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How Does a MOSFET Control a Load?

  • Basic
  • Article
  • Ages 15+
  • 4 min read
A black power MOSFET with a metal mounting tab and three leads sits beside an aluminum heat sink.

A MOSFET is a voltage-controlled semiconductor component. Common discrete MOSFETs usually expose three external terminals: Gate, Drain and Source; the substrate (body) is usually connected internally to Source. The gate electrode is insulated from the channel, so gate voltage relative to source controls the current running between drain and source. Its high input resistance means little current is needed to maintain the gate voltage, while switching requires charge to move into or out of the gate.

Contents
A common three-terminal model: the gate is insulated from the channel, whose ends are drain and source.
  • Gate
  • Drain
  • Source

What you will learn

Before you start

It helps to know what a BJT and a capacitor are.

What terminals and types does a MOSFET have?

The MOSFET is a type of solid-state semiconductor device, also known as an Insulated Gate Field Effect Transistor (IGFET). Common discrete MOSFETs expose three external terminals: Gate, Drain and Source; the substrate is usually connected internally to Source.

Both P-channel (PMOS) and N-channel (NMOS) MOSFET types are available. Both use gate-to-source voltage as the control.

Why can gate voltage control current?

The gate electrode is insulated from the N-channel or P-channel by a very thin layer of insulating material. The gate terminal controls the flow of current between the drain and source by creating an electric field.

This insulated gate electrode can be thought of as one plate of a capacitor. The gate's high input resistance means little current is needed to maintain its voltage, but a driver must add or remove charge to switch it.

A thin insulating layer sits between gate and channel, like the plates of a capacitor.
  • Gate electrode
  • Insulating layer
  • Channel
  • Voltage applied to the gate

In its linear (ohmic) region, the MOSFET also acts like a voltage-controlled resistor.

Teaching model: gate voltage controls the MOSFET channel and thereby the load current.
  • Control voltage
  • MOSFET
  • Main current to the load
  • Load

How do enhancement and depletion mode differ?

The enhancement mode MOSFET is equivalent to a normally open switch, while the depletion mode MOSFET is equivalent to a normally closed switch.

When selecting a part, identify which kind it is to know its state when gate-to-source voltage is zero.

At zero gate-to-source voltage, enhancement mode is like an open switch and depletion mode like a closed switch.
  • Normally open (enhancement)
  • Normally closed (depletion)

Real-world example

With an adult, look at a circuit that drives a small fan or lamp. In a suitable design, a control signal applies voltage to the gate so the MOSFET switches the current path through the load.

Common misunderstandings

A MOSFET is controlled by a continuous current into the gate. The gate voltage controls the MOSFET; a driver supplies or removes charge briefly when changing that voltage, then needs little current to maintain it.

Every MOSFET is off when gate-to-source voltage is zero. Only the enhancement mode is equivalent to a normally open switch, while the depletion mode is equivalent to a normally closed one.

Frequently asked questions

What terminals does a MOSFET have?

The three external terminals commonly found on discrete MOSFETs are Gate, Drain and Source; some structures also have a substrate terminal, usually connected internally to Source.

Does the MOSFET gate draw current?

The insulated gate needs charge added or removed to change its voltage. Because its input resistance is very high, little current is needed to hold a steady voltage.

What is an enhancement mode MOSFET?

It is the kind that is equivalent to a normally open switch.

What are NMOS and PMOS?

They are N-channel and P-channel MOSFETs, and both types are available.

Summary

  • Common discrete MOSFETs usually expose Gate, Drain and Source and are voltage-controlled.
  • The insulated gate has high input resistance; changing its voltage requires gate charge, while little current is needed to hold a steady voltage.
  • Enhancement mode is like a normally open switch and depletion mode like a normally closed one.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

Metal-oxide-semiconductor field-effect transistorMOSFET
A field-effect transistor with an insulated gate that controls current by voltage.
GateCực gate
The control terminal of a MOSFET, insulated from the channel.
Drain and sourceDrain và source
The two MOSFET terminals between which the main current runs.
Enhancement modeLoại tăng cường
A MOSFET that needs gate-source voltage to turn on, like a normally open switch.
Depletion modeLoại suy giảm
A MOSFET that needs gate-source voltage to turn off, like a normally closed switch.

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Sources

These sources are written for older readers. If they feel hard, read them with an adult.