Semiconductor Electronic Materials, Devices and Simple Circuits – Notes

Classification of materials
Vacuum tubes are also referred to as valves, as in these devices the flow of the electrons is only in one direction, that is, from the cathode to the anode.
In the semiconductor devices, the flow of charge carriers is within the solid itself.
Solid materials are broadly classified into three categories on the basis of their electrical conductivity or resistivity as:
• Metals
• Semiconductors
• Insulators

Energy Bands
In a crystal, each energy level or state is split up into a large number of closely spaced sub-levels. A group of such sub levels with continuous energy variation is called an energy band.
Valence band is the group of energy levels containing bonded electrons.
The group of energy levels occupied by free electrons that help in conduction is called the conduction band.
On the basis of energy bands, materials are classified as conductors, semiconductors and insulators, with conductors having a very small or nearly zero band gap, semiconductors having a finite and small band gap of less than 3 eV and insulators having a band gap of greater than 3ev.