How do an npn transistor and an n-channel MOSFET act as electronic switches?
The npn bipolar transistor and the n-channel enhancement MOSFET used as switches: how a small input controls a larger output current, the meaning of saturation and cut-off, and the differences between the two devices.
A focused answer to WJEC Eduqas GCSE Electronics on transistor and MOSFET switching, covering how an npn transistor and an n-channel enhancement MOSFET act as switches, saturation and cut-off, and the differences between the two devices.
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What this topic is asking
WJEC Eduqas wants you to know how two transistors act as electronic switches: the npn bipolar transistor and the n-channel enhancement MOSFET. You should explain how a small input controls a much larger output current, what saturation (fully on) and cut-off (fully off) mean, and the key differences between the two devices, including that the bipolar transistor is current-controlled and the MOSFET is voltage-controlled.
The transistor as a switch
Switching is different from amplifying: as a switch the transistor is driven hard into full-on or full-off, not held in between. In the on state the voltage across the transistor is small and the full supply appears across the load; in the off state no current flows and the full supply appears across the transistor. This on/off behaviour is exactly what is needed to drive lamps, motors, relays and buzzers from a small control signal.
The npn bipolar transistor
To switch a load, the load is placed in the collector circuit and the control signal is applied to the base through a resistor. When the base voltage rises above about , base current flows and the transistor turns on (saturates), so a large collector current passes through the load. When the base voltage is below , the transistor is cut off and no collector current flows. The base resistor is essential: without it the base current would be too large and could destroy the transistor.
The n-channel enhancement MOSFET
The MOSFET turns on when the gate-source voltage exceeds its threshold voltage; below that it is off. Because the gate is insulated, practically no current flows into it, so the controlling circuit does not have to supply current to keep the MOSFET on (unlike the bipolar transistor's base). This makes the MOSFET ideal for switching from a logic output, and a low on-resistance lets it switch large currents with little heat. The load is connected in the drain circuit.
Comparing the two switches
Both devices do the same job - a small input switches a large load - but they differ in how they are driven and in their terminal names (base/collector/emitter versus gate/drain/source). The bipolar transistor is simple and cheap; the MOSFET wastes no input current and can handle large load currents efficiently. You choose between them using the device data sheet, checking the maximum current, the threshold or turn-on requirement, and the on-state voltage.
Try this
Q1. State whether a bipolar transistor is current-controlled or voltage-controlled, and name its three terminals. [2 marks]
- Cue. Current-controlled; base, collector and emitter.
Q2. State why a MOSFET is easy to drive from a logic gate output. [1 mark]
- Cue. Its gate draws almost no current (high input resistance), so the logic output is not loaded.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas style4 marksExplain how an npn transistor is used to switch a lamp on when a sensor voltage rises, including the purpose of the base resistor.Show worked answer →
A Component 1 Explain question on transistor switching. The lamp is connected in the collector circuit and the sensor voltage is applied to the base through a resistor (1 mark for the connection). When the base voltage is high enough (above about ), a small base current flows, which switches the transistor on (saturated), allowing a much larger collector current to flow through the lamp, turning it on (2 marks for the small base current controlling the large collector current). The base resistor limits the base current to a safe value so the transistor is not damaged (1 mark). Markers reward the small-controls-large idea, saturation and the base resistor's role.
Eduqas style3 marksState two differences between an n-channel enhancement MOSFET and an npn bipolar transistor when used as a switch.Show worked answer →
A Component 1 compare question. Acceptable differences (1 mark each, up to 3): a MOSFET is switched on by a voltage on the gate and draws almost no input (gate) current, whereas an npn transistor needs a continuous base current to stay on; the MOSFET's controlling terminal is the gate (with source and drain), while the transistor's is the base (with emitter and collector); a MOSFET has a very high input resistance; a MOSFET typically has a low on-resistance so it can switch larger currents efficiently. Markers reward any clear, correct differences such as voltage-controlled versus current-controlled and the terminal names.
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Sources & how we know this
- WJEC Eduqas GCSE Electronics specification (from 2017) — WJEC Eduqas (2017)