text: The law of mass action also has implications in semiconductor physics. Regardless of doping, the product of electron and hole densities is a constant at equilibrium.  This constant depends on the thermal energy of the system (i.e. the product of the Boltzmann constant, , and temperature, ), as well as the band gap (the energy separation between conduction and valence bands, ) and effective density of states in the valence  and conduction  bands.  When the equilibrium electron  and hole  densities are equal, their density is called the intrinsic carrier density  as this would be the value of  and  in a perfect crystal.  Note that the final product is independent of the Fermi level :