Transcript Ionic lattice structures

Superconductors
1. Electrical resistance falls as temperature increases
2. Resistance falls to zero at the critical temperature
3. A current induced in a superconductor will flow forever
4. Superconductors repel magnetic fields
5. Superconductors are said to be magnetic because they repel
magnetic fields
6. Superconductors are used in magnetic resonance imaging
7. In the future, superconductors might be useful in power transmission and
in frictionless bearings.
Semiconductors
• solid-state materials with many technical applications
• most valuable property is increase in conductivity
with increasing temperature (p38 LTS)
• conductivity measured in Siemens, S, = ohm-1
Energy levels and bands
Isolated atom
solid
band
band gap
Increasing
energy
band
band gap
band
Energy levels in
an isolated atom
Energy bands in
solid
Energy levels and bands cont.
conductor
band
insulator
Increasing
energy
semiconductor
lowest unfilled band
small energy gap
large energy gap
highest occupied
band
valence band
Semiconductors
Elements such as silicon and germanium are
semiconductors and are part of a
set of elements called metalloids, occurring
at the division between metals and non-metals.
The bonding in silicon and germanium
is similar to diamond but their bonds are
weaker. This results in a conductivity higher
than that of a non-metal, which has no free
electrons, but lower than that of a true metal,
which has mobile electrons
Semiconductors cont.
Semiconductor conductivity
also increases with exposure
to light thus applications
include
- photographic light meters
- automated lighting sensors
- photocopiers
The absorption of photons of light by semiconductors promotes
electrons from the valence band into the conduction band, leaving
electron vacancies called positive holes.
-
+
electron
positive hole
If a voltage is applied, then both the electron and the hole can
contribute to a small current flow.
Doping
The conduction can be considerably affected by the presence of impurities.
Deliberate addition of impurities is called doping
1 atom of dopant per 1 x 109 atoms of parent element
N-Type Semiconductors
The addition of pentavalent impurities such as phosphorous, arsenic
or antimony contributes free electrons, greatly increasing the conductivity
of the intrinsic semiconductor.
P-Type Semiconductors
The addition of trivalent impurities such as boron, aluminium or gallium
to an intrinsic semiconductor introduces positive holes.
p & n-type semiconductors
Semi-conductor crystals
Crystals of germanium or silicon
Doped with group
5 element
n-type
Electrons in the lowest
unfilled (conduction) band
carry the charge
p-n junction
p-type
Doped with group
3 element
Positive holes in the highest
(valence) occupied band
carry the charge
Semi-conductor crystals
e.g. silicon
trichlorosilane + hydrogen
SiHCl3 + H2
diborane
B2H6
phosphine
PH3
silicon + hydrogen chloride
Si
+ 3HCl
boron
B
phosphorous
P
Photovoltaic cell