What Is a MOSFET?

A MOSFET is a type of field-effect transistor in which an electrically insulated gate controls a semiconductor channel between the drain and source terminals.

The three main terminals are:

• Gate (G)
• Drain (D)
• Source (S)

Power MOSFETs also have an internal body connection associated with the semiconductor structure. In many discrete power MOSFETs, this produces an intrinsic body-diode path between drain and source.

Why Is It Called a Field-Effect Transistor?

A MOSFET controls current using an electric field.

The voltage applied between the gate and source creates an electric field that changes the conductivity of the channel between drain and source.

Unlike a conventional BJT, the control input is primarily a voltage rather than a continuous base current.

MOSFET Terminals

Gate

The gate is the control terminal. It is electrically insulated from the semiconductor channel by a very thin dielectric layer. This insulation results in extremely low steady-state gate current.

Drain

The drain is one of the main current terminals. Current flows through the channel between drain and source when the MOSFET is conducting.

Source

The source is the reference terminal for the gate voltage in most discrete MOSFET circuits. The important control voltage is VGS, the voltage between gate and source.

Basic MOSFET Structure

A simplified N-channel MOSFET can be represented as a semiconductor device with an N-type source and drain separated by a region in which a conductive channel can be formed.

                 Gate
             ───────────
                 │
              Insulator
             ───────────
                 │
        N+       │       N+
      Source     │      Drain
         │       │        │
         └──── Channel ───┘
              P-type
             substrate

When the appropriate gate-to-source voltage is applied, an inversion layer forms near the semiconductor surface and creates a conductive path between source and drain.

N-Channel and P-Channel MOSFETs

MOSFETs are commonly divided into two channel polarities:

• N-channel MOSFET
• P-channel MOSFET

N-Channel MOSFET

An N-channel MOSFET uses electrons as its principal charge carriers. Enhancement-mode N-channel MOSFETs normally require the gate to be sufficiently positive relative to the source to turn the device ON.

N-channel MOSFETs are extremely common because electron mobility is higher than hole mobility, allowing many N-channel devices to achieve low on-resistance and high switching performance.

They are widely used in:

• Switch-mode power supplies
• DC-DC converters
• Motor controllers
• Battery systems
• Load switching
• Audio amplifier power stages
• Digital electronics

P-Channel MOSFET

A P-channel MOSFET uses holes as its principal charge carriers. An enhancement-mode P-channel device normally turns ON when the gate is sufficiently negative relative to the source.

P-channel MOSFETs are often useful for high-side switching because their source can be connected to a positive supply while the gate is pulled lower to turn the device ON.

They are commonly used where simple high-side control is more important than achieving the lowest possible conduction resistance.

Enhancement-Mode MOSFET

An enhancement-mode MOSFET is normally OFF when the gate-to-source voltage does not provide sufficient channel formation.

Applying an appropriate gate-to-source voltage creates or enhances the conductive channel.

Enhancement-mode MOSFETs are the most common type used in modern switching applications.

Depletion-Mode MOSFET

A depletion-mode MOSFET can conduct at zero gate-to-source voltage. Applying an appropriate gate voltage can reduce the channel conduction.

Depletion-mode MOSFETs are less common than enhancement-mode devices but have specialized applications in analog and electronic control circuits.

Gate-to-Source Voltage

The most important control voltage for a MOSFET is:

VGS = VG - VS

where:

• VG = gate voltage
• VS = source voltage
• VGS = gate-to-source voltage

The required gate voltage must always be considered relative to the source, not simply relative to ground.

This is particularly important when a MOSFET is used as a high-side switch and its source voltage changes with the switching state.

MOSFET Threshold Voltage

The datasheet parameter VGS(th) is called the gate threshold voltage.

It indicates the approximate gate-to-source voltage at which a specified small drain current begins to flow under the datasheet's test conditions.

It is important to understand that VGS(th) is not normally the voltage required to turn a power MOSFET fully ON.

