The+Band+Model+and+Crystal+Defects

=Band Model and Crystal Defects=

Rob Harvey, Ben Hamme
The band model assumes that the low energy molecular orbitals have valence electrons and make up the valence band. The empty, higher energy molecular orbitals are then the conduction band. Using lithium metal as an example, we know lithium has 2s orbitals and 2p orbitals in its valence. The filled 2s orbitals are valence band and the intermingling empty 2s and 2p orbitals make up the conduction band. In all metals, the valence and conduction bands are considered continuous. This indicates that the electrons in the valence band can jump to the molecular orbitals in the conduction band with very little energy. This basically leaves the electrons delocalized which explains why metals conduct electricity.
 * The Band Model**



Crystals by nature have a very regular, defined shape. This ideal shape is only achieved under favorable conditions, and when the crystals are grown slowly. Perfect crystals are rarely found in nature, in reality **crystal defects** are more commons. Crystal defects occur when crystals form rapidly and the planes can become misaligned. When this happens gaps can appear in the structure, possibly letting foreign particles get lodged in the structure. Most of the time these defects can weaken a metal, but sometimes they do not. For example welding creates gaps and the atoms farther down move up to take their place. The atoms in the two pieces of metal intermingle filling in each others gaps and this determines the strength of the weld.
 * Crystal Defects**

http://www.dycoind.com/mig-welding.jpg


 * Point defects** are defects that occur just at one spot on the crystal (these are the gaps that were discussed), atoms can be put in these gaps as well. In fact this is how silicon chips are made.By **doping** semiconductors can become more conductive. Doping is when other elements are added to increase or decrease the amount of valence electrons in the MO band. Silicon is actually a poor conductor of electricity at room temperature because of the large energy gap the valence electrons need to overcome. However once it is doped with phosphorus it creates extra negative charges. Because of the negative charges it is considered a //n-type semiconductor.// The opposite happened when it is doped with Gallium, it becomes a //p-type semiconductor.//

http://www.ndt-ed.org/EducationResources/CommunityCollege/Materials/Graphics/Chrystal-Defects.jpg

http://web.mit.edu/mna/Public/normal_Absolute_12_5416.jpg

__**Sources:**__ http://en.wikipedia.org/wiki/Crystallographic_defect#Point_defects The Band Model and Crystal Defects packet