Download Chapter 20 - New Technologies Research Center (NTRC)

Electrical Properties
• How are electrical conductance and resistance
characterized?
• What are the physical phenomena that distinguish
conductors, semiconductors, and insulators?
Chapter 20 - 1
Electrical Conduction
• Ohm's Law:
DV = I R
voltage drop (volts = J/C)
resistance (Ohms)
current (amps = C/s)
C = Coulomb
A
(cross
sect.
area)
e-
I
DV
L
• Resistivity, r and Conductivity, s:
-- geometry-independent forms of Ohm's Law
-- Resistivity is a material property & is independent of sample
conductivity

1
s
r
Chapter 20 - 2
Conductivity: Comparison
• Room T values (Ohm-m)-1 = ( - m)-1
METALS
CERAMICS
conductors
-10
Silver
6.8 x 10 7
Soda-lime glass 10 -10-11
Copper
6.0 x 10 7
Concrete
10 -9
Iron
1.0 x 10 7
Aluminum oxide <10-13
SEMICONDUCTORS
POLYMERS
Polystyrene
Silicon
4 x 10 -4
Polyethylene
Germanium 2 x 10 0
GaAs
10 -6
semiconductors
-14
<10
10 -15-10-17
insulators
Selected values from Tables 18.1, 18.3, and 18.4, Callister 7e.
Chapter 20 - 3
Energy States: Insulators &
Semiconductors
• Insulators:
• Semiconductors:
-- Higher energy states not
-- Higher energy states separated
accessible due to gap (> 2 eV). by smaller gap (< 2 eV).
Energy
Energy
empty
band
filled
valence
band
filled
band
?
GAP
filled states
filled states
GAP
empty
band
filled
valence
band
filled
band
Chapter 20 - 4
Intrinsic vs Extrinsic Conduction
• Intrinsic:
# 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)
• p-type Extrinsic: (p >> n)
Phosphorus atom
4+ 4+ 4+ 4+
s  n e e
4+ 5+ 4+ 4+
4+ 4+ 4+ 4+
Adapted from Figs. 18.12(a)
& 18.14(a), Callister 7e.
no applied
electric field
Boron atom
hole
conduction
electron
4+ 4+ 4+ 4+
valence
electron
4+ 4+ 4+ 4+
Si atom
4+ 3+ 4+ 4+
no applied
electric field
s  p e h
Chapter 20 - 5
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.
Adapted from Fig. 18.21,
• Results:
p-type
n-type
+ + +
+ +
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.
--Reverse bias: carrier
flow away from p-n junction;
carrier conc. greatly reduced
at junction; little current flow.
-
-
-
-
-
p-type
+
-
+ - n-type
+
++- - + -
+ p-type
+ +
+ +
n-type
-
-
-
-
+
-
Chapter 20 - 6
Properties of Rectifying Junction
Fig. 18.22, Callister 7e.
Fig. 18.23, Callister 7e.
Chapter 20 - 7
Transistor MOSFET
• MOSFET (metal oxide semiconductor field effect transistor)
Fig. 18.24,
Callister 7e.
Fig. 18.25, Callister 7e.
Chapter 20 -
Integrated Circuit Devices
Fig. 18.26, 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”
Chapter 20 - 9
Ferroelectric Ceramics
Ferroelectric Ceramics are dipolar below Curie TC = 120ºC
• cooled below Tc in strong electric field - make material
with strong dipole moment
Fig. 18.35, Callister 7e.
Chapter 20 - 10
Piezoelectric Materials
Piezoelectricity – application of pressure produces current
at
rest
compression
induces
voltage
applied voltage
induces
expansion
Adapted from Fig. 18.36,
Callister 7e.
Chapter 20 - 11
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).
Chapter 20 - 12
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?
Chapter 20 - 13
Applied Magnetic Field
• Created by current through a coil:
Applied
magnetic field H
N = total number of turns
L = length of each turn
current I
• Relation for the applied magnetic field, H:
N I
H
L
current
applied magnetic field
units = (ampere-turns/m)
Chapter 20 - 14
Response to a Magnetic Field
• Magnetic induction results in the material
B = Magnetic Induction (tesla)
inside the material
current I
• Magnetic susceptibility, c (dimensionless)
B
c >0
vacuum c = 0
c<0
H
c measures the
material response
relative to a vacuum.
Chapter 20 - 15
Magnetic Susceptibility
• Measures the response of electrons to a magnetic field.
• Electrons produce magnetic moments:
magnetic moments
electron
nucleus
electron
spin
Adapted from Fig. 20.4,
Callister 7e.
• Net magnetic moment:
--sum of moments from all electrons.
• Three types of response...
