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1. Conductors, semiconductors, and insulators

Materials can be divided into three categories according to their ability to conduct an electric current:

Conductors: Materials that easily conduct electricity (i.e., materials with high electrical conductivity and low electrical resistivity)
Semiconductors: Materials with an electrical conductivity value that falls between that of a conductor and that of an insulator
Insulators: Materials that do not readily conduct electricity (i.e., materials with high electrical resistivity)

Conductors have electrical resistivity on the order of 10^-8 to 10^-4 Ωcm whereas insulators have electrical resistivity on the order of 10⁸ to 10¹⁸ Ωcm. Semiconductors have an electrical resistivity value between those of conductors and insulators—10^-4 to 10⁸ Ωcm.

Electrical resistivity: Ωcm

Figure 1-1 Categories of materials according to their electrical resistivity

Figure 1-2 Definition of the electrical resistance of a material

Electrical resistance (R) is the resistance to a flow of electric current through a material. The electrical resistance of a material is proportional to its length (l) and inversely proportional to its cross-sectional area (A). Each material has also an intrinsic property called electrical resistivity (ρ). Electrical resistance (R) is expressed as follows as a function of ρ, I and A. Electrical resistivity (ρ) is determined by the energy level (band) of the electrons in the outermost shell of an atom, crystalline states, and other factors.