Covalent Bonding in the Atom
The chemical activity of an atom is determined by the number of electrons in its valence shell. When the valence shell is complete, the atom is stable and shows little tendency to combine with other atoms to form solids.
Only atoms that possess eight valence electrons have a complete outer shell. These atoms are referred to as inert or inactive atoms. However, if the valence shell of an atom lacks the required number of electrons to complete the shell, then the activity of the atom increases.
Silicon and germanium, for example, are the most frequently used semiconductors. Both are quite similar in their structure and chemical behavior. Each has four electrons in the valence shell.
Consider just silicon. Since it has fewer than the required number of eight electrons needed in the outer shell, its atomswill unite with other atoms until eight electrons are shared. This gives each atom a total of eight electrons in its valence shell; four of its own and four that it borrowed from the surrounding atoms.
The sharing of valence electrons between two or more atoms produces a COVALENT BONDING between the atoms. It is this bond that holds the atoms together in an orderly structure called a CRYSTAL.
A crystal is just another name for a solid whose atoms or molecules are arranged in a three-dimensional geometricalpattern commonly referred to as a lattice. The illustration below shows a typical crystal structure. Each sphere in thefigure represents the nucleus of an atom, and the arms that join the atoms and support the structure are the covalent bonds.
Typical crystal structure in covalent bonding.
As a result of this sharing process, the valence electrons are held tightly together. This can best be illustrated by the two-dimensional view of the silicon lattice in next illustration shown below.
The circles in the figure represent the nuclei of the atoms. The +4 in the circles is the net charge of the nucleus plus the innershells (minus the valence shell). The short lines indicate valence electrons.
Because every atom in this pattern is bonded to four other atoms, the electrons are not free to move within the crystal. As a result of this bonding, pure silicon and germanium are poor conductors of electricity.
The reason they are not insulators but semiconductors is that with the proper application of heat or electrical pressure, electrons can be caused to break free of their bonds and move into the conduction band. Once in this band, they wander aimlessly through the crystal.
A two dimensional view of a silicon cubic lattice.