We just learned about what a Anode and Cathode are.
We learned a while back that semiconductors are chemicals that do not conduct electricity very well.
Some scientists discovered a crazy thing, that if you mix up a semiconductor with another type of chemical,
then when you zap it with electricity it becomes a good conductor!
This is called doping or adding impurities into the semiconductor.
Remember we said silicon was a semiconductor.
The chemical arsenic can be added to silicon for doping.
![silicon](https://upload.wikimedia.org/wikipedia/commons/thumb/e/e9/SiliconCroda.jpg/320px-SiliconCroda.jpg)
(from: wikipedia - silicon)
![arsenic](https://upload.wikimedia.org/wikipedia/commons/8/8b/Native_arsenic.jpg)
(from: wikipedia - arsenic)