In solid-state physics and solid-state chemistry, a band gap, also called a bandgap or energy gap, is an energy range in a solid where no electronic states exist. The band gap is the minimum amount of energy required for an electron to break free of its bound state and become free to move about the semiconductor and participate in conduction. The band gap represents the minimum energy that is required to excite an electron up to a state in the conduction band where it can participate in conduction. The size and existence of this band gap allows one to visualize the difference between conductors, semiconductors, and insulators.

Key points about energy band gaps include:

• Valence Band: The lower energy level of a semiconductor is called the "valence band" (EV), where electrons are tightly bound or firmly attached to the nucleus.

• Conduction Band: The energy level at which an electron can be considered free is called the "conduction band" (EC), where electrons are free to move within the crystal lattice and serve as charge carriers to conduct electric current.

• Forbidden Energy Gap: The gap between the valence band and the conduction band is referred to as the "forbidden energy gap," where no electrons stay in this band.

• Band Gap Sizes: The size of the band gap gives materials some of their distinct properties. In insulators, the electrons in the valence band are separated by a large band gap from the conduction band, while in semiconductors, the gap is small enough that it can be bridged by some sort of excitation.

• Band Gap Engineering: Band-gap engineering is the process of controlling or altering the band gap of a material by controlling the composition of certain semiconductor alloys, such as GaAlAs, InGaAs, and InAlAs.

In summary, the energy band gap is the energy range in a solid where no electronic states exist, and it represents the minimum energy required for an electron to break free of its bound state and become free to move about the semiconductor and participate in conduction. The size and existence of this band gap allows one to visualize the difference between conductors, semiconductors, and insulators.