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I got this from Wikipedia. " In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material. So the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap."
A bandgap is the energy difference, generally in electron volts between the Conduction band minimum and the Valence band Maximum. Both the valence and conduction bands are divided into various energy levels and bandgap of a material is the minimum energy required to excite a charge carrier from an energy level in the valence band to the conduction band. In case of solar cells, when a photon of light with energy equal to greater than the bandgap of the material strikes a PN Junction, it can create an electron-hole pair. The hole stays in the valence band while the electron moves to the conduction band, then both these carriers have the tendency to lower their energy which leads to their motion and thus contribute towards current in the semicinductor
The term “band gap” refers to the energy difference between the top of the valence band to the bottom of the conduction band (See Figure 1); electrons are able to jump from one band to another. In order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition, the band gap energy.1,2 A diagram illustrating the band gap is shown in Figure 1. |dw:1362404895386:dw| Figure1 If the band gap is very small, the working value of V would be too small. If it is very large, the photocurrent would be too small.
Band gap is the energy difference between the Valence band and conduction band of the material. It is the energy required to create a electron-hole pair. The incoming photons having bandgap lesser or greater than the material will be either unabsorbed or wasted as heat. So, bandgap plays a crucial role in light absorption by the solar cells.The optimum bandgap for solar cells is around 1.5 eV.
Energy difference between the Vvalence band and conduction band of the material. Simply, forbidden gap where electrons can't exist.
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For semiconductor materials the energy gap to raise an electron from top region of valence band to lowest region of conduction band, some energy is required to rasie that electron so that desired electron-hole pair is generated for conduction purpose. There are different states at valence and conduction band where electron is allowed to reside but in between these states electron is forbiden to reside. So that forbiden region in which an electron is reqired to jump (move up to conduction) or descent (move down to valence) as it can't reside in between these two states is called band gap.
There are two most common types of band gaps, direct or indirect. For direct bandgap an electron jumps directly by absorbing photon but for indirect bandgap some work is needed to move the electron to the desired state.