Abstract: Based on the electrical neutrality of semiconductor, and taking into consideration the influence of the impurity concentration upon the impurity ioaization energies, the computer solving method is adopted to find out the Fermi level variation law of the position with the change of impurity concentration and temperature. And thus the variation law of the impurity ionization rate with the temperature at different impurity concentrations is worked out. And a detailed examination is carried out to analyze the freeze out effect of the semi-conductor low temperature carriers.