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An Atomic Description of Silicon

All matter is composed of atoms, which are made up of positively charged protons, negatively charged electrons, and neutral neutrons. Protons and neutrons, which are about the same size, are in the close-packed, central nucleus of the atom. The much lighter electrons orbit the nucleus. Although atoms are built of oppositely charged particles, their overall charge is neutral because they contain an equal number of positive protons and negative electrons whose charges offset one another.

Illustration of a silicon crystal with its 14 electrons.

As depicted in this simplified diagram, silicon has 14 electrons. The four electrons that orbit the nucleus in the outermost "valence" energy level are given to, accepted from, or shared with other atoms.

Electrons orbit at different distances from the nucleus, depending on their energy level. For example, an electron with less energy orbits close to the nucleus, whereas one with greater energy orbits farther away. The higher energy electrons farthest from the nucleus are the ones that interact with neighboring atoms to form solid structures.

Illustration of a crystalline silicon solid demonstrating the valence electrons.

In the basic unit of a crystalline silicon solid, a silicon atom shares each of its four valence electrons with each of four neighboring atoms.

A silicon atom has 14 electrons, but their natural orbital arrangement allows only the outermost four electrons to be given to, accepted from, or shared with other atoms. These outermost four electrons, called valence electrons, play a very important role in the photoelectric effect.

Large numbers of silicon atoms bond with each other by means of their valence electrons to form a crystal. In a crystalline solid, each silicon atom normally shares one of its four valence electrons in a covalent bond with each of four neighboring silicon atoms. The solid thus consists of basic units of five silicon atoms: the original atom plus the four other atoms with which it shares valence electrons.

The solid silicon crystal is thus made up of a regular series of units of five silicon atoms. This regular, fixed arrangement of silicon atoms is known as the crystal lattice.

See these pages for more information on crystalline silicon solar cells:

To learn more about PV physics, see: