I/V characteristic of a semiconductor diode

Class practical

An example of the behaviour of a simple component, giving students opportunities to construct a circuit, gather data and perform some analysis.

Apparatus and Materials

• Semiconductor diode - e.g. IN 5401
• Protective resistor, at least 10 ohm
• Power supply, 0 to 12 V, DC (or, better, small smooth stabilized 5 V supply)
• Leads, 4 mm
• Multimeters, 2, or 1 ammeter and 1 voltmeter of suitable ranges
• Rheostat

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Set up the circuit as shown below.

2. Use the variable power supply and the variable resistor to vary the potential difference across the diode from 0 V to +0.8 V in intervals of about 0.1 V. Record pairs of potential difference and current values.
3. Repeat in the range 0 V to -4.0 V in intervals of 0.5V, by reversing the connections on the diode.
4. Analysis. Plot a graph of current/A (y-axis) against potential difference/V (x-axis). Remember to include the readings for ‘negative’ voltages.
5. The resistance of the diode at a particular voltage = potential difference/current reading.
6. Use the graph to calculate the resistance of the diode at a number of different potential differences.
7. Describe how the resistance changes with potential difference. Is the resistance of the diode the same for ‘positive’ voltages and ‘negative’ voltages?
8. The conductance of the diode at a particular potential difference = current/potential difference.
9. Use the graph to calculate the conductance of the diode at a number of potential differences.

Teaching Notes

• The aim of this experiment is to develop confidence in setting up simple circuits and in taking careful measurements. The analysis is fairly straightforward but students may well need reminding to convert mA into A where necessary.
• It is often stated that the resistance of a component is the gradient of a V against I graph. This is not usually the case. Resistance is the ratio of V/I so it is best to encourage students to takeV/I ratios at specific points.
• The main learning point here is that the diode only allows current flow in one direction.
• Using a potential divider, as shown below, will enable students to get a full range of readings.

• You could discuss the miniaturization that is possible by building integrated circuits onto a wafer of semiconductor. Students may have heard of Moore’s law in which Intel’s co-founder Gordon Moore proposed the trend that the number of components on an integrated circuit would approximately double every two years. It has held from 1972 to at least 2006.
• This experiment comes from AS/A2 Advancing Physics. It has been re-written for this website by Lawrence Herklots, King Edward VI School, Southampton.

This experiment was safety-tested in January 2007