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Semiconductor Structure

Explanation of Semiconductor Structure

Pure silicon’s crystal structure is three dimensional. Silicon (and germanium) belongs to the IVa column of the Periodic Table, which is the carbon family of elements. The main properties for these elements is that every atom has four electrons to share with nearby atoms in creating bonds.

For a simple description, the type of a bond between two atoms of silicon is a one in which each atom offers an electron for sharing with the other atom. Therefore the two electrons shared are actually shared between the two atoms equally. This sort of sharing is called a covalent bond which is a very stable bond, and tightly holds together the two atoms, and a lot of energy is required as a result for breaking this bond.

This forms the silicon crystal, but not the semiconductor. In the silicon crystal, all the outer electrons of every silicon atom are used for creating covalent bonds with other atoms. So no electrons are available to travel from one position to another as an electrical current. Therefore, a pure silicon crystal is considered a really good insulator. A pure silicone crystal is called an intrinsic crystal.

To make the silicon crystal conduct electricity, the electrons must be allowed to move from one position to another inside the crystal, regardless of the covalent bonds between atoms. One method to do this is by introducing an impurity into the crystal structure similar to Arsenic or Phosphorus. These elements belong to the Va group of the Periodic Table, and possess five outer electrons for sharing with other atoms. In this method, four out of the five electrons bond with nearby silicon atoms like before, but a bond can be formed with the fifth electron. Just with a small applied electrical voltage this electron can be easily moved. Since the resulting crystal has extra current carrying electrons, with a negative charge each, it is called a N type  silicon.

 

Other elements – like Gallium- have only three electrons that can be shared with nearby atoms. The three electrons  create a covalent bond with nearby silicon atoms, but the anticipated fourth bond cannot be created thus leaving a  hole in the crystal’s structure.This way, holes seem to move as a positive charge through crystals. So, this type of semiconductor material is called P type silicon

 

Semiconductor Composition

Semiconductors, like (Si) Silicon are composed of separate atoms bonded together in an even, periodic structure to create an arrangement in which every atom is encircled by 8 electrons. Each individual atom is made up of a nucleus consisting of a core of positively charged particles (protons) and particles having no charge (neutrons) surrounded by the electrons. The number of protons and electrons is the same, so that the atom is electrically neutral in total. The electrons which surround every atom in the semiconductor are a part of a covalent bond.

A covalent bond contains two atoms which share two electrons. Every atom creates 4 covalent bonds with the surrounding 4 atoms. Hence, 8 electrons are being shared between every atom and the 4 surrounding atoms. Understanding the arrangement of these atoms is important to understand properties of the different materials of semiconductors. Earlier, semiconductors were manufactured from the Germanium element, but Silicon is now preferred in modern applications.

 

 

 

 

 

 

References:

https://www.pveducation.org/pvcdrom/pn-junctions/semiconductor-structure

 

 

 

 

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