AQA A-Level Physics Paper 1, June 2025: Question 16

1 mark · Medium difficulty · Multiple Choice

Identify the correct statement regarding the I–V characteristic curve of a semiconductor diode.

Practise this question

Question

Current-voltage (I-V) characteristic graph for a semiconductor diode. The vertical axis is current I and the horizontal axis is voltage V. For negative voltage, the current is zero, with a point T marked along the negative V-axis. For positive voltage, current remains zero until the threshold voltage, after which it curves sharply upwards. A point S is marked at voltage V_s and current I_s. The area under the curve between 0 and V_s is shaded and labelled P.
Question text

16 The graph shows the I–V characteristic for a semiconductor diode.

Which is true for this graph?

[1 mark]

A The resistance of the diode decreases as V increases for V > Vs.

B The resistance at S is equal to

gradient

C The resistance at T is equal to zero.

D At I = Is, the power dissipated in the diode is equal to area P.

Mark scheme

Show the mark scheme Mark scheme table row for question 16 indicating the correct option is A: 'The resistance of the diode decreases as V increases for V > V_s', categorized as AO1.

16 A The resistance of the diode decreases as V increases for V > Vs. AO1

How to answer it

Diode I–V Characteristics & Resistance Analysis

📋 What this question tests

This question assesses your conceptual understanding of non-ohmic components, specifically the forward and reverse bias behaviour of a semiconductor diode. You must be able to:

  • Distinguish between static resistance ( R = V / I ) and dynamic resistance ( ΔV / ΔI ).
  • Explain how resistance changes along an I–V curve as potential difference increases.
  • Recognise infinite vs zero resistance conditions from graphical data.
  • Evaluate the physical meaning of area under an I–V curve versus electrical power dissipation ( P = IV ).

Question 16 (Multiple Choice)

Analysis of Statement Options [1 Mark]

✅ Correct Answer: A

"The resistance of the diode decreases as V increases for V > Vs."

Resistance is defined as R = V / I . Beyond the threshold voltage ( V > Vs ), the current I increases at a far higher rate than voltage V (the curve gets steeper). Therefore, the ratio V / I continues to decrease as V increases.

Award: 1 mark for option A selected

💡 Key Knowledge: Static vs Dynamic Resistance

  • Static Resistance: At any point on the graph, resistance is always R = V / I . This corresponds to the reciprocal of the gradient of a straight chord drawn from the origin (0,0) to that point.
  • Dynamic Resistance: r = dV / dI = 1 / (tangent gradient) . Dynamic resistance is not the definition of electrical resistance used in A-Level Physics.
  • Reverse Bias (Point T): Very little or no current flows ( I ≈ 0 ), which means the resistance is extremely high (theoretically approaching infinity, not zero).

📐 Detailed Evaluation of Incorrect Options

Option Statement Validity Why it is incorrect
B The resistance at S is equal to 1 / gradient ❌ Incorrect The gradient of the tangent at point S gives ΔI / ΔV . Its reciprocal is dynamic resistance ΔV / ΔI . Actual resistance is RS = Vs / Is (reciprocal of chord from origin, not the tangent).
C The resistance at T is equal to zero ❌ Incorrect At point T, V < 0 while I ≈ 0 . Since R = V / I , dividing a non-zero voltage by an almost-zero current yields an infinitely large resistance ( R → ∞ ), not zero.
D At I = Is, the power dissipated is equal to area P ❌ Incorrect Power dissipated is P = Is × Vs , which is represented by the entire rectangle with sides Is and Vs . Area P is the integral ∫ I dV , which is strictly less than IsVs .

❌ Common Traps & Student Misconceptions

  • Confusing "Zero Current" with "Zero Resistance": Many students see a flat horizontal line along the voltage axis at T and assume R = 0 . Remember: if current cannot flow despite a voltage being applied, the resistance must be enormous (infinite), not zero!
  • Assuming R = 1 / (tangent gradient): A common error carried over from linear graphs. For non-ohmic components, resistance is not the tangent gradient. It is the coordinates evaluated as V / I .
  • Area Under I–V Curve Fallacy: Area under a force-extension graph or velocity-time graph has physical meaning (work done, displacement). However, area under an I–V curve ( ∫ I dV ) is not the total power dissipated. Total power is simply the product P = IV .

🧠 Exam Technique: Elimination Strategy

For multiple choice questions on graph interpretation:

  1. Check definitions first: Instantly disqualify statements that define resistance as the tangent gradient for curved lines (eliminates B).
  2. Test extreme values: At T, current is blocked. A blocked path cannot have zero resistance; it acts as an open circuit ( R = ∞ ) (eliminates C).
  3. Check dimensions/geometry: Power is a product of single values ( P = I × V , a rectangle), not an area bounded under a curve (eliminates D).
  4. This confirms A with high confidence within 60 seconds.

Topics

Physics · 3.5 Electricity

Question and mark scheme from the AQA A-Level Physics examination, Paper 1, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.