A gap in allowed energies
In a solid, many electronic states form closely spaced bands separated by possible forbidden intervals. A band gap describes energy structure, not an empty physical channel between atoms. Band occupancy and other conditions matter alongside gap size, so electron abundance alone does not explain electrical behaviour.
References: [1]
Valence and conduction bands
In a typical semiconductor, excitation into the conduction band leaves a hole in the valence band. Temperature, illumination and material properties affect this process. Metals have different occupancy and available-state arrangements; not every conductor requires electrons to cross the same kind of gap before carrying current.
What doping changes
Dopants introduce donor or acceptor states near band edges and alter carrier populations and the Fermi level. They need not eliminate the original band gap. Device descriptions must therefore account for bands, impurity states and carrier distributions rather than rely only on an n-type or p-type label.
References: [2]
Connecting electrical and optical properties
Light absorption can excite carriers across suitable energy differences, while recombination releases energy. Band structure consequently guides choices for solar cells, detectors and emitting materials. Transition conditions, defects and fabrication also influence results, so one gap value does not directly specify efficiency or a unique emission colour.