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Conductivity of germanium


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.


a Connect the mounted slice in series with the three cells and the lamp. The lamp does not light. 

b 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

1 You will need to check in advance that the cells and lamp are correctly matched to your germanium slice to show the desired effect. 

2 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.  


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