1. I. ELECTRICAL CONDUCTION
• Ohm's Law:
V = I R in most cases V = DV = V2 – V1
voltage drop (volts)
resistance (Ohms)
current (amps)
• Resistivity, r and Conductivity, s:
--geometry-independent forms of Ohm's Law
E: electric
field
intensity
resistivity
(Ohm-m)
J: current density
conductivity
• Resistance: 3

2. c12f01
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3. c12f02
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4. c12f03
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5. c12f04
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6. c12f05 12.6 Conduction in Terms of Band and Atomic Bonding Models
Metallic character
Excitation barrier is much smaller than heat (RT), noise or any background excitations. Practically anything excite electrons to the conduction band
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7. An intrinsic semiconductor has a band gap or barrier for the electrons to get into the conduction band
This barrier need a photon excitation, an external bias potential
Usually RT cannot excite the electron to the conduction band.
For instance, a photon of light ~4eV
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8. c12f07 12.7 Electron Mobility Scattering Events J = sE
Electric field drifts electrons in the opposite direction to the field
b/c electrons are –ve
The actual speed of electrons is much higher than the drift velocity
Scattering is due to the strike with the nuclei
When the actual velocity of the electrons is similar to the drift velocity, the system is in the ballistic regime
In the ballistic regime there is practically no barrier for the electrons
Examples: in vacuum tubes, ~in carbon nanotubes (CNT), superconductors*
J = sE
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9. 12.10 Intrinsic Semiconduction
II. SEMICONDUCTIVITY
Conductivity of semiconducting materials is lower than from metals
Sensitive to minute concentrations of impurities
Intrinsic: pure material
Extrinsic: doped with impurity atoms
12.10 Intrinsic Semiconduction
Band structure
Si (1.1 eV)
Ge (0.7 eV)
GaAs (IIIA-VA)
InSb (IIIA-VA)
CdS (IIB-VIA)
ZnTe (IIB-VIA)
We have electrons as carriers in metals and “electrons” and ‘holes’ as carriers in semiconductors

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n-type extrinsic semiconductor
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11. c12f14
p-type extrinsic semiconductor
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12. c12f15
p-type extrinsic semiconductor
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13. Intrinsic carrier increases rapidly with temperature
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14. c12f17
n-type silicon
1021/m3
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15. 12.14 The Hall Effect
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How we can determine the type of carriers?
Use magnetic fields
The Hall voltage
The Hall coefficient
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16. p-n rectifying junction
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Flow of electrons in one direction only convert alternating current to direct current
Processing: diffuse P into one side of a B-doped crystal.
• Results:
--No applied potential, no net current flow. --Forward bias: carriers flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows.
--Reverse bias: carrier
flow away from p-n junction;
carrier conc. greatly reduced
at junction; little current flow.
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19. The Junction Transistor
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20. The Junction Transistor
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21. c12f26
The MOSFET
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22. c12eqf01 12.17 Electrical properties of polymers
Usually poor conductors of electricity
Mechanism not well-understood
Conduction in polymers of high purity is electronic
Conducting Polymers
Conductivities of 1.5x107 (W-m)-1
Even polyacetylene
Due to alternating single-double bonds

23. c12f28 Dielectric Behavior Do = eoE Do = eoE + P
Dielectric material
Electric dipole structure
Charge separation
Do = eoE
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eo= permittivity of vacuum
= 8.85x10-12 F/m
Do = eoE + P
Dielectric Behavior

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25. c12f29 p = qd Do = eoE D= e E 12.19 Field vectors and polarization
Surface charge density or dielectric displacement (C/m2)
Do = eoE
For the dielectric case
D= e E

26. c12f31 Do = eoE + P where P is the polarization (C/m2) P= eo(er – 1)E
or total dipole moment per unit of volume of the dielectric
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P= eo(er – 1)E

27. c12f32 Electronic Polarization Ionic polarization
Orientation polarization

28. 12.21 Frequency Dependence of the dielectric constant
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Response to alternating fields
Reorientation time --- relaxation frequency

29. 12.22 Dielectric Strength Substance Dielectric Strength (MV/m) Air 3
Quartz 8
Strontium titanate
Neoprene rubber 12
Nylon 14
Pyrex glass
Silicone oil 15
Paper 16
Bakelite 24
Polystyrene
Teflon 60

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Frequency dependence of the dielectric constant
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31. SUMMARY --material parameters.
• Electrical conductivity and resistivity are:
--material parameters.
--geometry independent but not at small scale (nano)
• Electrical resistance is:
--a geometry and material dependent parameter.
• Conductors, semiconductors, and insulators...
--different in whether there are accessible energy
states for conductance electrons.
• For metals, conductivity is increased by
--reducing deformation
--reducing imperfections
--decreasing temperature.
• For pure semiconductors, conductivity is increased by
--increasing temperature
--doping (e.g., adding B to Si (p-type) or P to Si (n-type). 15