How is a MOSFET used as a voltage-controlled switch, and how does it compare with a bipolar transistor?
MOSFET switching: the MOSFET as a voltage-controlled switch, the gate threshold voltage, why it draws no steady gate current, and choosing between a MOSFET and a bipolar transistor.
An Eduqas GCSE Electronics answer on the MOSFET as a switch: voltage control by the gate, the gate threshold voltage, why a MOSFET draws essentially no steady gate current, switching higher-current loads, and choosing between a MOSFET and a bipolar transistor for an output stage.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Eduqas wants you to use a MOSFET as a voltage-controlled switch: how a gate voltage above the threshold turns it on, why it draws essentially no steady gate current, and when to choose a MOSFET over a bipolar transistor. The MOSFET is the modern choice for switching higher-current loads from a weak control signal.
The answer
The MOSFET as a voltage-controlled switch
The gate threshold voltage
Why it draws no steady gate current
Choosing a MOSFET or a bipolar transistor
Examples in context
MOSFETs are the standard high-current output device in modern electronics: they switch motors, heaters, high-power LEDs and lamps directly from a microcontroller pin that could never supply enough base current for a bipolar transistor. Their negligible gate current and low on-resistance make them efficient, which is why they dominate power-switching and motor-driver stages in the non-exam assessment. Like the bipolar switch, a MOSFET driving an inductive load still needs a flyback diode to absorb the back-EMF.
Try this
Q1. State what kind of quantity controls a MOSFET. [1 mark]
- Cue. A voltage (the gate-source voltage), not a current.
Q2. State the condition on the gate-source voltage for an n-channel MOSFET to turn on. [1 mark]
- Cue. must exceed the threshold voltage .
Q3. State one advantage of a MOSFET over a bipolar transistor for switching a large load from a microcontroller. [1 mark]
- Cue. It draws essentially no steady gate current, so a weak output can switch it fully on (and its on-resistance is low).
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20214 marksExplain how an n-channel MOSFET is turned on, and state one advantage of using a MOSFET rather than a bipolar transistor as a switch.Show worked answer →
Turning it on (up to 3 marks): a MOSFET is a voltage-controlled switch. When the gate-source voltage is raised above the threshold voltage, a conducting channel forms between the drain and source, so the MOSFET turns on and current flows through the load. Below the threshold no channel exists and the MOSFET is off.
Advantage (1 mark): the gate draws essentially no steady current (the gate is insulated), so almost no power is needed to hold it on, and it can be driven directly from a high-impedance source such as a logic output or sensor. (Other acceptable advantages: very low on-resistance for high-current loads, fast switching.)
Markers reward the gate voltage exceeding the threshold to form the channel, and a valid advantage such as negligible gate current.
Eduqas 20223 marksA designer must switch a motor from a microcontroller output that can supply only . Explain why a MOSFET is a better choice than a single bipolar transistor here.Show worked answer →
Reasoning (up to 3 marks): a single bipolar transistor needs a base current of about ; for a load with a typical gain of that is about , more than the the microcontroller can supply, so it would not saturate. A MOSFET is voltage-controlled and draws essentially no steady gate current, so the output can switch it fully on; its low on-resistance also dissipates little power at .
Markers reward the bipolar base-current demand exceeding the available drive, and the MOSFET needing negligible gate current (voltage control), with low on-resistance for the high current.
Related dot points
- Transistor switching: the bipolar transistor as a switch, cut-off and saturation, current gain, and choosing the base resistor to drive a load.
An Eduqas GCSE Electronics answer on using a bipolar transistor as a switch: the cut-off and saturation states, current gain relating collector and base current, and choosing the base resistor so a small input current turns on a larger load current.
- Diodes: forward and reverse bias, the forward voltage drop, the LED, rectification, and protecting circuits against reverse current and back-EMF.
An Eduqas GCSE Electronics answer on diodes: forward and reverse bias, the forward voltage drop, the light-emitting diode, half-wave rectification of a.c. to d.c., and using a diode to protect a circuit from reverse polarity and from the back-EMF of a relay or motor.
- Comparators: comparing two voltages, the reference set by a potential divider, the digital output, and using a comparator to make a sensing system switch at a threshold.
An Eduqas GCSE Electronics answer on comparators: how a comparator compares two input voltages and switches its output high or low, setting the reference with a potential divider, the digital nature of the output, and combining a sensor divider with a comparator to switch a circuit at a chosen threshold.
- Microcontrollers: the microcontroller as a programmable processing subsystem, inputs and outputs, and planning a control program with a flowchart.
An Eduqas GCSE Electronics answer on microcontrollers: the microcontroller as a programmable processing subsystem with input and output pins, the advantages of programming over fixed logic, and planning a control program with a flowchart using the standard symbols.
Sources & how we know this
- WJEC Eduqas GCSE (9-1) Electronics specification (C490) — WJEC Eduqas (2017)