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Introduction: Semiconductors

<^< Assignment: Passive Components | Course Index | Diodes >^>

An isolated silicon atom contains four electrons in its outer valence shell. When silicon atoms combine to form a solid crystal, each atom positions itself between four other silicon atoms in such a way that the valence shells overlap from one atom to another. This causes each individual valence electron to be shared by two atoms. By sharing the electrons between four adjacent atoms, each individual silicon atom appears to have eight electrons in its valence shell. This sharing of valence electrons is called covalent bonding.

In its pure state, silicon is an insulator because the covalent bonding rigidly holds all of the electrons leaving no free (easily loosened) electrons to conduct current.If, however, an atom of a different element (i.e. an impurity) is introduced that has five electrons in its valence shell, a surplus electron will be present. These free electrons become available for use as charge carriers and they can be made to move through the lattice by applying an external potential difference to the material.

Conversely, if the impurity element introduced into the pure silicon lattice has only three electrons in its valence shell, the absence of the fourth electron needed for proper covalent bonding will produce a number of spaces into which electrons can fit. These spaces are referred to as holes. Once again, current will flow when an external potential difference is applied to the material.

Regardless of whether the impurity element produces surplus electrons or holes, the resulting material will have the properties that we associate with a semiconductor (it is neither an insulator nor a conductor).

The process of introducing an atom of another (impurity) element into the lattice of an otherwise pure material is called doping. When the pure material has been doped with an impurity with five electrons in its outer valence shell (i.e. a pentavalent impurity) the material is known as an N-type material.If, however, the pure material is doped with an impurity having three electrons in its outer valence shell (i.e. a trivalent impurity) it will become P-type material.

Putting this another way, N-type semiconductor material contains an excess of negative charge carriers whilst P-type material contains an excess of positive charge carriers.

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