MSE- Lecture 12 - Dr. P. Lucas U of A MSE 110 Band...

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Unformatted text preview: Dr. P. Lucas U of A MSE 110 Band Structures SUMMARY FROM LAST CLASS There are three types of band structures associated with each types of primary bonding which determine the properties of electrical conductivity of solid materials. Metals are conductors and have either a partially filled s, p, or d band or an overlapping full and empty bands which generate an overall partially filled energy band. Ionic solids are insulators. They are composed of atoms of very different electronegativity having orbitals of very different energy which generate two separate bands. An empty conduction band and a full valence band separated by a large band gap. Semiconductors are tetrahedrally connected covalent solids. They are formed of a network of sp3 hybrid orbitals generating a full bonding valence band and a empty antibonding conduction band separated by a small band gap (<4eV) that some electrons can cross. Binary and ternary semiconductor have an average of four electrons per atom that can form four covalent bonds each. Visible light is not sufficiently energetic to excite electrons across the gap of insulators which appear transparent, but can excite electrons across the bandgap of semiconductors which appear opaque. The transparency cut-off wavelength of a material is then g c E hc = Dr. P. Lucas U of A MSE 110 (macroscopic) definition of conductivity : CONDUCTIVITY IN SOLIDS When a potential V is applied across a material, it induces a flow of charge: the current I (number of charge per second) that depends on the resistance R. I R V = We can express the same equation for a solid of dimensions l=length and A=cross-sectional area according to: j E = l V E = Where E is the electric field created by a potential V is applied to a material of length l : A I j = j is the current density or flow of charge per unit area A. l A R = And is the resistivity or resistance per unit distance ( m). The quantity R depends on the shape and size of a material while is define for a certain type of material regardless of its size and shape. Dr. P. Lucas U of A MSE 110 (macroscopic) definition of conductivity : CONDUCTIVITY IN SOLIDS The quantity that is experimentally measured is R while the quantity that describes the physical property of the material is . Hence the need to convert one into the other....
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MSE- Lecture 12 - Dr. P. Lucas U of A MSE 110 Band...

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