School-college Physics Notes: Electricity 3.4 The diode - the I-V graph

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Electricity Section 3: 3.4 Investigating the current - voltage characteristics of a diode

[Author © Dr WP Brown PhD: Doc Brown's physics exam revision notes suitable for students of UK IGCSE & GCSE level physics courses, ~ US grades 9-10 physics, electricity page updated Feb 8th 2026]

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INDEX for electricity section 3 notes on current, voltage, resistance, energy & charge transfer in circuits

3.4 Investigating the current

The voltage characteristics of a diode
 

circuit diagram to investigate the current - voltage behavior chracteristics of a diode The current through a diode flows in one direction only - see graph 3.

The resistance in the opposite direction is very high - hence effectively a 'one way' system.

Circuit 43 shows how you can investigate the current - potential difference characteristics of a diode.

The voltmeter is wired in parallel with the thermistor, the p.d. V is measured in volts (V).

The variable resistor allows you to vary the p.d. and current flow.

The ammeter, wired in series, gives you the current I reading in amps (A).

 

A diode has a very high resistance in the reverse direction.

current-voltage graph for a diode in an electrical circuit

There is also a threshold p.d. (e.g. 1.4 V) before any current flows at all - look at the graph carefully - there is a short horizontal portion before the current rises from zero and eventually becomes linear.

Therefore you get a top right portion of the graph 3 compared to graphs 1 an 2 above.

This is because when you do the experiment using the circuit described above, on reversing the connections, you will find no current flows as you vary the p.d.

Its a non-linear graph.

If the gradient is changing, then the resistance is changing.

 

When the current (A) is NOT proportional to the p.d (V), the diode is described as a non-ohmic conductor (doesn't obey Ohm's Law!).

The phrase non-linear component may be used.

The graph (3) is constructed on a crosswire axis. The top right half is your first set of results, you then reverse the terminals on the power supply and repeat the experiment giving the bottom left part of the graph.

Since the current only flows one way through a diode, it can be used to convert an ac current into a dc current

 

INDEX of electricity section 3 notes on current, voltage, resistance, energy & charge transfer in circuits including Ohm's Law investigations

Key points about electric circuits in physics courses - a diode circuit

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations.

Here's a structured, exam-board-aligned summary on the current-voltage (I–V) behaviour of a diode, tailored for GCSE/IGCSE Physics students:

Diode – Current-Voltage Behaviour

What is a Diode?

A diode is an electronic component that allows current to flow in one direction only.

It has:

  • Low resistance in the forward direction (forward bias)
  • Very high resistance in the reverse direction (reverse bias)

This makes it a non-ohmic component, meaning it does not obey Ohm’s Law.

I–V graph characteristics of a Diode

Region Description
Forward Bias Current flows after a threshold voltage (~0.6V for silicon diodes); resistance drops sharply
Reverse Bias Almost no current flows; diode acts as an insulator
Graph Shape Flat near zero (no current), then sharp rise after threshold; no current in reverse

Graph Tip: The I–V graph is non-linear and asymmetric. It shows:

  • A sharp curve in the forward direction after threshold
  • A flat line near zero in the reverse direction

Required Practical (All Boards)

Aim: Investigate the I–V characteristics of a diode

Method Overview:

  • Connect diode in series with ammeter and variable power supply
  • Use voltmeter in parallel across diode
  • Measure current and voltage in both forward and reverse directions
  • Plot I versus V graph

Expected Result: Non-linear, directional graph showing diode’s one-way behaviour

Typical Exam Board Syllabus Content for a diode

Key Focus
Maybe required practical on I–V characteristics; diode as non-ohmic component
Emphasis on interpreting I–V graphs and understanding directionality
Includes diode in non-linear component studies and practicals
Focus on circuit diagrams and diode behaviour in forward/reverse bias
Investigates diode function and current flow direction
Strong emphasis on I–V graphs, definitions, and practical understanding

Student Tips

  • Understand directionality: Diodes only conduct in one direction
  • Sketch the I–V graph: Show threshold behaviour and no reverse current
  • Use correct terms: “Forward bias”, “reverse bias”, “threshold voltage”
  • Explain non-ohmic: Diodes don’t follow ( V proportional to I )
  • Revise practicals: Be able to describe method, apparatus, and expected results
  • Think real-world: Diodes are used in rectifiers (AC to DC converters)

Keywords, phrases and learning objectives for the current-voltage behaviour of a diode

Be able to describe the circuit and explain the method of how to investigate the electrical resistance of diode.

Be able to describe and explain the current-voltage graph for electrical behaviour of a diode.

Be able to do any calculations of results for the current-voltage graph for the diode, a non-ohmic conductor.

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