Conductivity of germanium

Demonstration

A slice of germanium conducts when heated.

Apparatus and Materials

• Cells, 1.5 V, with holders, 3
• Lamp in holder, 6 V 3 W approx
• Mounted slice of germanium, see technical note
• 25 W soldering iron or beaker of boiling water

Health & Safety and Technical Notes

Read our standard health & safety guidance

Typically, the germanium slice (n type) should be approximately 5 mm square, 1-2 mm thick and with leads soldered as shown; its resistance cold is of the order of 300 Ω..

If the method of mounting the slice without using solder is adopted, then heating with a match becomes possible. If not, the match is liable to melt the solder.

Procedure

1. Connect the mounted slice in series with the three cells and the lamp. The lamp does not light.
2. Heat the slice by touching it with a small soldering iron (25 watt) or by immersing in boiling water. The lamp then lights.

Teaching Notes

• You will need to check in advance that the cells and lamp are correctly matched to your germanium slice to show the desired effect.
• Pure germanium belongs to a type of semiconductor which behaves as an insulator until the rise in temperature suddenly allows it to conduct. This temperature is much lower than for a true insulator. It is called an intrinsic semiconductor. When it is heated, more charge carriers are released and so the current increases.