Semiconductor
A class of crystalline solids possessing electrical conductivity between that of conductors and insulators, controllable by temperature, light, or doping.
Governing formula
σ = e (n μ_e + p μ_h), n_i = √(N_c N_v) e^(-E_g / (2 k_B T))
SI unit
S/m (Electrical Conductivity)
In depth
Semiconductors possess a narrow energy band gap (E_g ~ 0.5 to 3.0 eV) between the valence and conduction bands. At absolute zero (0 K), they act as perfect insulators. At room temperature, thermal energy excites electrons across E_g into the conduction band, leaving mobile holes in the valence band.
Examples in the real world
Silicon (Si, E_g = 1.12 eV) and Germanium (Ge, E_g = 0.67 eV) forming the substrate of integrated circuits and solar cells.