Superconductors 2. Resistance falls to zero at the critical temperature 1. Electrical resistance falls as temperature increases 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 3. A current induced in a superconductor will flow forever 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 Increasing energy Energy levels in an isolated atom solid band band gap Energy bands in solid

Energy levels and bands cont. semiconductor small energy gap Increasing energy insulator lowest unfilled band highest occupied band large energy gap valence band conductor 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. If a voltage is applied, then both the electron and the hole can contribute to a small current flow. electronpositive hole + -

The conduction can be considerably affected by the presence of impurities. Doping Deliberate addition of impurities is called doping 1 atom of dopant per 1 x 10 9 atoms of parent element

The addition of pentavalent impurities such as phosphorous, arsenic or antimony contributes free electrons, greatly increasing the conductivity of the intrinsic semiconductor. N-Type Semiconductors

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 n-type p-type Doped with group 5 element Doped with group 3 element Electrons in the lowest unfilled (conduction) band carry the charge Positive holes in the highest (valence) occupied band carry the charge p-n junction

Semi-conductor crystals e.g. silicon trichlorosilane + hydrogensilicon + hydrogen chloride SiHCl 3 + H 2 Si + 3HCl diborane boron B2H6B2H6 B phosphine phosphorous PH 3 P

Photovoltaic cell