Semiconductors are solids and, like metals, their electrical conductivity is based upon movable electrons. The primary consideration here is the level of conductivity. Materials are known as:
– Conductors at a conductivity of σ > 104 (Ωcm)−1;
– Semiconductors at a conductivity of 104 > σ > 10−8 (Ωcm)−1;
–Insulators (Non-conductors) at a conductivity of σ < 10−8 (Ωcm)−1
This simple categorization is, however, hardly an adequate criterion for a definition, and it is predominantly other characteristics, in particular the thermal behavior of conductivity, that form the basis for classification. This is where metals and semiconductors behave in an opposing manner. Whereas the conductivity of metals decreases with increasing temperature, in semiconductors it increases greatly. So what is a crystalline solid? At this point, we wish to differentiate between two separate categories. On the one hand, there are the so-called amorphous substances. In these, the structure of individual atoms and molecules displays almost no periodicity or regularity.
Crystalline solids, on the other hand, are distinguished by a perfect (or near perfect) periodicity of atomic structure. These materials naturally make it much easier to understand the physical characteristics of solids. Therefore, the explanation of semiconductor characteristics and the physical principles of photovoltaics is normally based upon crystalline semiconductors, and in particular crystalline silicon.
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