L13 - SUMMARY FROM LAST CLASS Conductivity n is the number...

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Dr. P. Lucas U of A MSE 110 Conductivity SUMMARY FROM LAST CLASS n is the number of charge carriers (electrons) per unit volume (m -3 ). e is the charge of the electron (1.6x10 -19 C). μ is the mobility of the electron. Metals have high n but low μ , semiconductors have high μ but low n and insulators have n =0. The mobility μ of an electron moving in a solid is finite due to collision with structural defects and vibrating atoms. Vibrations amplitude increases with temperature hence the conductivity of metals decrease with increasing temperature. µ σ ne = = kT E N n g c 2 exp In semiconductors, the number of charge carrier n increase with increasing temperature according to: hence the conductivity of semiconductors increase with increasing temperature . In semiconductors both the electron and holes contribute to the conductivity: σ =n h h +n e e
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Dr. P. Lucas U of A MSE 110 Crystals: CRYSTALLINE STRUCTURE Evidently this can only be achieved by assuming a highly ordered structure characterized by regular arrays of atoms. If the crystal can grow freely without confinement, the internal order is reflected by the external macroscopic appearance of the crystal For all type of solids, metallic, ionic, covalent and molecular, the bonding energy is maximum at a specific equilibrium bonding distance r o . A system of atoms or molecule interacting together to form a solid will therefore attempt to maximize its total bonding energy by all adopting the same equilibrium distance.
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Dr. P. Lucas U of A MSE 110 Packing of atoms: In other words, the atoms tend to fill the available volume with high efficiency, they maximize the packing density. CRYSTALLINE STRUCTURE AB C When the atoms or molecule do not favor bonding in any particular direction like elemental metals or ionic compounds, the atoms behave as hard sphere.
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This note was uploaded on 11/19/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at University of Arizona- Tucson.

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L13 - SUMMARY FROM LAST CLASS Conductivity n is the number...

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