Page 35 - Curriculum Visions Dynamic Book

Crystals of ultra-pure silica
Ultra-pure silicon – needed for semiconductors – is obtained from silica by growing a large crystal from a tub of molten silica. This is done using the Czochralski method of crystal growth: A rod is pulled slowly out of the melt, leaving a growing crystal behind it.
diode: a semiconducting device that allows an electric current to flow in only one direction.
doping: the adding of metal atoms to a region of silicon to make it semiconducting.
electron: a tiny, negatively charged particle that is part of an atom. The flow of electrons through a solid material such as a wire produces an electric current.
semiconductor: a material of intermediate conductivity. Semiconductor devices often use silicon when they are made as part of diodes, transistors or integrated circuits.
Solar cells
The photoelectric effect is the release of electrons from semiconductors when light falls on their surfaces. Its use
to generate electricity from sunlight has given some semiconductors the nickname “solar cells”.
When light falls on the junction of a junction diode semiconductor, photons are absorbed, providing enough energy to allow some electrons to move across the junction. This produces an electric current; light is converted directly
into electricity.
The solar cell junction diode is made so that as much light as possible falls on the junction. The surface layer of the diode sandwich is therefore made very thin and transparent.
A solar cell produces between 0.6 and 1.0 volts and a tiny current (a few milliAmps). To make the cells more powerful and give a better working voltage, cells are connected together.
 A pure silicon crystal, which will be sliced up to make “chips”.
 Solar cell arrays have to be placed in large banks if they are to generate substantial electricity. They also have to be placed in parts of the world where sunshine is common and the sun shines from as close to overhead as possible. Sites on or near the equator are ideal.
 Light emitting diodes can be used for low- energy consumption rear view lights designed to be fitted to bicycles.
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