Chapter 20 - 16
3 Types of Magnetism
B  (1  c)oH
permeability of a vacuum:
(1.26 x 10-6 Henries/m)
Magnetic induction
B (tesla)
(3) ferromagnetic e.g. Fe3O4, NiFe2O4
ferrimagnetic e.g. ferrite(), Co, Ni, Gd
( c as large as 106 !)
(2) paramagnetic ( c ~ 10 -4)
e.g., Al, Cr, Mo, Na, Ti, Zr
vacuum (c = 0)
(1) diamagnetic ( c ~ -10 -5)
e.g., Al 2 O3 , Cu, Au, Si, Ag, Zn
Strength of applied magnetic field (H)
Plot adapted from Fig. 20.6, Callister 7e. Values and
(ampere-turns/m)
materials from Table 20.2 and discussion in Section
20.4, Callister 7e.
Chapter 20 - 17
Magnetic Moments for 3 Types
none
opposing
(2) paramagnetic
random
aligned
(3) ferromagnetic
ferrimagnetic
aligned
Applied
Magnetic Field (H)
aligned
No Applied
Magnetic Field (H = 0)
(1) diamagnetic
Adapted from Fig.
20.5(a), Callister 7e.
Adapted from Fig.
20.5(b), Callister 7e.
Adapted from Fig. 20.7,
Callister 7e.
Chapter 20 - 18
Ferro- & Ferri-Magnetic Materials
• As the applied field (H) increases...
--the magnetic moment aligns with H.
B sat
H
Magnetic
induction (B)
H
H
H
H
0
• “Domains” with
aligned magnetic
moment grow at
expense of poorly
aligned ones!
Adapted from Fig. 20.13,
Callister 7e. (Fig. 20.13
adapted from O.H.
Wyatt and D. DewHughes, Metals,
Ceramics, and
Polymers, Cambridge
University Press, 1974.)
Applied Magnetic Field (H)
H=0
Chapter 20 - 19
Permanent Magnets
• Process:
B
3. remove H, alignment stays!
=> permanent magnet!
Adapted from Fig. 20.14,
Callister 7e.
4 . Coercivity, HC
Negative H needed to demagnitize!
B
Soft
• Hard vs Soft Magnets
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)
2. apply H, cause
alignment
Applied Magnetic
Field (H)
1. initial (unmagnetized state)
Adapted from Fig. 20.19,
Callister 7e. (Fig. 20.19 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)
small coercivity--good for elec. motors
(e.g., commercial iron 99.95 Fe)
Chapter 20 - 20
Magnetic Storage
• Information is stored by magnetizing material.
• Head can...
recording medium
-- apply magnetic field H &
align domains (i.e.,
magnetize the medium).
-- detect a change in the
magnetization of the
Image of hard drive courtesy
medium.
Martin Chen.
• Two media types:
Reprinted with permission
from International Business
Machines Corporation.
-- Particulate: needle-shaped
g-Fe2O3. +/- mag. moment
along axis. (tape, floppy)
Adapted from Fig.
20.24, Callister 7e.
(Fig. 20.24
courtesy P. Rayner
and N.L. Head, IBM
Corporation.)
recording head
Adapted from Fig. 20.23, Callister 7e.
(Fig. 20.23 from J.U. Lemke, MRS
Bulletin, Vol. XV, No. 3, p. 31, 1990.)
--Thin film: CoPtCr or CoCrTa
alloy. Domains are ~ 10 - 30 nm!
(hard drive)
Adapted from Fig. 20.25(a),
~2.5 m ~120 nm
Callister 7e. (Fig. 20.25(a)
from M.R. Kim, S.
Guruswamy, and K.E.
Johnson, J. Appl. Phys.,
Vol. 74 (7), p. 4646, 1993. )
Chapter 20 - 21
Superconductivity
Hg
Copper
(normal)
4.2 K
Adapted from Fig. 20.26,
Callister 7e.
• Tc = temperature below which material is superconductive
= critical temperature
Chapter 20 - 22
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. 20.27,
Callister 7e.
Chapter 20 - 23
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
Chapter 20 - 24
Meissner Effect
• Superconductors expel magnetic fields
normal
superconductor
Adapted from Fig. 20.28,
Callister 7e.
• This is why a superconductor will float above a magnet
Chapter 20 - 25
Current Flow in Superconductors
• Type I
current only in outer skin
- so amount of current limited
• Type II
current flows within wire
Type I
M
Type II
complete
diamagnetism
HC1 HC
mixed
state
HC2
H
normal
Chapter 20 - 26
Summary
• A magnetic field can be produced by:
-- 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)
Chapter 20 - 27
Chapter 20 - 28