For power switching, the MOSFET's specified RDS(on) should be checked at the actual gate-drive voltage being used.

RDS(on)

When a MOSFET is fully enhanced, the resistance between drain and source is called RDS(on).

The conduction loss can be approximated by:

P ≈ I² × RDS(on)

where:

• P = conduction power loss
• I = drain current
• RDS(on) = drain-source ON resistance

This is one of the most important parameters when selecting a MOSFET for power switching.

MOSFET Switching

MOSFETs are widely used as electronic switches because the gate requires very little steady-state current.

A simplified low-side switching arrangement is:

             +V
              │
             LOAD
              │
              D
             MOSFET
              S
              │
             GND

Control ───── G

When the gate-to-source voltage is sufficiently high for the selected MOSFET, the channel becomes conductive and current flows through the load.

Why MOSFETs Need Gate Current During Switching

Although a MOSFET gate ideally draws almost no DC current, the gate behaves electrically like a capacitive load.

When the gate voltage changes, the driver must supply or remove charge from the gate.

Therefore a MOSFET can require substantial instantaneous gate current during fast switching even though its steady-state gate current is extremely small.

Gate Charge

The total gate charge, commonly specified as Qg, indicates how much charge must be moved into or out of the gate to switch the MOSFET under specified conditions.

For high-frequency switching, gate charge can be just as important as RDS(on).

A MOSFET with very low RDS(on) may not necessarily be the best choice for a high-frequency converter if its gate charge is excessive.

Miller Plateau

During MOSFET switching, the gate voltage can temporarily remain within a relatively narrow voltage range while the drain voltage changes substantially.

This behavior is associated with the gate-drain capacitance and is commonly called the Miller plateau.

The gate driver must supply sufficient current to move the required charge through this portion of the switching process.

MOSFET Body Diode

Power MOSFETs have an intrinsic body-diode path resulting from their semiconductor structure.

The body diode can provide a current path when the MOSFET is reverse biased in certain circuit configurations.

Its forward voltage, reverse-recovery behavior and current rating can be important in switching converters and motor-control applications.

MOSFET as a Low-Side Switch

The N-channel MOSFET is especially convenient for low-side switching.

        +V
         │
        LOAD
         │
         D
        N-MOSFET
         S
         │
        GND

Control ── G

The source is connected to ground, making the required gate voltage easy to reference to the controller ground in many applications.

This is one reason N-channel MOSFETs are extremely common in switching circuits.

P-Channel High-Side Switching

A P-channel MOSFET can provide a relatively simple high-side switching arrangement.

        +V
         │
         S
       P-MOSFET
         D
         │
        LOAD
         │
        GND

Control ── G

The source is connected to the positive supply. Pulling the gate sufficiently below the source voltage turns the device ON.

A P-channel MOSFET may have higher RDS(on) than an equivalent N-channel device, but it can simplify gate-drive circuitry.

N-Channel High-Side Switching

N-channel MOSFETs can also be used for high-side switching, but the gate must normally be driven to a voltage above the source voltage by the required VGS.

When the source rises toward the supply voltage, a gate voltage referenced only to ground may no longer be sufficient.

This is why high-side N-channel MOSFET circuits often use:

• Bootstrap gate drivers
• Charge-pump drivers
• Isolated gate drivers
• Dedicated high-side driver ICs

MOSFET Operating Regions

MOSFET terminology differs somewhat between device physics, datasheets and circuit applications. In practical switching circuits, it is useful to distinguish the OFF state, the resistive ON state and the conditions where the device is being used as an amplifier.

The terminology should always be interpreted according to the MOSFET type and the definitions used in the manufacturer's datasheet.

MOSFETs in Power Supplies

MOSFETs are fundamental components in switch-mode power supplies.

They can be used as the main switching devices in:

• Buck converters
• Boost converters
• Buck-boost converters
• Flyback converters
• Forward converters
• Half-bridge converters
• Full-bridge converters
• Inverters

Their low conduction resistance and fast switching capability make them particularly useful for high-frequency power conversion.

