For pure crystals, ionic or covalent bonds are formed, there are no free charge carriers such as metals available. The reason is that the electrons, which are exchanged for Valence shell of individual atoms rebuild heavily bound. Through energy intake, these electrons from the composite, however, be resolved and then to the cargo transport is available.
In a semiconductor, the thermal energy of the particles at temperatures significantly below the melting point, to a large number of electrons as a charge from the commitment to liberate. Each free electron leaves behind a gap in the crystal lattice, also known as the defect electron or hole which is like the one shown in Figure 6 shows how a positive charge carriers can see.
In the semiconductor growing number of free carrier with the temperature, so that unlike the metal in this case, the resistance decreases.

Absent: Figure 6 to explain the electron transport in semiconductors