MOSFET (metal-oxide semiconducting field-effect transistor) (A-level only)

What you'll learn

• The basic structure and terminals of an N-channel enhancement mode MOSFET.
• How to interpret the characteristic voltages and currents ($V_{DS}$, $V_{GS}$, $I_{DSS}$, and $V_{th}$).
• Why the MOSFET's near-infinite input resistance makes it a brilliant electronic switch.
• How to solve potential divider circuits designed to turn a MOSFET on and off.

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Meet the MOSFET

A MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) is a semiconductor device that acts as an electrically controlled switch or amplifier. While you might have seen bipolar junction transistors (BJTs) before, which are controlled by current, a MOSFET is controlled purely by voltage.

For AQA A-Level Physics, you only need to know about one specific type: the N-channel, enhancement mode MOSFET.

To understand how it works, we first need to look at its internal structure.

Notice the insulating oxide layer (usually silicon dioxide) sitting immediately directly below the Metal Gate. This is the most important structural feature of the MOSFET! Because the Gate is completely insulated from the rest of the semiconductor, no current can flow into the Gate terminal.

How the MOSFET Turns On (Enhancement Mode)

By default, an N-channel enhancement mode MOSFET is OFF. If you apply a voltage between the Drain and the Source ($V_{DS}$) but leave the Gate alone, no current will flow because the P-type substrate blocks the path between the two N-type regions.

To turn it on, we must apply a positive voltage to the Gate relative to the Source. This is called the Gate-Source Voltage, or $V_{GS}$.

When $V_{GS}$ is positive, it creates an electric field that pushes positive "holes" away from the oxide layer and attracts negatively charged electrons up toward it. This gathers a thin, temporary bridge of electrons directly under the Gate. Because this bridge is made of negative charge carriers, it is called an N-channel.

Since we are "enhancing" the conductivity of the device by applying this voltage, this is called enhancement mode.

• If $V_{GS}<V_{th}$, the MOSFET is OFF (acts like an open switch).
• If $V_{GS}\ge V_{th}$, the N-channel forms, and the MOSFET turns ON (acts like a closed switch).

Characteristic Voltages and Currents

Once the N-channel is open, current can flow. The current flowing into the Drain is called the Drain Current, $I_D$. The amount of current that flows depends on both $V_{GS}$ (how "wide" you open the channel) and $V_{DS}$ (how hard you push the electrons across it).

We can plot how $I_D$ changes as we increase $V_{DS}$. This gives us a family of output characteristic curves.

Look closely at the graph:

1. The rising slope: At low values of $V_{DS}$, the current $I_D$ increases roughly proportionally to the voltage. Here, the N-channel is behaving a bit like a standard ohmic resistor.
2. The plateau (Saturation): As $V_{DS}$ increases further, the current stops rising and flattens out into a horizontal line. The MOSFET has reached its maximum current capacity for that specific Gate voltage.

Using the MOSFET as a Switch

The most common A-Level application for a MOSFET is using it as an electronic switch controlled by a sensor (like a thermistor or an LDR).

Because the Gate has virtually infinite input resistance, it draws zero current. This is perfect for potential divider circuits, because we can connect the Gate to the middle of the divider without it "stealing" any current and messing up the voltage calculation!

In the circuit above, the LDR and the fixed resistor form a potential divider across the supply voltage. The Gate is connected to the midpoint. When the light level changes, the resistance of the LDR changes, which changes the voltage at the midpoint. If that voltage (which is $V_{GS}$) rises above $V_{th}$, the MOSFET switches on, allowing a large current to flow through the Drain and light the lamp.