A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: MOSFETs: Structure, Characteristics, and Applications — mark scheme explained
The short answer
Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) are fundamental components in modern electronics, particularly in digital circuits and power electronics. This section focuses on the simplified structure, behavior, and characteristics of N-channel enhancement mode MOSFETs, as well as their use as switches and devices with high input resistance.
The question
A MOSFET has a threshold voltage (V th ) of 2 V. If the gate-source voltage (V GS ) is 3 V, will the MOSFET be in the cutoff region, triode region, or saturation region? Explain your answer.
[Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
How the examiner actually awards the marks on this topic.
Gradora's own decode of the marking approach — not the exam board's published mark scheme.
How marks are awarded
For questions involving the operating regions of a MOSFET, allocate marks for correctly identifying the given voltages (1 mark), comparing V GS with V th (1 mark), and determining the correct operating region (2-3 marks).
What the command words demand
- Identify
- Determine which voltages are given and what they represent.
- Compare
- Compare V GS with V th to determine if the MOSFET is on or off.
- Determine
- Calculate V GS - V th and compare it with V DS to identify the operating region.
Model answer
A full-mark response to the question above, worked through step by step.
Timing: Allocate about 5 minutes to answer questions on MOSFETs, ensuring you have enough time to carefully compare voltages and identify the operating regions.
- 1. Identify the threshold voltage: V th = 2 V1 mark
- 2. Identify the gate-source voltage: V GS = 3 V0 marks
- 3. Compare V GS with V th : Since V GS (3 V) > V th (2 V), the MOSFET is not in the cutoff region.1 mark
- 4. Determine if the MOSFET can be in the triode or saturation region: To do this, we need to know V DS . However, since V GS > V th , the MOSFET is definitely on.1 mark
- 5. Conclusion: The MOSFET will be in either the triode or saturation region, depending on the value of V DS .1 mark
Final answer: The MOSFET will be in either the triode or saturation region, but not in the cutoff region.
Work through every step correctly and you earn all 4 marks.
Another worked example
A MOSFET is operating with a drain-source voltage (V DS ) of 5 V and a gate-source voltage (V GS ) of 6 V. The threshold voltage (V th ) is 2 V. Determine the region in which the MOSFET is operating.
- 1. Identify the given voltages: V DS = 5 V, V GS = 6 V, V th = 2 V1 mark
- 2. Check if V GS > V th : Since 6 V > 2 V, the MOSFET is on.1 mark
- 3. Calculate V GS - V th : 6 V - 2 V = 4 V (the overdrive voltage)1 mark
- 4. Compare V DS with V GS - V th : Since V DS (5 V) > V GS - V th (4 V), the MOSFET is in the saturation region.1 mark
- 5. Conclusion: The MOSFET is operating in the saturation region.1 mark
Final answer: The MOSFET is operating in the saturation region.
Work through every step correctly and you earn all 5 marks.
Common mistakes
Confusing V DS with V GS
Why it happens: Students often mix up the drain-source voltage (V DS ) and the gate-source voltage (V GS ).
Fix: Always label the voltages clearly and remember that V DS is between the drain and source, while V GS is between the gate and source.
Forgetting to check if V GS > V th
Why it happens: Students sometimes skip this crucial step when determining the operating region of a MOSFET.
Fix: Always start by comparing V GS with V th . If V GS is less than V th , the MOSFET is off (cutoff region).
Misidentifying the triode and saturation regions
Why it happens: Students may confuse the conditions for the triode and saturation regions.
Fix: Remember that in the triode region, V DS < V GS - V th , while in the saturation region, V DS > V GS - V th .
Forgetting that the gate has high input resistance
Why it happens: Students might overlook this important characteristic of MOSFETs.
Fix: Always remember that the gate terminal is insulated by a thin oxide layer, making it have very high input resistance.
Not understanding the role of V th
Why it happens: Students may not fully grasp the significance of the threshold voltage (V th ).
Fix: Understand that V th is the minimum gate-source voltage required to start conducting current between the drain and source.
Confusing the cutoff region with the triode region
Why it happens: Students might think that any V GS < V th means the MOSFET is in the triode region.
Fix: Remember that if V GS < V th , the MOSFET is in the cutoff region and no current flows between the drain and source.
Where the marks go
The question types you’ll meet on this topic and the marks each one carries — so you know what to expect and where to focus.
| Question type | What you’re asked to do | Marks |
|---|---|---|
| MOSFET Operating Region | Compare gate-source voltage with threshold voltage to determine the MOSFET's operating region. | 4 |
| MOSFET Operating Region | Compare the MOSFET's terminal voltages to threshold and overdrive to identify its operating region. | 5 |
| MOSFET Operating Region | Compare gate-source voltage with threshold voltage to decide whether the MOSFET conducts. | 3 |
| MOSFET Operating Region | Identify a MOSFET's operating region by comparing gate-source and drain-source voltages against the threshold voltage. | 5 |
| Total across these question types | 17 | |
Question types and mark tariffs are Gradora’s guidance based on how this topic is typically examined — not the board’s official paper structure.