Semiconductor electronics: materials, devices and simple circuits (part-2)

What is semi conductor? What are the properties of semi conductors? How do semi conductors behave at low temperature? What are energy levels? What are energy bands? What is Valence Band?

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There are two distinct band of energies (called valence band and conduction band) in which the electrons in a material lie. Valence band energies are low as compared to conduction band energies. All energy levels in the valence band are filled while energy levels in the conduction band may be fully empty or partially filled. The electrons in the conduction band are free to move in a solid and are responsible for the conductivity. The extent of conductivity depends upon the energy gap (Eg) between the top of valence band (EV ) and the bottom of the conduction band EC. The electrons from valence band can be excited by heat, light or electrical energy to the conduction band and thus, produce a change in the current flowing in a semiconductor.