325_Sp2011_6_Dielectrics

325_Sp2011_6_Dielectrics - 6. Dielectrics EE325 Mikhail...

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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 1 6. Dielectrics EE325 Mikhail Belkin
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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 2 Dielectrics a large bandgap usually gives a small number of free charged carriers dielectrics: “large” bandgap, n free ~ 0 cm -3 conduction band (allowed states) valence band (allowed states) band gap (forbidden states) energy fermi level
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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 3 - - - + + + - - - + + + Dielectric materials: partial field cancellation when an electric field is applied to a material the charges that make up the atoms in the material will respond in an insulator, or dielectric, which contains NO free charges, the charges cannot move all the way to the surfaces but the charge displacement at the electron polarizes atoms (create dipoles out of atoms) and this charge separation can still produce an induced field that PARTIALLY cancels the applied field inside the material - - - + + + E applied E induced imagine a small piece of the material
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© Copyright Dean P. Neikirk 2004-2009 Mikhail Belkin, EE 325, ECE Dept., UT Austin 4 Dielectric materials: polarization density imagine a small piece of the material the charge displacement at the atomic level produces an induced field that PARTIALLY cancels the applied field inside the material the separation of induced charge is viewed as an “induced” dipole. We can introduce induced “polarization density” or “polarization” P W e can write for polarization: P = ε 0 χ E total_inside = ε 0 χ( E applied - E induced 29 where χ is the dimensionless dielectric susceptibility of the material (this is just a linear response approximation; it holds for many, but not all, materials and electric fields << atomic electric fields) N p P = where p is the dipole moment of an individual atom and N is dipole density per unit volume
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This note was uploaded on 04/03/2012 for the course EE 325 taught by Professor Brown during the Fall '08 term at University of Texas at Austin.

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325_Sp2011_6_Dielectrics - 6. Dielectrics EE325 Mikhail...

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