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
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
1. Identify the threshold voltage: V th = 2 V
- S2
2. Identify the gate-source voltage: V GS = 3 V
- S3
3. Compare V GS with V th : Since V GS (3 V) > V th (2 V), the MOSFET is not in the cutoff region.
- S4
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.
- S5
5. Conclusion: The MOSFET will be in either the triode or saturation region, depending on the value of V DS .
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
1. Identify the threshold voltage: V th = 2 V
- S2
2. Identify the gate-source voltage: V GS = 3 V
- S3
3. Compare V GS with V th : Since V GS (3 V) > V th (2 V), the MOSFET is not in the cutoff region.
- S4
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.
- S5
5. Conclusion: The MOSFET will be in either the triode or saturation region, depending on the value of V DS .
Final answer: The MOSFET will be in either the triode or saturation region, but not in the cutoff region.
Common mistakes
- Confusing V DS with V GS — 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 — 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 — 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 — 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 — 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 — 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
- Full worked solution (all marking points)4 marks