MOSFETs in Motor Control

MOSFETs are widely used to control DC motors, brushless motor systems and other electrically controlled loads.

They can be arranged into half-bridge and full-bridge circuits to control current direction and motor speed.

Important considerations include:

• Drain-source voltage
• Continuous and peak current
• RDS(on)
• Gate charge
• Switching frequency
• Thermal performance
• Body-diode characteristics

MOSFETs in Audio Amplifiers

MOSFETs can be used in both small-signal and power audio circuits.

Power MOSFETs are found in some Class AB and Class D amplifier designs, while MOSFETs are also used in driver and switching stages.

In linear audio applications, device transconductance, bias conditions, linearity, thermal behavior and safe operating area are important.

MOSFET Gate Protection

The MOSFET gate oxide is thin and can be damaged by excessive gate voltage or electrostatic discharge.

A MOSFET must therefore be operated within the maximum VGS rating specified by the manufacturer.

Practical circuits may use:

• Gate resistors
• Gate-source resistors
• Zener or TVS gate protection
• Controlled gate-drive circuits
• ESD precautions during handling

Gate Resistor

A resistor placed between a gate driver and MOSFET gate can control the speed of gate charging and discharging.

It can help reduce:

• Gate ringing
• Electromagnetic interference
• Uncontrolled switching transients
• Oscillation caused by parasitic inductance

The optimum value depends on the MOSFET, driver and circuit layout.

Gate Pull-Down Resistor

A gate-source pull-down resistor can keep an N-channel MOSFET OFF when the control circuit is disconnected or in a high-impedance state.

For a P-channel MOSFET, an appropriate gate-source pull-up arrangement can serve the corresponding purpose.

This prevents the MOSFET gate from being left electrically floating.

MOSFET Power Dissipation

MOSFET power loss generally consists of several components, including conduction losses and switching losses.

For a simplified conduction-loss calculation:

Pconduction ≈ I² × RDS(on)

Switching losses depend on factors such as:

• Switching frequency
• Drain voltage
• Drain current
• Rise and fall times
• Gate charge
• Gate-driver capability

The total device temperature must remain within the manufacturer's specified limits.

MOSFET Thermal Management

Even a MOSFET with low RDS(on) can become hot when carrying large current.

Heat dissipation depends on:

• Power loss
• Package thermal resistance
• PCB copper area
• Heatsink size
• Ambient temperature
• Airflow

The datasheet's thermal specifications should be used when designing the cooling system.

MOSFET Safe Operating Area

The Safe Operating Area (SOA) specifies combinations of voltage, current and time for which the MOSFET can safely operate under the manufacturer's specified conditions.

Simply checking the maximum drain current is not sufficient for many power applications.

A MOSFET can exceed its safe operating conditions even when its current is below the headline maximum rating if the drain-source voltage and pulse duration are also significant.

Avalanche Rating

Some MOSFETs are specified with an avalanche-energy rating.

Avalanche occurs when the drain-source voltage rises beyond the normal blocking condition and the device enters controlled breakdown.

Although an avalanche rating can provide useful design information, it should not automatically be interpreted as permission to operate the MOSFET continuously in avalanche.

MOSFET Testing With a Multimeter

A MOSFET can often be checked with a digital multimeter, although the exact test depends on the MOSFET structure and the device's internal body diode.

For a conventional power N-channel MOSFET, the body diode normally conducts in one direction between drain and source.

A basic test should be performed with the device isolated from the circuit whenever possible.

The gate should also be discharged before making resistance or diode measurements.

Testing the MOSFET Gate

The insulated gate should normally have extremely high resistance to the source and drain when measured with a conventional resistance test.

A very low resistance between gate and source or gate and drain can indicate damage to the gate oxide.

However, meter measurements should be interpreted carefully because some devices contain additional internal structures.

