What are semiconductors?
As the name suggests a semiconductor is neither a true conductor nor an insulator, but half way between. A number of materials exhibit this property, and they include germanium, silicon, gallium arsenide, and a variety of other substances.
To understand how it acts as a semiconductor it is necessary to first look at the atomic structure of pure silicon, a good insulator. It consists of a nucleus with three rings or orbits containing electrons, each of which has a negative charge. The nucleus consists of neutrons that are neutral and have no charge, and protons that have a positive charge. In the atom there are the same number of protons and electrons so the whole atom has no overall charge.
The electrons are arranged in rings with strict numbers of electrons. The first ring can only contain two, and the second has eight. The third and outer ring has four. The electrons in the outer shell are shared with those from adjacent atoms to make up a crystal lattice. When this happens there are no free electrons in the lattice, making silicon a good insulator. A similar picture can be seen for germanium. It has two electrons in the inner most orbit, eight in the next, 18 in the third, and four in the outer one. Again it shares its electrons with those from adjacent atoms to make a crystal lattice without any free electrons.