operate in either enhancement mode (requiring a nonzero gate-to-source voltage to turn on) or depletion mode (turning off when a certain gate-to-source voltage is reached).

MOSFET Symbol

Symbol for MOSFET in an electric circuit is:

Structure of MOSFET

MOSFETs are constructed using a semiconductor material, typically silicon, with a thin layer of insulating oxide separating the gate from the semiconductor channel. MOSFET is a four-terminal device consisting of the following components:

• Gate: An insulated gate made of polysilicon or metal that controls the flow of current between the source and drain.
• Drain: A heavily doped p⁺ region or n⁺region that receives the majority charge carriers when the transistor is turned ON.
• Source: A lightly doped p⁺ region or n⁺ region that supplies the majority charge carriers when the transistor is turned ON.
• Body (substrate): A p-type or n-type semiconductor material that serves as the foundation for the transistor and provides electrical connection to the source.

Other than these parts there are some more components in MOSFET:

Channel: The oxide layer, which forms between the source and drain, is the insulator. The gating of this channel is determined by the electric field, which is produced by the gate voltage.

Oxide Layer: An insulating layer (normally silicon dioxide) is put between the gate and the semiconductor, so that the gate cannot make a direct electrical contact with the semiconductor.

MOSFET Diagram

The diagram of a MOSFET in a circuit as a switch is given as follows:

Working of MOSFET

Working of MOSFET can be understand as follows:

• Formation of Channel: The application of voltage to the gate produces an electric field which, in its turn, attracts or repels carriers of charges in the semiconductor. This alters the carrier density in a region known as the channel, creating a conduction path for current flow.

• Controlling the Flow: For an N-channel MOSFET, a positive supply to the gate lures electrons, therefore, it becomes a conductive channel between the source and drain terminals. In a P-channel MOSFET, a negative voltage applied on its gate brings holes to experience attraction.

• Operation Modes: MOSFETs can function in three modes – cut-off, saturation, and triode. As there is no current in the cutoff mode since the path is blocked. When the channel is fully opened, it is in saturation and maximum current can flow. Triode has a channel that is partly open and hence allows only a controlled current flow.

• Voltage Control: The essential advantage of MOSFET is its voltage controlled conductance operation that allows the current to be modulated precisely according to the input signal voltage. This feature is important for them being able to act as components in digital circuits.

Operation Region of MOSFET

The functioning of a MOSFET can be classified into different regions depending on the voltages applied at its terminals. The three main operation regions are:The three main operation regions are:

• Cut-off Region (V_GS < V_t): In this area, the gate-to-source voltage (V_GS) is lower than the threshold voltage (V_t) and the MOSFET is in the off state. The channel is not conducting and the current from the drain to the source is very small. MOSFET is an open switch in this region.

• Triode Region (V_GS > V_t, V_DS < V_GS – V_t): In the triode region the gate-to-source voltage (V_GS) exceeds the threshold voltage (V_t), and the drain-to-source voltage (V_DS) is also significant but less than the difference between V_GS and V_t. In this region, the MOSFET behaves as a variable resistor. Both V_GS and V_DS control the drain current (I_D) and the MOSFET is not fully on.

• Saturation Region (V_GS > V_t, V_DS ≥ V_GS – V_t): In the saturation region, both V_GS and V_DS voltages are significant. The MOSFET is completely on, and there is a linear relationship between I_D and V_DS.The MOSFET acts as an voltage-controlled current source in this region. The saturation is the region of interest for many applications like amplifiers and digital logic circuits.

Types of MOSFET

MOSFET can be categorized based on different parameters i.e.,

Based on the polarity of the channel, MOSFET can be classified as:

• n-MOSFET
• p-MOSFET

Based on the gate voltage, MOSFET can be classified as:

• Depletion Mode
• Enhancement Mode

Note: In each mode, depletion and enhancement both n and p MOSFET can be possible.

Lets Discuss these types in detail as follows:

n – MOSFET

An n-channel MOSFET (also known as n-MOSFET) is a type of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) wherein the channel consists primarily of negatively charged electrons as current carriers i.e., n-type semiconductor.

