6.22: Semiconductors and Insulators

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We have seen that insulators do not conduct because there are no uncompensated electrons. Considering both the conduction and valence band in the form shown in Figure 6.22.1, it is evident that the material is an insulator when the Fermi energy lies in the bandgap. But if we introduce electrons to the conduction band, or remove electrons from the valence band, what was once an insulator can be transformed into a conductor. In fact if the Fermi energy is close to either band edge, then even a small movement in the Fermi energy can significantly modulate the conductivity. Such materials are known as semiconductors because it is easy to modulate them between the metallic and insulating regimes.

Sometimes, it can be difficult to distinguish between insulators and semiconductors. But insulators tend to have band gaps exceeding several electron-Volts and a Fermi energy close to the center of the band gap.