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Properties of metals
Metals (75% of elements)
• Lustrous (reflect light)
• (almost) all solids
• malleable & ductile
• good conductors of heat and electricity
• oxides are basic, ionic solids
• aqueous cations (n+)
Bonding in metals
Free Electron Model
Metals are positive ions in “sea” of nearly free electrons
Electrons bond metal ions together but are free to roam the crystal lattice.
Explains high electrical and thermal conductivity
Bonding in Metals
Band Theory
•Atomic orbitals (AO) mix to form molecular orbitals (MO).
•Start with 2 AO, end with 2 MO
•Start with n AOs, end up with n MOs
•In metals energy difference between orbitals in valence band is small.
•Orbital form a continuous “band” of allowed energy states.
Conduction and Insulation
Metal
Valence electrons do not fill available orbitals
(not enough electrons)
Insulator or semiconductor
Valence band is full (or completely empty). Energy gap
separates valence band from empty orbitals.
Band Gaps
Insulators: The energy gap is > 3.0 eV (= 290 kJ/mol) in
insulators
Semiconductors: The energy gap is between 0.05 and 3.0 eV
in semiconductors
(kJ/mol)
SEMICONDUCTORS
Add impurities (dopants) to semi-conductor
If impurities donate extra electrons, then the semiconductor is n-type
e.g. P impurities in Si.
If impurities accept electrons, then the semiconductor is p-type
e.g. B impurities in Si.
n-type: negative charge carriers (electrons).
p-type: apparent positive charge carriers (holes).
Silicon
Properties:
shiny, silvery gray
brittle
poor thermal conductor
semiconductor
Uses:
alloy (with Al, Mg)
silicone polymers
electronics, solar cells:
very pure silicon (<1ppb) is required.
Zone refining
to get pure Si
Diodes
A diode is a semiconductor with a p-type material bonded
to an n-type material.
Solar cells (photovoltaics) and light emitting diodes (LEDs)
are both diode devices.
When no current flows
Diodes
A diode allows current to flow in only one direction
Electrons can flow from n-type to p-type under forward bias
In a solar cell, light excitation makes current flow in the opposite
direction
Current flows when the diode is forward biased
Light Emitting Diode
When electrons combine with holes, light is emitted.
The energy of light (E = h) is the same as the band gap energy Eg
The band gap energy depends on the material used to make the
diode.
LED Materials
Wavelength
Color
Material and structure of LEDs
700
red
GaP:ZnO/GaP
660
red
Ga0.65Al0.35As/GaAs
630
red
GaAs0.35P0.65:N/GaP
610
orange
GaAs0.25P0.75:N/GaP
590
yellow
GaAs0.15P0.85:N/GaP
565
green
GaP:N/GaP
555
green
GaP/GaP
LEDs: Light Emitting Diodes
More energy efficient than incandescent lighting
LEDs producing visible light are typically made from doped
Aluminum-Gallium-Arsenide (AlGaAs)
Where are LEDs used?