All modern day electronics are build using a special class of materials called semiconductors. These materials have an electrical resistivity between a conductor & an insulator.

They are the foundations of all electronics which are computerized(computers, ipods, etc) & ones which use radio waves(radio, cell phones, etc), silicon being the heart of all these devices.

The elements like Silicon & Germanium having 4 valence electrons are elemental semiconductors. The 4 valence electrons can easily bond with 4 neighbouring electrons to give rise to a lattice structure with no free electrons(at zero temperature).

Since, there are no free electrons at zero temperature, Intrinsic(pure/elemental) Semiconductors behave as insulators at zero temperature.

Then how do they differ from insulators? Well, the difference is in terms of the energy gap between the valence & conduction bands.

This energy gap is zero in case of conductors, very high for insulators & very small for semi conductors(about 1 eV)

Hence, on increasing the temperature, the electrons in the valence band of the semiconductor gain energy & some of them get sufficient energy to move to the conduction band.

This is what happens physically inside the lattice. In terms of the energy bands, we could show this as follows...

These electrons leave behind empty spaces called holes. The holes appear to move in a direction opposite to that of the electron & hence, are the positive charge carriers of the semiconductor.

Hence, a semiconductor conducts only at high temperatures & the conduction is due to both electrons & holes, also, the electrons & holes are equal in number.

However, the conductivity of the semiconductors can be changed drastically by adding certain impurities to the semiconductor materials. This process is called doping & is explained in the next post.

Semiconductors find their major application in manufacturing transistors. The first transistor was made of Germanium. Germanium, in fact, would have more free electrons at a particular temperature than silicon. But Silicon is preferable as it can be used at extremely high temperatures.