2_1_lect7 - EE 2 Fall 2007 Class 7 slides 1 Outline 1 2 3 4...

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1 EE 2 Fall 2007 Class 7 slides
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2 Outline 1. Review of last class 2. Classification of solids 3. Covalent bonding 4. Crystal structure of silicon 5. Bonding model 6. Band structure of typical semiconductors 7. Generation of electron hole (e-h) pairs
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5 Classification of solids Electron structure of the constituent atoms determines the physical properties of the solid Inter-atomic spacing is determined by the balance between the attractive force and the repulsive force. The attractive force can arise due to several mechanisms Ionic crystal Metallic crystal Covalent crystal Van der Waals crystal
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6 Classes of solids Ionic solid : example NaCl. Sodium gets positively ionized by losing a s electron and Chlorine gets negatively ionized by gaining the electron which makes the electron configuration the same as that of the inert element that follows Chlorine in the Periodic Table. The electrostatic attractive force between Na + ion and the Cl - ion is the binding force. The electrons are localized around each ion and as such are not free to move around in the crystal. The characteristics of ionic solid is i) high melting point (because of the high binding energy) ii) strong infra-red absorption
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7 Classes of solids Metallic crystal : The solid made up of atoms having one or two s electrons in the outer shell. The s electrons are not confined to the vicinity of the parent atom alone and are free to roam around in the entire solid and thus a ‘sea’ of electrons is formed.
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