When a positive voltage is applied to the gate relative to the source, an electric field is created in the channel, controlling the flow of electrons from the source to the drain. Typically, the threshold voltage for an n-MOSFET is negative, meaning a positive voltage must be applied to the gate to enable conduction.

p-MOSFET

A p-MOSFET (p-type Metal-Oxide-Semiconductor Field-Effect Transistor) is a type of MOSFET where the semiconductor channel is made of p-type material. It operates by applying a negative voltage to the gate terminal, creating an electric field in the channel. When the gate voltage is less than the threshold voltage, the MOSFET conducts current from the drain to the source.

Depletion Mode

A Depletion Mode MOSFET is a type of MOSFET that is normally on at zero gate-source voltage. Depletion Mode MOSFETs are used as load “resistors” in logic circuits and can be turned off by applying a negative gate voltage. These MOSFETs are ideal for applications like power supply startup power, over-voltage protection, and in-rush-current limiting.

They provide a constant current source and can function as voltage regulators. Infineon Technologies offers a range of N-channel Depletion Mode MOSFETs with voltages from 60 V to 600 V, providing higher efficiency and ruggedness for various applications.

Enhancement Mode

An Enhancement Mode MOSFET is a type of MOSFET that is normally off at zero gate-source voltage. When a positive voltage is applied to the N-channel gate terminal, the channel conducts and the drain current flows through the channel. If this bias voltage increases to more positive, then the channel width and drain current through the channel increases. If the bias voltage is zero or negative, then the transistor may switch off and the channel is in non-conductive mode.

Enhancement Mode MOSFETs are commonly used as switches in electronic circuits because of their low ON resistance and are used to make logic gates and in power switching circuits, such as CMOS gates, which have both NMOS and PMOS Transistors.

Applications of MOSFET

Some common applications of MOSFETs are:

• Amplification: MOSFETs in amplifiers are used to intensify electronic signals, making them the key parts of the audio amplifiers, RF amplifiers and operational amplifiers.

• Switching Circuits: MOSFETs are used to guide the current flow as electronic switches, which is important in digital circuits. These are used in logic gates, memory cells, and microprocessor designs.

• Voltage Regulators: MOSFETs are used in the voltage regulation and voltage regulation circuits, which are responsible for providing stable and regulated output voltages in power supplies.

• Power Amplifiers: High-power MOSFETs are employed in power amplifiers of audio systems, RF transmitters and power inverters.

• Radio Frequency (RF) Devices: MOSFETs are critical to RF amplifiers, oscillators and mixers for the wireless communication systems such as cell phones, Wi-Fi routers and satellites.

• Analog Signal Processing: MOSFETs are widely adopted in many analog signal processing circuits like filters, oscillators, and analog-to-digital converters.

MOSFET as Switch

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) can be utilized as a switch in electronic circuits. The MOSFET has three terminals: the gate (G), the source (S), and the drain (D). Depending on the voltage applied to the gate terminal, the MOSFET can either be in an “on” state (conducting) or an “off” state (non-conducting), effectively acting as a switch to control the flow of current between the source and drain terminals.

MOSFET as Capacitor

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) can be utilized as a component in a circuit that behaves like a capacitor, although it’s not typically used for this purpose due to its primary function as a voltage-controlled switch. However, under certain conditions, the gate-source capacitance (Cgs) of a MOSFET can exhibit capacitor-like behavior.

Difference between MOSFET and BJT

The key differences between MOSFET and BJT are:

Feature	MOSFET	BJT
Full Form	Metal-Oxide-Semiconductor Field-Effect Transistor	Bipolar Junction Transistor
Structure	Three-terminal device: Source, Drain, Gate	Three-terminal device: Collector, Base, Emitter
Principle of Operation	Majority carrier device: controlled by gate voltage	Minority carrier device: controlled by base current
Current Carrying Mechanism	Majority carrier (electrons or holes)	Minority carrier (holes or electrons)
Input Impedance	High (insulated gate)	Low (input current controls output current)
Output Impedance	Moderate to High (depends on circuit)	Low (depends on circuit)
Switching Speed	Faster	Slower
Gain	Voltage-controlled	Current-controlled
Saturation Voltage	Typically lower (near 0V)	Typically higher (around 0.2V to 0.7V)
Temperature Sensitivity	Low	Moderate to high
Thermal Stability	Generally more stable	Less stable

