Explain diode current–voltage directionality
Sketch a diode's forward and reverse behaviour with explicit polarity and avoid assigning a universal threshold or exact zero leakage.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Read positive and negative I–V axes.
- Distinguish current direction from magnitude.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What do positive and negative graph voltages indicate?
What you will learn
- Sketch and explain a diode I–V pattern.
- Distinguish directional conduction from resistor and filament-lamp behaviour.
Identify the forward reference
A diode conducts much more readily in its forward direction than its reverse direction. Choose the graph's positive voltage reference as forward bias. The forward current is initially small and rises rapidly as forward p.d. increases over a suitable region. Reverse current remains very small on the ordinary plotted scale within the stated operating range.
The device's exact forward behaviour depends on its type, temperature and current. Do not assign one universal turn-on voltage from an uncalibrated qualitative sketch. The simple pattern omits reverse breakdown and small leakage; it is not a claim that real reverse current is exactly zero in every condition.
Compare with other components
A constant-resistance resistor has a straight symmetric origin line, and an ordinary filament lamp has broadly matching positive and negative curved branches in the school model. The diode is strongly asymmetric under reversal. This is the useful direction-selecting behaviour rather than a source creating charge.
A current–voltage graph alone does not set a complete safe device circuit: operating ratings and any current-limiting component must be specified for a practical arrangement. Here the task is a qualitative pattern and interpretation. Circuit symbols and LED applications belong to the next circuit-component section.
Sources for this lesson
Pause and explain
What is justified by an uncalibrated ordinary diode sketch?
Worked example
A diode's positive voltage reference is forward bias. Describe its two branches within an ordinary range without breakdown.
Show the worked solution
- Forward current rises strongly over a suitable positive-voltage region.
- Reverse current is very small on this plotting scale.
- The graph is strongly asymmetric under polarity reversal.
Answer Strong forward conduction; very small reverse current in the stated non-breakdown range.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Describe a diode I–V sketch with positive V defined as forward bias.
Give me a hint
Include a rapid forward rise and the small reverse response within the stated range.
Compare my reasoning
- Label I vertically and V horizontally.
- Draw strong forward growth in the positive-voltage region.
- Show very small reverse current on the scale, excluding breakdown.
An asymmetric pattern with strong forward growth and very small reverse current in the stated range.
Look for these in your work
- Specify forward polarity.
- Retain leakage and breakdown limits.
Compare diode, constant-resistance resistor and filament-lamp behaviour under voltage reversal.
Give me a hint
Distinguish directionality from thermal curvature.
Compare my reasoning
- The resistor has a symmetric straight origin relation.
- The lamp has broadly matching curved polarity branches.
- The diode conducts far more readily forward than reverse.
Resistor symmetric straight; lamp broadly symmetric curved; diode strongly asymmetric.
Look for these in your work
- Cover all three components.
- Name the reason for the diode difference.
A learner reads an unlabelled schematic knee as an exact universal diode threshold and assumes reverse current is zero at unlimited voltage. Assess both claims.
Give me a hint
Check calibration, device variation and the excluded reverse region.
Compare my reasoning
- The knee has no universal calibrated value.
- Device and temperature affect forward behaviour.
- Small leakage and reverse breakdown invalidate the unlimited zero-current claim.
Neither follows; use device-specific data and the stated operating range.
Look for these in your work
- Reject universal threshold inference.
- Keep leakage and breakdown explicit.
Common mistakes
- Specify which polarity is forward.
- No universal threshold follows from an uncalibrated sketch.
- Tiny omitted leakage is not exact zero under every condition.
What can you explain now?
What distinguishes a diode from the ordinary resistor and lamp patterns?
Compare with the explanation
Strongly different forward and reverse conduction.
Its I–V response is asymmetric rather than a matching two-polarity conducting pattern.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Sketch and explain diode directionality without overstating a schematic threshold or reverse-current idealisation.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.