1. Electrical Properties
• How are electrical conductance and resistance
characterized?
• What are the physical phenomena that distinguish
conductors, semiconductors, and insulators?

2. Electrical Conduction
• Ohm's Law:
DV = I R
voltage drop (volts = J/C)
C = Coulomb
resistance (Ohms)
current (amps = C/s) I e - A
(cross
sect.
area) DV L
• Resistivity, r and Conductivity, s:
-- geometry-independent forms of Ohm's Law
conductivity
-- Resistivity is a material property & is independent of sample

3. Conductivity: Comparison-1 -1
• Room T values (Ohm-m)
= ( - m)
METALS
conductors
Polystyrene <10
-14
Polyethylene
-15
-10
-17
Soda-lime glass 10
Concrete -9
Aluminum oxide <10
-13
CERAMICS
POLYMERS
insulators
-11 7
Silver
6.8 x 10 7
Copper
6.0 x 10 7
Iron
1.0 x 10
Silicon
4 x 10 -4
Germanium
2 x 10
GaAs 10 -6
SEMICONDUCTORS
semiconductors
Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e.

4. Energy States: Insulators & Semiconductors
-- Higher energy states not
accessible due to gap (> 2 eV).
• Semiconductors:
-- Higher energy states separated
by smaller gap (< 2 eV).
Energy
filled
band
valence
empty
filled states
GAP ?
Energy
filled
band
valence
empty
filled states
GAP

5. Intrinsic vs Extrinsic Conduction
# electrons = # holes (n = p)
--case for pure Si
• Extrinsic:
--n ≠ p
--occurs when impurities are added with a different
# valence electrons than the host (e.g., Si atoms)
• n-type Extrinsic: (n >> p)
no applied
electric field 5+ 4 +
Phosphorus atom
valence
electron
Si atom
conduction
hole
• p-type Extrinsic: (p >> n)
no applied
electric field
Boron atom 3 + 4
Adapted from Figs (a) & 18.14(a), Callister 7e.

6. p-n Rectifying Junction
• Allows flow of electrons in one direction only (e.g., useful
to convert alternating current to direct current.
• Processing: diffuse P into one side of a B-doped crystal.
• Results: + -
p-type
n-type
Adapted from Fig , Callister 7e.
--No applied potential:
no net current flow.
--Forward bias: carrier
flow through p-type and
n-type regions; holes and
electrons recombine at
p-n junction; current flows. + -
p-type
n-type
--Reverse bias: carrier
flow away from p-n junction;
carrier conc. greatly reduced
at junction; little current flow. + -
p-type
n-type

7. Properties of Rectifying Junction
Fig , Callister 7e.
Fig , Callister 7e.

8. Transistor MOSFET
MOSFET (metal oxide semiconductor field effect transistor)
Fig , Callister 7e.
Fig , Callister 7e.

9. Integrated Circuit Devices
Fig , Callister 6e.
Integrated circuits - state of the art ca. 50 nm line width
> 100,000,000 components on chip
chip formed layer by layer
Al is the “wire”

10. Ferroelectric Ceramics
Ferroelectric Ceramics are dipolar below Curie TC = 120ºC
cooled below Tc in strong electric field - make material with strong dipole moment
Fig , Callister 7e.

11. Piezoelectric Materials
Piezoelectricity – application of pressure produces current
at rest
compression induces voltage
applied voltage induces expansion
Adapted from Fig , Callister 7e.

12. Summary • Electrical conductivity and resistivity are:
-- material parameters.
-- geometry independent.
• Electrical resistance is:
-- a geometry and material dependent parameter.
• Conductors, semiconductors, and insulators...
-- differ in accessibility of 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).

13. Chapter 20: Magnetic Properties
ISSUES TO ADDRESS...
• How do we measure magnetic properties?
• What are the atomic reasons for magnetism?
• How are magnetic materials classified?
• Materials design for magnetic storage.
• What is the importance of superconducting magnets?

14. Applied Magnetic Field
• Created by current through a coil:
Applied
magnetic field H
current I
N = total number of turns
L = length of each turn
• Relation for the applied magnetic field, H:
applied magnetic field
units = (ampere-turns/m)
current

15. Response to a Magnetic Field
• Magnetic induction results in the material
current I
B = Magnetic Induction (tesla)
inside the material
• Magnetic susceptibility, c (dimensionless)
c measures the
material response
relative to a vacuum. H B
vacuum c
= 0
> 0
< 0

16. Magnetic Susceptibility
• Measures the response of electrons to a magnetic field.
• Electrons produce magnetic moments:
Adapted from Fig. 20.4, Callister 7e.
magnetic moments
electron
nucleus
spin
• Net magnetic moment:
--sum of moments from all electrons.
• Three types of response...

17. 3 Types of Magnetism Magnetic induction B (tesla)
permeability of a vacuum:
(1.26 x 10-6 Henries/m)
Magnetic induction
B (tesla)
ferromagnetic e.g. Fe3O4, NiFe2O4
ferrimagnetic e.g. ferrite(), Co, Ni, Gd
(3) ( c
as large as 10 6 !)
(2)
paramagnetic
e.g., Al, Cr, Mo, Na, Ti, Zr ( c
~ 10 -4 )
vacuum ( c
= 0) -5
diamagnetic ( c
~ -10 )
(1)
e.g., Al 2 O 3
, Cu, Au, Si, Ag, Zn
Strength of applied magnetic field (H)
(ampere-turns/m)
Plot adapted from Fig. 20.6, Callister 7e. Values and materials from Table 20.2 and discussion in Section 20.4, Callister 7e.

