AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Drain, source and gate.
Full specification wording
MOSFET (metal-oxide semiconducting field-effect transistor) (A-level only): Drain, source and gate.
The MOSFET has three terminals.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The MOSFET has three terminals. The source is the terminal from which charge carriers enter the channel; in an n-channel device electrons flow from source to drain, so conventional current flows from drain to source. The drain is the terminal that collects the carriers and connects to the load or supply. The gate is the control terminal, separated from the channel by a thin insulating oxide layer, so it draws almost no steady current but its voltage sets the channel conductivity. In circuit diagrams the arrow on the source terminal indicates the direction of conventional current for that device type. Correct identification matters because the gate-source voltage controls operation, while the drain-source path carries the output current.
What earns the marks
- Names the three terminals as gate, source and drain and links each to its function in the device.
- States that the gate is the control terminal and is insulated from the channel by an oxide layer.
- States that the source is the terminal through which carriers enter the channel and the drain is the terminal that collects them.
- Uses the gate-source voltage as the controlling input and the drain-source path as the output current path.
- Interprets the arrow on the source terminal as showing the direction of conventional current for the device type.
- Applies terminal identification when connecting a MOSFET in a circuit as a switch or amplifier.
Common mistakes and how to put them right
Labelling the gate as the output terminal.
Put it right by identifying the drain-source path as the output current path and the gate as the control input.
Reversing source and drain in an n-channel circuit.
Put it right by tracking electron flow from source to drain and conventional current from drain to source.
Ignoring the oxide layer when describing the gate.
Put it right by stating that the insulation gives the gate its very high input resistance.
Exam tips
- Label terminal names directly on any circuit or cross-section diagram you draw.
- State the carrier type and then give the conventional current direction to avoid ambiguity.
- Link each terminal to its function in one short clause rather than listing names alone.
Try the lesson
The start of the MOSFET (metal-oxide semiconducting field-effect transistor) (A-level only): Drain, source and gate. lesson, made from checked teaching notes for this specification point.
Have a go first
Which terminal of an n-channel enhancement MOSFET is the region from which charge carriers enter the channel?
Listen
The MOSFET has three terminals. The source is the terminal from which charge carriers enter the channel; in an n-channel device electrons flow from source to drain, so conventional current flows from drain to source. The drain is the terminal that collects the carriers and connects to the load or supply. The gate is the control terminal, separated from the channel by a thin insulating oxide layer, so it draws almost no steady current but its voltage sets the channel conductivity. In circuit diagrams the arrow on the source terminal indicates the direction of conventional current for that device type. Correct identification matters because the gate-source voltage controls operation, while the drain-source path carries the output current. A strong answer earns marks for these points. Names the three terminals as gate, source and drain and links each to its function in the device. States that the gate is the control terminal and is insulated from the channel by an oxide layer. States that the source is the terminal through which carriers enter the channel and the drain is the terminal that collects them. Uses the gate-source voltage as the controlling input and the drain-source path as the output current path. A common slip: Labelling the gate as the output terminal. Instead, identifying the drain-source path as the output current path and the gate as the control input. Exam tip: Label terminal names directly on any circuit or cross-section diagram you draw.
See it
Key words
Say what each word means before you turn the card.
Source
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Drain
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Gate
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Gate-source voltage
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See every definition
- Source
- The terminal from which charge carriers enter the channel; in an n-channel device electrons flow from source to drain.
- Drain
- The terminal that collects the carriers and connects to the load or supply.
- Gate
- The control terminal, separated from the channel by a thin insulating oxide layer, whose voltage sets the channel conductivity.
- Gate-source voltage
- The controlling input, while the drain-source path carries the output current.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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