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A semiconductor is a solid material whose electrical conductivity falls in between conductors and insulators. Its conductivity can be controlled by adding impurities (a specific amount of other materials) to the semiconductor.
Like other solids, the electrons in semiconductors can have energies only within certain bands (i.e. ranges of energy levels) between the energy of the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material, and the free electron energy, which is the energy required for an electron to escape entirely from the material.