Well, let's see. Galena mineral PbS with fine wire acts as semiconductor diode detector. Without looking anything up the galena is providing minority charge carriers and the copper is providing the majority electron charge carriers with enough potential difference to act as a diode.
Google homemade semiconductor and you'll see several pages of examples.
One such is:
http://thesciencecupboard.com/page3.htm [Broken]
Notice though it seems they all try to make diodes the simplest or a transistor no integrated circuits yet that would require a lot more expertise and equipment.