The incomplete ionization of impurities in semiconductors is one of the key and difficult points in the course of Physics of Semiconductor. By the profound analysis on the incomplete ionization of impurities，this paper deduces the relationship among the four essential parameters of the ionization rate η，the temperature T，the doping concentration ND and the ionized energy ΔED. Furthermore，the maximum doping concentration for complete ionization as well as the dependence of the ionization rate upon the ionized energy are proposed，and the introduction of the Lambert W-function simplifies the solution of temperature. Moreover，the issue of the incomplete ionization in 4H-SiC is discussed. The discussions and conclusions are beneficial for the students to have the systematic comprehension on the basic problems concerning incomplete ionization of impurities，and to develop the ability of analysis and innovation for future research and design work in the fields of semiconductor devices.