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Semiconductors possess a unique electronic band structure, which means that the electrons in a semiconductor material are separated into energy bands. The two important energy bands in semiconductors are the valence band and the conduction band. The valence band is the highest energy band that is normally filled with electrons, while the conduction band is an empty higher energy band. In semiconductors, there is an energy gap (called the bandgap) between the valence and conduction bands, which electrons need to overcome to move from the valence band to the conduction band.

As the temperature of a semiconductor increases, the kinetic energy of the electrons increases. This increase in kinetic energy makes it more likely for the electrons to gain enough energy to cross the bandgap and move from the valence band to the conduction band. The process of electrons moving from the valence band to the conduction band is called electron excitation.

When there are more electrons in the conduction band, electrical conductivity increases. This is because there are now more charge carriers (electrons) available to contribute to the flow of current through the semiconductor material.

As the temperature of the semiconductor increases, more electrons get excited and move from the valence band to the conduction band. This increases the number of charge carriers in the conduction band, which in turn increases the material's conductivity. It is important to note that the conductivity increases up to a certain point because as the temperature increases further, the semiconductor may undergo structural changes or degradation, resulting in a decrease in conductivity. In summary, the increase in conductivity of semiconductors when heated is due to the increased number of excited electrons moving to the conduction band, which results in a higher availability of charge carriers and increased electric current flow through the material.