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Intrinsic Silicon

The word semiconductor, which features so prominently in present-day electronics, means rather generally a material which has electrical conductivity half-way between that of a metallic conductor, and that of an insulator. However, there are some specific properties that distinguish semiconductors used for electronic devices from materials which generally might be said to have semiconducting characteristics (for example, a wet insulator may very well be a semiconductor in.some general sense, but it is regarded as hazardous rather than as useful electrically).

Silicon is the most widely used of semiconductors. In the Earth's crust, it is the second most plentiful element, next to oxygen, but it appears only in oxide compounds known as silicates. Quartz is one form of silicon oxide, and sand, that very common stuff, is mostly composed of fine particles of silicon oxide. To be useful for electronic device manufacture, the silicon must be obtained in its elemental form, that is, free of oxygen, and also in its single crystal form.

Single crystal means that the atomic lattice structure making up the silicon is regular throughout; in effect, the lattice as "seen" from any lattice point within the silicon appears the same in all directions. Other impurities, especially boron which associates easily with silicon, must be removed or reduced to negligible level. A high-purity silicon is specified as semiconductor grade silicon. In later stages of processing, certain chemical elements, or impurities, are purposely added again in precisely controlled amounts to alter the electrical conductivity.

The manner in which the conductivity is altered is all-important in determining the characteristics of devices fabricated in the silicon, but before we consider this we shall compare the resistivity of intrinsic silicon with that of a good electrical conductor, copper, and that of a good electrical insulator, ceramic. Within an order of magnitude, the volume resistivities are:

In the three cases just considered, conduction, to the extent that it does occur ² is a result of the movement of electrons under the influence of an applied electric field. In copper, there is an abundance of what are termed conduction-band electrons, which are only very loosely bound to parent atoms. They move relatively freely under the influence of any externally applied electric field, and account for the high conductivity of copper. In ceramic, almost all electrons are very tightly bound to parent atoms. The density of conduction-band electrons is negligible, with the result that conductivity is also negligible. Of course, the function of an insulator is to prevent conduction, and most practical problems with insulators result from unwanted conduction through surface contaminants.

With intrinsic silicon, two distinct condition mechanisms must be taken into account. Conduction-band electrons are present which contribute to conduction, although the density is many orders of magnitude less than that in copper. These conduction-band electrons originate by being shaken loose from parent atoms, by the thermal energy which is present naturally because the semiconductor is at a finite (e.g. room) temperature.

The electron energy band from which they are shaken loose is termed the valence band. Vacancies, or holes, will be left in the valence band by those electrons which leave and these holes allow the valence-band electrons to contribute to the conduction also by a “hopping” process under the influence of an applied electric field. Rather than describe the process as one of electrons hopping from atom to atom wherever holes permit this, it is vastly more convenient to describe it in terms of the movement of holes. In this way, the concept of hole conduction arises.

The two distinct conduction mechanisms in intrinsic silicon are known, therefore, as electron conduction and hole conduction.

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