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 Figure: 22-50

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

Current flows when the diode is forward biased 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.

Material and structure of LEDs LED Materials Wavelength Color Material and structure of LEDs 700 red GaP:ZnO/GaP 660 Ga0.65Al0.35As/GaAs 630 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 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?