Until relatively recently all substances according to their electrical properties were divided into conductors and dielectrics. Such a unit is advisable, because these materials differ greatly from each other in conductivity (rice. 21.1).

For conductors the value of resistivity is in the range from 10^-5 to 10^-8 Ohm*m, and for insulators it varies from 10¹⁰ to 10¹⁶ Ohm*m. These numbers show, how big is the interval of values of resistivity of conductors and dielectrics.

Further study of the conductivity of the substances led to the discovery of such materials, in which the conductivity was intermediate between conductors and insulators (rice. 21.1). These substances are called semiconductors. To them primarily include the elements of group IV of the periodic table germanium and silicon, and also silicon carbide, selenium, compounds of elements of III group to V group elements and many other substances. The resistivity of semiconductors is in the range from 10⁴ to 10^-5 Ohm*m.

Note, the resistance of various substances, including semiconductors, depends on their purity. The presence of metal in conductors of impurities has little effect on the concentration of mobile charge carriers, but seriously affects their mobility. This is due to the, what impurities create defects in the crystal lattice, which increase the resistance of metals to electric current. Impurities in metals, as a rule, increase the resistance of the latter.

In insulators, atoms of impurities are usually the electrons, which are loosely bound to atoms. These electrons are easily detached from the atoms and move to a free state. Thus, the conductivity of dielectrics is mainly determined by the content of impurities. Therefore, impurities in the dielectric, as a rule, reduce the resistance.

In semiconductors, as with dielectrics, the impurities significantly reduce their resistance. Special selection of impurities can change the resistance of semiconductors in the right direction. So impurity semiconductors have wide application in modern technology.

It is interesting to compare the dependence of the resistance of various substances on the temperature. Remember, that metal resistance increases when heated, when cooling decreases and becomes zero when superconductivity. The resistance of insulators decreases when heated, but still great. In the dielectric layer to remove an electron from the atoms needs a lot of energy, therefore, solid dielectrics for the most part have time to melt before, than get a large enough conductivity.

In semiconductors the energy, required to remove an electron from atoms, much less, than that of dielectrics. So at heating of semiconductors the number of mobile charge carriers in them is increasing rapidly, and their resistance is greatly reduced. When temperature decreases, the resistance of semiconductors increases, and at low temperatures their resistance is as great, as with dielectrics. The phenomenon of superconductivity in semiconductors is missing.

Experience has shown, the resistance of semiconductors is strongly influenced by not only temperature. The lighting of the semiconductor reduces its resistance, because the radiation brings energy, sufficient for the formation of mobile charge carriers in the semiconductor.

So, the conductivity of semiconductors is strongly dependent on temperature Yot light. These characteristics of semiconductors are of practical importance.