The energy difference between the outer electrons of an atom and the energy level where they can move freely, which determines if a material conducts electricity.
From 'band' (a range of energy levels in physics) and 'gap' (space between), coined in the 20th century as physicists studied how electrons behave in solids.
The bandgap is why silicon is perfect for computer chips—it's in the Goldilocks zone where it can be easily switched between conducting and insulating electricity!
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