18. Magnetic Moments for 3 Types
No Applied
Applied
Magnetic Field (H = 0)
Magnetic Field (H)
(1) diamagnetic
none
opposing
Adapted from Fig. 20.5(a), Callister 7e.
Adapted from Fig. 20.5(b), Callister 7e.
(2) paramagnetic
random
aligned
Adapted from Fig. 20.7, Callister 7e.
(3) ferromagnetic
ferrimagnetic
aligned

19. Ferro- & Ferri-Magnetic Materials
• As the applied field (H) increases...
--the magnetic moment aligns with H. B
sat H
Adapted from Fig , Callister 7e. (Fig adapted from O.H. Wyatt and D. Dew-Hughes, Metals, Ceramics, and Polymers, Cambridge University Press, 1974.) H H
• “Domains” with
aligned magnetic
induction (B) H
Magnetic
moment grow at
expense of poorly H
aligned ones!
H = 0
Applied Magnetic Field (H)

20. Permanent Magnets • Process: • Hard vs Soft Magnets B B
2. apply H, cause
alignment
3. remove H, alignment stays!
=> permanent magnet!
Adapted from Fig , Callister 7e. 4
Negative H needed to demagnitize!
. Coercivity, HC
Applied Magnetic
Field (H)
1. initial (unmagnetized state)
large coercivity
--good for perm magnets
--add particles/voids to
make domain walls
hard to move (e.g.,
tungsten steel:
Hc = 5900 amp-turn/m)
• Hard vs Soft Magnets
small coercivity--good for elec. motors
(e.g., commercial iron Fe)
Adapted from Fig , Callister 7e. (Fig from K.M. Ralls, T.H. Courtney, and J. Wulff, Introduction to Materials Science and Engineering, John Wiley and Sons, Inc., 1976.)
Applied Magnetic
Field (H) B
Hard
Soft

21. Magnetic Storage • Information is stored by magnetizing material.
• Head can...
-- apply magnetic field H &
align domains (i.e.,
magnetize the medium).
-- detect a change in the
magnetization of the
medium.
recording head
recording medium
Adapted from Fig , Callister 7e. (Fig from J.U. Lemke, MRS Bulletin, Vol. XV, No. 3, p. 31, 1990.)
Image of hard drive courtesy Martin Chen.
Reprinted with permission
from International Business Machines Corporation.
• Two media types:
-- Particulate: needle-shaped
g-Fe2O3. +/- mag. moment
along axis. (tape, floppy)
Adapted from Fig , Callister 7e. (Fig courtesy P. Rayner and N.L. Head, IBM Corporation.)
~2.5 mm
Adapted from Fig (a), Callister 7e. (Fig (a) from M.R. Kim, S. Guruswamy, and K.E. Johnson, J. Appl. Phys., Vol. 74 (7), p. 4646, )
~120 nm
--Thin film: CoPtCr or CoCrTa
alloy. Domains are ~ nm!
(hard drive)

22. Superconductivity Hg
Copper (normal)
4.2 K
Adapted from Fig , Callister 7e.
Tc = temperature below which material is superconductive
= critical temperature

23. Limits of Superconductivity
26 metals + 100’s of alloys & compounds
Unfortunately, not this simple:
Jc = critical current density if J > Jc not superconducting
Hc = critical magnetic field if H > Hc not superconducting
Hc= Ho (1- (T/Tc)2)
Adapted from Fig , Callister 7e.

24. Advances in Superconductivity
This research area was stagnant for many years.
Everyone assumed Tc,max was about 23 K
Many theories said you couldn’t go higher
1987- new results published for Tc > 30 K
ceramics of form Ba1-x Kx BiO3-y
Started enormous race.
Y Ba2Cu3O7-x Tc = 90 K
Tl2Ba2Ca2Cu3Ox Tc = 122 K
tricky to make since oxidation state is quite important
Values now stabilized at ca. 120 K
Suddenly everyone was doing superconductivity. Everyone was doing similar work, making discoveries, & rushing to publish so they could claim to have done it first. Practically, daily new high temp. records were set.

25. Meissner Effect Superconductors expel magnetic fields
This is why a superconductor will float above a magnet
normal
superconductor
Adapted from Fig , Callister 7e.

26. Current Flow in Superconductors
Type I current only in outer skin
- so amount of current limited
Type II current flows within wire M H
Type I
Type II
complete diamagnetism
mixed state
HC1
HC2 HC
normal

27. Summary • A magnetic field can be produced by: • Magnetic induction:
-- putting a current through a coil.
• Magnetic induction:
-- occurs when a material is subjected to a magnetic field.
-- is a change in magnetic moment from electrons.
• Types of material response to a field are:
-- ferri- or ferro-magnetic (large magnetic induction)
-- paramagnetic (poor magnetic induction)
-- diamagnetic (opposing magnetic moment)
• Hard magnets: large coercivity.
• Soft magnets: small coercivity.
• Magnetic storage media:
-- particulate g-Fe2O3 in polymeric film (tape or floppy)
-- thin film CoPtCr or CoCrTa on glass disk (hard drive)