Testing the MOSFET Channel

A MOSFET's apparent resistance between drain and source can change depending on the charge stored on the gate.

This is why simple resistance measurements do not always provide a complete MOSFET test.

A more useful bench test can involve:

• Discharging the gate
• Checking the body diode
• Applying an appropriate gate voltage
• Checking whether the drain-source channel responds
• Removing the gate voltage
• Confirming that the device turns OFF

The maximum VGS rating must never be exceeded during such testing.

Common MOSFET Faults

Common MOSFET failures include:

• Drain-source short circuit
• Gate-source short circuit
• Gate-drain short circuit
• Excessive leakage
• Damaged gate oxide
• Thermal failure
• Body-diode failure
• Increased conduction resistance

A MOSFET that has failed short-circuit in a power supply may cause fuses, current-sense resistors, gate-driver components or other semiconductor devices to fail as well.

MOSFET Replacement

When replacing a MOSFET, the replacement should not be selected solely by matching the part number's voltage and current ratings.

Important specifications include:

• Drain-source voltage rating (VDS)
• Continuous drain current
• RDS(on)
• Gate threshold voltage
• Maximum gate-source voltage
• Total gate charge
• Switching characteristics
• Body-diode characteristics
• Power dissipation
• Safe Operating Area
• Package
• Pin configuration

The replacement MOSFET must also be suitable for the actual gate-drive voltage used by the circuit.

Logic-Level MOSFETs

A logic-level MOSFET is designed to provide useful conduction at relatively low gate-drive voltages.

This can make such devices suitable for control by microcontrollers and other low-voltage logic circuits.

However, the term "logic-level" should not be used as a substitute for checking the datasheet. The important specification is the RDS(on) value at the actual gate voltage being applied.

MOSFET Selection for Switching

When selecting a MOSFET for a switching application, consider the complete set of electrical and thermal requirements.

• VDS rating
• Required drain current
• RDS(on) at the actual VGS
• Total gate charge
• Switching frequency
• Body-diode performance
• Power dissipation
• Thermal resistance
• Package
• SOA

A voltage rating with adequate safety margin should be selected for the actual circuit's transient conditions rather than only its nominal supply voltage.

MOSFET Selection for a Power Supply

For switch-mode power supplies, both conduction and switching losses are important.

A MOSFET with very low RDS(on) can reduce conduction losses, while a device with lower gate charge and suitable switching characteristics can reduce switching losses.

The optimum choice is therefore a balance between:

• RDS(on)
• Gate charge
• Drain voltage rating
• Switching speed
• Thermal performance
• Cost

N-Channel vs P-Channel MOSFET

MOSFET Advantages

• Very high input impedance
• Very low steady-state gate current
• Fast switching capability
• Low conduction resistance in many modern devices
• Available for low-voltage and high-voltage applications
• Excellent for switch-mode power conversion
• Widely available in many package types

MOSFET Limitations

• Gate oxide can be damaged by excessive voltage or ESD
• Gate charge must be supplied during switching
• Switching losses increase with frequency
• RDS(on) generally changes with temperature
• High-side N-channel switching requires suitable gate-drive circuitry
• Body-diode behavior must be considered in power applications

Common MOSFET Applications

Key Points

• MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor.
• The main terminals are gate, drain and source.
• The gate is electrically insulated from the channel.
• MOSFETs have very high input impedance.
• N-channel and P-channel MOSFETs are the two main channel polarities.
• Enhancement-mode MOSFETs are normally OFF without sufficient gate drive.
• VGS is the key control voltage.
• VGS(th) should not be confused with the voltage required for low RDS(on).
• RDS(on) is an important parameter for power switching.
• Gate charge is important for high-frequency switching.
• Power MOSFETs have an intrinsic body-diode path.
• Always check the MOSFET's ratings at the actual gate-drive voltage used.
• Thermal management is essential in high-current applications.
• The datasheet should always be checked before replacing a MOSFET.