JFET and MOSFET

Common differences between JFET and MOSFET are:

Feature	JFET	MOSFET
Full Form	Junction Field-Effect Transistor	Metal-Oxide-Semiconductor Field-Effect Transistor
Structure	Three-terminal device: Source, Drain, Gate	Three-terminal device: Source, Drain, Gate
Channel Type	Majority carrier device: controlled by gate-source voltage	Majority carrier device: controlled by gate-source voltage
Channel Composition	Semiconductor channel formed by reverse-biased pn-junction	Semiconductor channel formed by insulated gate
Gate Construction	No insulating layer between gate and channel	Insulating layer (oxide) between gate and channel
Voltage Applied	Gate-source voltage controls channel conductivity	Gate-source voltage controls channel conductivity
Conductivity Control	Voltage-controlled resistor	Voltage-controlled resistor
Input Impedance	High (insulated gate)	High (insulated gate)
Output Impedance	Moderate to High (depends on circuit)	Moderate to High (depends on circuit)
Switching Speed	Faster than MOSFETs	Generally slower than JFETs
Voltage Ratings	Low to medium voltage applications	High voltage applications
Temperature Sensitivity	Moderate	Low
Thermal Stability	Generally more stable	Generally more stable

Advantanges and Disadvantages of MOSFETs

Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) offer several advantages over other transistors, making them widely used in various electronic applications. Some of the common advantages of MOSFETs are listed below:

• Power Efficiency: MOSFETs provide exceptional power efficiency due to their low on-resistance and negligible static power consumption.

• High Input Impedance: MOSFETs have a very high input impedance due to the insulation layer between the channel and the gate electrode, allowing them to operate without any gate current.

• High Drain Resistance: They exhibit high drain resistance due to the lower resistance of the channel, contributing to their efficiency.

Despite their numerous advantages, MOSFETs also come with some drawbacks that need to be considered:

• Fragile Gate-Channel Layer: The layer between the gate and channel in MOSFETs is delicate and susceptible to electro-static damage during installation, requiring well-designed circuits to prevent issues.

• Susceptibility to Overload Voltages: MOSFETs are very sensitive to overload voltages, necessitating special handling during installation to avoid damage.

• Sensitivity to Static Electricity: MOSFETs are highly sensitive to static electricity, which can lead to damage if not handled carefully during assembly and installation.
• Cost: MOSFETs can be more expensive than other transistor types, impacting their suitability for cost-sensitive applications.

Conclusion

In summary, MOSFETs are like tiny electronic switches that help control the flow of electricity in gadgets we use every day. They’re super important because they make our devices work better and smarter. As technology gets even more advanced, MOSFETs will keep playing a big role in making our gadgets smaller, faster, and more powerful.

MOSFET Frequently Asked Questions

What is a MOSFET?

MOSFET, or Metal Oxide Semiconductor Field Effect Transistor, is a voltage-controlled four-terminal device that is used for switching and amplification.

Will a MOSFET conduct in both directions?

Yes, MOSFETs are bidirectional.

In what way, are there many types of MOSFETs?

MOSFETs are of two classes: The two types of modes are enhancement mode and depletion mode. These classes are available in two types, either n-channel or p-channel; therefore, these classes make up four types of MOSFETs.

At what location do MOSFETs operate?

There are three operating regions and they are named as: cut off, resistive and saturation regions.

What is the difference between Mosfet and BJT?

MOSFET is a voltage-dependent device while BJT is a current-controlled device.

What does MOSFET stand for?

MOSFET abbreviates Metal Oxide Semiconductor Field Effect Transistor.

What do a MOSFET do?

MOSFETs operate through control of the conductivity of a semiconductor channel between the source and drain terminals by an electric field created by the voltage applied to the gate.

What is the function of a gate in a MOSFET?

The gate in a MOSFET is used to control the current flow between the source and drain terminals by producing a field across the semiconductor channel.

What is the Vt (threshold voltage) in a MOSFET?

The gate threshold voltage is the lowest voltage at the gate which is necessary to enable the conduction of the channel of the MOSFET.