Some impurities are known to modify the properties of grown-in microdefects in Si crystals at relatively low concentrations when the only operating mechanism is trapping of either vacancies (V) or self-interstitials (I), or both, by the major impurity state. First of all, doping affects the critical value of the ratio V/G, of the growth rate and the axial temperature gradient, that separates the range of V-based microdefects (at higher V/G) from the range of I-based microdefects (at lower V/G). The two representative impurities studied -carbon and antimony -exhibit opposite effects: carbon increases (V/G) cr thus expanding the interstitial region of a crystal, while antimony reduces (V/G) cr thus expanding the vacancy region. Another important effect of impurities is that microdefect formation may occur under a condition of a strong trapping which results in an reduction of the microdefect size and accordingly in an increase in the density. For carbon, a strong self-interstitial trapping (in the I-mode) is evidenced by an increased microdefect density. For antimony, vacancy trapping (in the V-mode) is apparently insignificant; void size is increased, due to an increased concentration of incorporated vacancies.