Using defect thermodynamics, we discuss physical factors that affect doping limits in semiconductors. The dependencies of the defect formation enthalpy on the atomic chemical potentials and on the electron Fermi energy are demonstrated. These dependencies, in particular on the Fermi energy, lead to spontaneous formation of charge-compensating defects that can limit doping. Experimental data compiled for III-V, II-VI, andI-III-VI2compounds support this view and further provide insight into the connections among different host materials. We argue that what matters is not the magnitude of the band gap that determines the dopability of a material, but rather, the relative position of the conduction-band minimum (in the case of n-doping) and the valence-band maximum (in the case of p-doping) with respect to vacuum. ©1999 American Institute of Physics.