LECTURE 15 Crystalline structures

LECTURE 15 Crystalline structures - Dr. P. Lucas U of A MSE...

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Unformatted text preview: Dr. P. Lucas U of A MSE 110 Semiconductor devices SUMMARY FROM LAST CLASS The Hall effect is based on the Lorentz force whereby a magnetic field B exerts a force on a moving charged particle. In P-type semiconductors, deflected holes create a positive voltage V H while in N-type semiconductors, deflected electrons generate a negative V H . A diode allows current to flow only in one direction. The Fermi level is the energy level which has a probability of occupancy of ½. Due to different concentrations of holes and electrons, the Fermi level is higher in the N-type and lower in the P-type. In the depletion layer at the p-n junction, there is no charge carriers as electrons and holes recombine to form positive and negative dopant ions and build up a space charge. The space charge shifts and equalizes the Fermi energy on both sides. In forward bias, many electrons on the N side and holes on the P side are available to flow through the junction while in reverse bias no electrons and holes are available and no current can flow. Dr. P. Lucas U of A MSE 110 Crystals: • Evidently this can only be achieved by assuming a highly ordered structure characterized by regular arrays of atoms. CRYSTALLINE STRUCTURE • If the crystal can grow freely without confinement, the internal order is reflected by the external macroscopic appearance of the crystal • For all types 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. 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 The figure illustrate different ways of packing identical atoms. There is the same number of atoms in each box....
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This note was uploaded on 10/06/2009 for the course MSE 110 taught by Professor Lucas during the Spring '08 term at Arizona.

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LECTURE 15 Crystalline structures - Dr. P. Lucas U of A MSE...

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