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ELECTRICAL PROPEORTIES OF
SOLIDS
ATOMIC STRUCTURE
nucleus: protons (+) & electrons
electrons (-): electron cloud
h
h
DE BROGLIE
wave model of particles   
p mv
ELECTRONS IN ATOMS
energy levels in atoms – energy quantized
electron behaves as a wave
calculate the probability of finding the electron within an atom or solid
sodium Na Z = 11
1s2 2s2 2p6 3s1
overlapping
bands
4d
4s
3p
3s
conduction band
valence band
3s
empty
2p
2s
1s
energy levels - sodium atom
2s
filled
1s
energy bands for sodium
metal – good conductor
electrons at the top of highest filled band can easily
gain energy by moving into the conduction band and
act as charge carriers  electric current
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ELECTRONS IN SOLIDS
ENERGY BANDS IN SOLIDS – energy quantized – electrons in atoms
Electronic configurations (atoms)
diamond C
Z=6
aluminum Al Z = 13
silicon Si
Z = 14
phosphorus P Z = 15
gallium Ga
Z = 31
germanium Ge Z = 32
arsenic As
Z = 33
1s2 2s2 2p2
1s2 2s2 2p6 3s2 3p1
1s2 2s2 2p6 3s2 3p2
1s2 2s2 2p6 3s2 3p3
1s2 2s2 2p6 3s2 3p6 3p10 4s2 4p1
1s2 2s2 2p6 3s2 3p6 3p10 4s2 4p2
1s2 2s2 2p6 3s2 3p6 3p10 4s2 4p3
Energy band theory of solids
Conductivity (resistance), relative abundance of free electrons (conduction electrons)
A
I  nqv A
R
L
insulators
high resistance
conductors
resistance increase with temperature
semiconductors
resistance decrease with temperature
Diamond is a very good insulator. The electronic configuration in the ground state is 1s2
2s2 2p2. It might appear that diamond is a conductor because it has only two electrons in the
2p energy level and that the 2p band is only partly filled. However, there are two distinct 2p
energy bands separated from each other by an energy gap of 6 eV. The lower 2p band is
completely filled. At room temperature, energy due to thermal motion is only about 0.03 eV,
much small than the energy gap, so virtually no electrons will be found in the upper 2p
energy band. Silicon has a much smaller band gap and therefore less energy is required for
electrons to be free electrons and take part in conduction.
energy bands for diamond
insulator
C Z=6
energy bands for silicon
semiconductor
Si Z = 14
1s2 2s2 2p2
forbidden band exists
between two 3p bands
forbidden band exists
between two 2p bands
2p
conduction band
Egap
2p
Egap
valence band
3s
2p
2s
1s
3p
3p
valence band
1s2 2s2 2p6 3s2 3p2
Egap ~ 6 eV
2s
Egap ~ 1 eV
1s
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electron energy
In a semiconductor – holes drift in the
direction of the externally applied
electric field and the free electrons move
in the opposite direction.
electron energy
The greater the number of charged
carriers in the conduction band, the
better the conduction.
insulator
semiconductor
conductor
Intrinsic Semiconductors
valence 4 Ge Si
germanium Ge
first used in transistors (other pure materials not available at the time)
becomes conducting very easily with temperature
silicon Si
today, most widely used semiconductor material (at first was difficult to purify)
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p3.13
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Extrinsic (Doped) Semiconductors
see Periodic Table
n type semiconductors – electrons
valence 5 P, As
donor levels
n type semiconductor – a donor impurity atom has a 5th valence electron that does
not participate in the covalent bonding and is very loosely bound. It is relatively easy
for an electron to be excited from the donor level into the conduction band where it
acts as a charge carrier – charge carriers are mainly negative electrons in the
conduction band.
p type semiconductors – holes (absence of electron)
valence 3 Al, Ga, In
acceptor levels
p type semiconductor – acceptor impurity atom has only 3 valence electrons, so it can
borrow an electron from a neighbouring atom. An electron can move from the valance
band to the acceptor level creating a hole in the valance band. The resulting hole is free to
move about the crystal – positive charge carriers moving in valence band.
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p3.55
p3.18
p3.22
p3.23
ELECTRONIC COMPONENTS
np junction – diodes  rectifiers – AC to DC (current single direction)
p
+
n
p
n
I
I=0
+
Transistor – switches & amplifiers
invention lead to the development of integrated circuits
(microchips, microprocessors)
Junction transistors (npn, pnp)
collector base emitter
Valves
thermionic devices - evacuated glass containers (gas at low pressure)
electrons released from hot filament – thermionic emission
large (bulky)
hot – used lots of energy
diodes – rectification
triodes – amplifiers, switches
Solid state devices - miniaturisation
diodes (pn junctions)
transistors (npn, pnp)
IC - integrated circuits
consume little power
may contain millions of transistors
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p3.24
p3.30
p3.77
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p.3.44
p3.52
p3.90
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Solar cells – photocells – photovoltaic effect - pn junction
Photons are absorbed creating electron-hole pairs if the photon energy is greater than the
band gap energy. The holes and electrons produce a current that when connected to an
external circuit becomes a source of emf and power.
conduction band
conduction band
energy gap Eg
electrons in conduction band
evalance band
valance band
light (photon) f > Eg / h
holes in valance band
p type
p
n
n type
p
+
n
-
I
emf produces
a current to
load
+
load
Solar cell – photovoltaic effect - current across a load (resistance) generated
when light shines on the p-side of a pn junction
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p3.39
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p3.27
p.3.95
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