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Solar Energy, Solar Cells Photovoltaic, Principles Two





















The above diagram represents an insulator, semiconductor and conductor based on their prohibited band and the electron energy required to move one electron from the valance band into the conduction band, where they can interchange position with electrons in neighbor atoms within the crystal lattice.



The energy required to move one electron from the valence band into the conduction band is very high, normally it is not possible without destroying or severely damaging the material.



On semiconductors in their pure intrinsic state, the energy to move one electron from the valence band into the conduction band, is several orders of magnitude less as compared to an insulator. Even at room temperature, some electrons can acquire sufficient energy to jump into the conduction band.



In conductors the prohibited band is so small that for practical purposes do no exist, any small electrical potential would be sufficient to liberate several electrons and start an electrical current.


Intrinsic Semiconductors

Elements with atoms in which the valence band is full or almost full but the conduction band is empty or almost empty show semiconductor properties as in the silicon and germanium.

At low temperatures intrinsic semiconductors have few conduction electrons because the electrons in the valence band are tightly linked due to the covalent bonds. The silicon has only four electrons in the valence band of the 18 it should have if the valence band was full, this creates a deficit of 14 electrons, the atom will try to complete this band by grabbing  electrons from other nearby atoms as showed in the picture, this makes the intrinsic semiconductors behave more like an insulator, however at higher than room temperature some electrons can escape the covalent bonds and act as conductors with high resistance, conduction is also possible if an electrical potential or light is applied, when an electron leaves the covalence band creates a void or positive charge  that is then filled by a new electron, the voids move to the negative electrode, while the electrons or negative charges move to the positive electrode creating in this way an electrical current.      




















The covalent bonds between atoms permit the crystalline structure on silicon and germanium elements.



Extrinsic Semiconductors

Extrinsic semiconductors are also called doped semiconductors, this means that the intrinsic pure semiconductors were doped or contaminated with a very small of controlled amount of other elements at levels of 10-5 to 10-7  or one impurity particle for each 100,000 to 10,000,000 atoms of silicon, this doping process changes on it some characteristics such a conductivity. We have seen that for the silicon the valence band has only 4 electrons of the 18 it should have to complete a full third band, in accordance with atomic physics, we have seen also that in the intrinsic state, the silicon forms covalent bonds with all other neighboring atoms within the crystalline structure, and that thanks to this the semiconductors act more like an insulator at low temperatures. In this estate the silicon is not ready to be used for electronic devices, therefore the doping process is applied to improve the conductivity capabilities at a much wider range of temperatures or to changes some electronic properties.













     Prohibited and Conduction Bands in Insulators, Semiconductors and Conductors


Electron Energy

Valence Band

Conduction Band


Band basically cero


Electron Energy

Valence Band

Conduction Band

Prohibited Band


Electron Energy

Valence Band

Conduction Band

Prohibited Band

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Valence band

Valence electrons shared

with nearby atoms

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This picture shows how silicon atoms share they electrons in a covalent bond